Low-Beta SC Linacs: Past, Present, and Future*


This review of superconducting linacs for low-velocity heavy ions outlines history of the subject, technical problems and achievements, characteristics of SC low- linacs, and ideas for future applications. Initial motivation for SC low- linacs was to accelerate beams from existing tandem electrostatic accelerators without destroying the excellent quality of their D.C. beams. Primary technical challenges were to accelerate effectively beams of very low velocity and the need to improve greatly beam bunching. Because of the low velocities involved, many geometries are usable for accelerating structures. To date, SC structures have been used in the range 0.008 < < 0.3. Nine low- SC linacs have been in operation and 2 more are under construction. All were built as tandem-beam boosters, but in 1992 the Argonne system was vastly upgraded by replacing the tandem with a SC-linac injector fed by an ECR ion source. Present work aimed at future applications include improvements in accelerator-structure fabrication, development of RFQ's and other SC structures for < 0.008, use of superconductors other than Nb or Pb, and development of structures for a high-current intermediate- (0.2 < < 0.8) linac to serve as a driver for a radioactive-ion accelerator.

*Research supported by the U.S. DOE Nuclear Physics Division under contract W-31-109-Eng-38.

Classification Category: A02

Status of the TESLA Design


The status of the layout of the linear collider project TESLA, which employs superconducting accelerating structures, will be presented. Latest results from the R&D program on 1.3 GHz superconducting cavities, achieved accelerating gradients and quality factors will be shown as well as the performance of the TESLA Test Facility linear accelerator.

Classification Category: A03

Research and Development for an X-Band Linear Collider


At SLAC and KEK research is advancing toward a design for a linear collider based on 11.4 GHz (X-Band) RF accelerator technology. The nominal acceleration gradient of the main linacs is nearly four times that in the SLAC Linear Collider (SLC). The design targets a 1.0 TeV center-of-mass energy but envisions initial operation at 0.5 TeV and allows for expansion to 1.5 TeV. A 1034 (cm-2 sec-1) level luminosity is achieved by colliding multiple bunches per pulse with bunch emittances about two orders of magnitude smaller than those in the SLC. The key components that are needed to realize such an accelerator are under development at SLAC and KEK. In this paper we review recent progress and discuss plans for the future.

Classification Category: A03

Scaling Laws for e+e- Linear Colliders


Design studies of a future TeV e+e- Linear Collider (TLC) are presently being made by five major laboratories within the framework of a world-wide collaboration. A figure of merit is defined which enables an objective comparison of these different designs. This figure of merit is shown to depend only on a small number of parameters. General scaling laws for the main beam parameters and linac parameters are derived and prove to be very effective when used as guidelines to optimize the linear collider design. By adopting appropriate parameters for beam stability, the figure of merit becomes nearly independent of accelerating gradient and RF frequency of the accelerating structures. In spite of the strong dependence of the wake-fields with frequency, the single bunch emittance preservation during acceleration along the linac is also shown to be independent of the RF frequency when using equivalent trajectory correction schemes. In this situation, beam acceleration using high frequency structures becomes very advantageous because it enables high accelerating fields to be obtained, which reduces the overall length and consequently the total cost of the linac.

*on sabbatical leave from SLAC

Classification Category: A03

An Induction Linac for the Second Phase of DARHT


The Dual Axis Radiographic Hydrodynamics Test facility (DARHT) is under construction at Los Alamos National Laboratory. The facility will contain two electron accelerators arranged perpendicular to each other. The second accelerator is a long pulse induction linac using Metglas core technology and will provide a beam pulse at 20MeV with flat top current of 4KA. The focal spot should be less than 1.2mm in diameter. Generation of beam breakup (BBU) and corkscrew motion at the focal spot must be minimal. Very flat beam energy, excellent alignment of transport magnets, and low values for TM mode transverse impedances in the accelerator cavities are needed. The accelerator will consist of a diode injector using a dispenser cathode, providing 3MeV energy together with a linac with 88 acceleration cells. Marx generators will provide pulsed power for both injector and linac. The pulse will be transported to a kicker (designed by Lawrence Livermore National Laboratory) which selects four 60ns pulses for transport to the final focus and the conversion target. The status of the design of the accelerator system will be presented along with results from prototype tests. Effect of operational requirements on the design of the accelerator will also be discussed.

Classification Category: A01

High-Power Proton Linac for APT; Status of Design and Development*


The accelerator for the APT (Accelerator Production of Tritium) Project is a hybrid normal-conducting (NC)/superconducting (SC) proton linac that drives a spallation neutron source with up to 170 MW of CW beam power. The high-energy SC linac employs elliptical-type niobium accelerating cavities, while the low-energy NC portion is constructed from copper cavities. The result is an integrated design making optimum use of the two technologies in their appropriate regions of application. The NC linac consists of an injector, RFQ, CCDTL, and CCL; it accelerates a 100-mA beam to 211 MeV. The SC linac, which will accelerate the beam to 1030 or 1700 MeV (depending on tritium requirements), is built in two sections optimized for different beam velocity spans. Each section is made up of cryomodules containing two, three, or four 5-cell cavities, driven by 1-MW 700-MHz klystrons. The SC linac focusing lattice consists of conventional doublets in the warm inter-module spaces. The accelerator design is supported by a comprehensive Engineering Development and Demonstration program , whose key elements are 1) construction and operation of a Low-Energy Demonstration Accelerator which will test the prototype APT front end up to 10 MeV, and 2) development and transfer to industry of prototype cryomodules for the high-energy SC linac. The status of the accelerator design and the ED&D program will be reported.

* Work Supported by U.S. Department of Energy.

Classification Category: A02

Heavy-Ion Fusion Experiments at LBNL and LLNL*

L. AHLE (presented for the groups at LBNL and LLNL)

The long-range goal of the U.S. Heavy-Ion Fusion (HIF) program is to develop heavy ion accelerators capable of igniting inertial fusion targets to generate fusion energy for electrical power production. Accelerators for heavy ion fusion consist of several subsystems: ion sources, injectors, matching sections, combiners, induction acceleration sections with electric and magnetic focusing, beam compression and bending sections, and a final-focus system to focus the beams onto the target. We are currently assembling or performing experiments to address the physics of all these subsystems. This paper will discuss some of these experiments.

* Supported by the Director, Office of Energy Research, Office of Fusion Energy, U.S. Dept of Energy, under contract DE-AC03-76SF00098.

Classification Category: U03

LINACs for Exotic Beams


One of the new frontiers in nuclear science is the use of radioactive ion beams. In the past nuclear reaction studies were restricted to the use of stable projectiles or a few long lived isotopes. The possibility of producing intense radioactive ion beams (RIB) opens a wide variety of research opportunities in nuclear astrophysics, nuclear physics, material sciences, etc. This field has grown considerably in the past ten years owing to progress in the production techniques of radioactive ion beams and more particularly in the field of heavy-ion accelerators. One of the major breakthrough is the possibility to accelerate very low energy heavy-ion using a low frequency RFQ at the front end of a linear accelerator.

Several RIB facilities are under construction and proposed. This paper will review the major issues of those facilities, and the types of linear accelerators to suit them. It will discuss their rational in function of the mass range and energies contemplated and of pre-existing laboratory constraints.

Classification Category: A06

Muon Collider: Muon Generation, Capture, and Cooling


A muon collider requires a high-intensity proton source for pi-production, followed by a high-acceptance pi-mu decay channel, and a mu-cooling system, which must result in compressed bunches of muons suitable for acceleration to high-luminosity collisions in a collider ring. Critical problems exist in developing and compressing high-energy proton bunches for producing pi's, in capturing pi's in decay to mu's, and in cooling the mu's into a compressed phase space at which high-luminosity collisions are possible. These problems and some paths to solutions are discussed in this paper.

Classification Category: A07

Advanced Concepts for High-Gradient Acceleration*


The promise of high-gradient accelerator research is a future for physics beyond the 5 TeV energy scale. Looking beyond what can be engineered today we examine basic research directions for colliders of the future, from mm-waves to lasers, and from solid-state to plasmas, with attention to material damage, beam-dynamics, a workable collision scheme, and energetics.

*Work supported by U.S. Department of Energy Contract DE-AC03-76SF00515.

Classification Category: A07

High Intensity Injector Linacs for Spallation Sources


A review is given about the layout and the design problems for recently proposed spallation neutron sources with up to 5 MW average beam power. The accelerator part consists out of a H- injector linac followed by compressor rings. Different to the design of high intensity proton linacs are the low energy front end and the restrictions at high energy for loss free ring injection. Due to intensity limitation of the H- ion source, a funneling line is needed at the front end. For ring injection the linac pulse has to be chopped after the first RFQ. In addition an unfilamented linac beam in longitudinal phase space is needed at ring injection. Concerning particle loss in the linac itself the loss rate has to be smaller than 10-7/m for unconstrained hands-on maintenance. Design criteria are discussed for 10% pulsed RF systems. Comments are given about the use of pulsed superconducting cavities above 200 MeV beam energy.

Classification Category: A02

Advances in High Brightness Electron Sources


Advances in experimental design and theoretical understanding of electron sources are reviewed. Much of the emphasis in this talk is on production of high brightness beams from rf photoinjectors. The physics underpinning the design principles of these devices, such as longitudinal and transverse effects in violent accelerating fields, beam-plasma (space charge) driven emittance oscillations and bunch lengthening are discussed. Optimization of performance in the presence of these effects is examined. Recent experimental results from basic beam physics experiments on these devices are reported. Future prospects for ultra-high beam brightness, based on the scaling these devices to short rf wavelength, and high accelerating gradient, are discussed.

Classification Category: T01

A High Intensity Proton Linac Development for the JAERI Neutron Science Project


JAERI has been proposing Neutron Science Project (NSP) which will be composed of a high intensity proton accelerator and various research facilities. The accelerator with an energy of 1.5GeV and a beam power of 8MW is required to operate both with pulse mode for basic researches and CW mode for nuclear waste transmutation study. A superconducting (SC) linac is the main option for the high energy portion from 100MeV to 1.5GeV. A beam test with an ion source and an RFQ has been performed with a current of 80mA and a duty factor of 10% at the energy of 2MeV. A hot test model of DTL has been fabricated and tested with a duty factor of 20%. A test stand for SC linac cavity with equipment of cryogenics, vacuum, RF and cavity processing and cleaning system has been prepared to test the fabrication process and physics issues. The vertical test of a beta=0.5 (145MeV) single cell SC cavity has been made resulting in a maximum electric field strength of 44MV/m at 2K.

Classification Category: A02

The TESLA Free Electron Laser -- Concept and Status

J. ROSSBACH, DESY, for the TESLA FEL collaboration

The concept of the TESLA Free Electron Laser (FEL) is to develop and realize an Angstrom wavelength, high gain FEL in parallel with the TESLA superconducting e+/e- linear collider. As a first step, an FEL for the VUV wavelength regime is now under construction at DESY, making use of the TESLA Test Facility (TTF). The TESLA linac is indeed exceptionally well suited for a short-wavelength Free Electron Laser: Excellent beam quality can be maintained during acceleration and a large variety of pulse train patterns can be provided to potential users.

The VUV FEL at the TTF comes in two phases, which are both approved. Phase 1 is the proof-of-principle experiment to demonstrate the Self-Amplified Spontaneous Emission (SASE) principle at wavelengths down to 42 Nanometers and to cultivate the technology necessary, such as small emittance photoinjectors, longitudinal bunch compression, precise undulators and appropriate diagnostics. First FEL tests for this phase 1 are scheduled for 1999.

Phase 2 aims at 6 Nanometers wavelength and provides photon beams for users. To this end, the linac will be upgraded to >1 GeV, a further bunch compression stage will be added aiming at 50 Micrometers rms bunch length, and the undulator will be 30 m long to reach saturation in the SASE mode.

The talk describes the over-all layout of each phase and the status of components. Also, recent results of simulation codes on coherent radiation effects and on the SASE process will be mentioned.

Classification Category: A04

The Linac Coherent Light Source at SLAC*


A design study has been completed at SLAC for a linac-based Free Electron Laser (FEL) known as the Linac Coherent Light Source (LCLS), which aims at the production of high peak power coherent X-rays with a wavelength of 0.15 nm. A 1-nC electron beam produces coherent radiation through self-amplified spontaneous emission (SASE) in a long undulator with a projected peak output power of ~10 GW. The design utilizes the last kilometer of the SLAC linac, not used by the PEP-II project, to compress and accelerate a single electron bunch, generated by a photocathode rf gun, to ~3400 Amperes at 15 GeV. We describe here the acceleration, compression and preservation of this high brightness electron beam through the S-band linac and related transport lines, and outline some of the required beamline modifications. Specific challenges such as emittance growth generated by coherent synchrotron radiation, compensation of the correlated electron energy spread after compression, and overall pulse-to-pulse machine stability issues drive the parameter choices and system designs outlined here.

* Work supported by U.S. Department of Energy, contract DE-AC03-76SF00515

Classification Category: A04

Computational Modeling of Klystrons


We have developed a new method for a realistic and accurate simulation of klystrons by applying the 2.5D PIC (particle-in-cell) code MAGIC. All parts of klystron from electron gun to collector are simulated with use of MAGIC only. This integrated method allows a smooth and matched transfer of simulation results from one section of klystron to another. The beam-cavity interaction is computed with no equivalent circuit model (port approximation) assumed, just by solving Maxwell's equations directly in time domain for a given cavity geometry in the presence of particles. This allows a straightforward simulation of a Traveling Wave (TW) type output structure as that of any type of cavity, while the accurate modeling of TW structure by equivalent circuit is extremely difficult and has been unsuccessful so far. Simulation results for the X-band klystrons, KEK XB72K series and SLAC XL-4, both utilizing TW output structures, are in excellent agreement with the experimental data. A 120 MW-class X-band klystron was developed based on a new design philosophy and simulations through MAGIC. Testing of the klystron is to begin in November 98.

Classification Category: D04

Temperature Anisotropy Instabilities in Space Charge Dominated Beams*


An unstable electrostatic mode has been observed in particle-in-cell simulations that, for intense beams with sufficient thermal anisotropy, transfers thermal energy from the transverse to the longitudinal directions [1]. The essential features of this mode have been observed to be similar for both initial K-V and semi-Gaussian distribution functions with the unstable mode seeded from noise. Moreover, full 3D simulations of alternating gradient transport channels have yielded similar results to idealized, R-Z simulations of uniform focusing channels. These results, together with the fact that large transverse/longitudinal thermal anisotropy can be expected in many intense beams applications suggest that this instability must be better understood in certain applications. Here we present a theory that promises to increase our understanding of this instability. This theory is on a continuous focusing model in the limit of strong thermal anisotropy with a warm, transverse KV distribution and a cold, longitudinal distribution. It is assumed that a single unstable mode of the form first analyzed by Wang and Smith [2] dominates the evolution. This mode has finite axial wavenumber and is similar in radial structure to a single transverse KV mode. The predicted mode structure is found to compare favorably with simulations carried out over a wide range of space-charge strength. Implications of these results are discussed in the context of practical machine design.

*Work supported at LLNL under U.S. DOE contract W-7405-ENG-48.

[1] I. Haber, D. Callahan, A. Friedman, D. Grote, S. Lund, and T.-F. Wang, "Characteristics of an Electrostatic Instability Driven by Transverse-Longitudinal Temperature Anisotropy," Proc. 1997 Symp. on Heavy Ion Fusion, to appear in IEEE Trans. Nuc. Sci., in press (1998).
[2] T.-F. Wang and L. Smith, "Transverse-Longitudinal Coupling in Intense Beams," Part. Accel., 12, 247 (1998).

Classification category: D01

Decelerating and Accelerating RFQs


This paper presents an overview of RFQ working principles, highlights the relevant parameters and summarises the different design approach for the high, medium and low intensity cases. Attention is then focused on the beam dynamics design in decelerating RFQs and, in particular, on how to cope with the intrinsic problems of deceleration (e.g., physical emittance increase and reduction of the longitudinal stable area). Fields of application for decelerating RFQs and their advantages with respect to conventional decelerating techniques will also be highlighted. The beam dynamics of the `Shin-pro' RFQ, the post decelerator for the CERN Antiproton Decelerator (AD) ring, will be presented in detail. This RFQ is intended to decelerate the 5.3 MeV antiproton beam coming from the AD down to an energy of virtually zero. Several decelerating schemes have been studied to fit the experimenters' need for a high quality beam with the final energy varying in the range 0 to 100 keV. Various potential solutions will be presented and discussed, with particular attention given to the intended approach.

Classification Category: D02

A Low-charge-state CW RFQ


The superconducting heavy-ion accelerator ATLAS provides heavy-ion beams with state-of-the-art quality, particularly with respect to longitudinal emittance. A related characteristic of ATLAS is very large transverse acceptance. Both of these features make ATLAS highly suitable for accelerating radioactive beams. To efficiently accelerate radioactive beams, however, a new injector section is required, to permit acceleration of very low charge state beams. This paper first briefly reviews why the best technology for such an injector seems to be a normally-conducting low-frequency RFQ. Secondly, the design of a 12 MHz split-coaxial RFQ intended for singly charged ions as heavy as mass 132 is presented. Finally, the construction and results of cw tests of a prototype structure are discussed.

Classification Category: T05

Survey Talk - New Laser and Optical Radiation Diagnostics*


Laser and optical radiation based methods for electron beam monitoring, capable of providing information on ultrafast time and micron or even submicron spatial scales are reviewed. The radiation can be produced through interaction of the electron beam with a medium (e.g., transition radiation), or through interaction with fields from a bending magnet (e.g., wiggler) or from a high intensity laser pulse. Laser based beam monitoring techniques will be discussed which, through analysis of the radiated beam properties, allow detailed measurement of transverse and longitudinal distributions of relativistic electron beam. As a particular example, results of 90 degree laser Thomson scattering experiments at LBNL will be shown [1]. Experimental results will be presented of a novel technique based on fluctuational characteristics of the radiation, for single shot non-destructive measurement of the electron beam bunch length [2]. Finally, a new technique will be discussed which relies on non-linear optical mixing of laser radiation with electron bunch emission [3]. Results will be presented of applying this technique to measuring electron bunch lengths in a storage ring [4].

*Work supported by the U.S. Department of Energy under contract No. AC03-76SF00098.

[1] W.P. Leemans et al., Phys. Rev. Lett. 77, p. 4182 (1996).
[2] P. Catravas et al., submitted to Phys. Rev. Lett.
[3] W.P. Leemans, Proc. Advanced Accelerator Concepts Workshop, AIP 398, Woodbury, NY (1997).
[4] M. Zolotorev et al., private communication.

Classification Category: T02

High Gradient Superconducting RF Structures


Superconducting (s.c.) RF structures have been known for over 30 years (HEPL, Stanford). More than 10 years after their successful start, a new generation (i.e., standing wave, pi-mode cavities), made from niobium and operated at frequencies between 352 MHz and 3 GHz, was established at accelerators being operated for high energy physics (CERN, DESY, KEK) as well as nuclear physics (CEBAF, Darmstadt) experiments. Although the HEPL s.c. cavities were limited to a 10% duty cycle and the 2nd generation now allows continuous wave operation, the increase of the accelerating gradient was remarkably small with respect to the >50 MV/m limit given by the physics of RF superconductivity.

Thirty years ago HEPL cavities reached about 2 MV/m; 15 years later DESY cavities achieved 4 MV/m. And the large 338-cavity CEBAF installation is based on a 5-MV/m gradient, the commissioning of the accelerator being less than 10 years ago. Since 1992 the TESLA (TeV Energy Superconducting Linear Accelerator) collaboration has studied the fundamental problems in cavity fabrication as well as operation. Last year an 8-mA electron beam was successfully accelerated in a 15-MV/m module containing 8 s.c. 9-cell cavities. Two more modules (20 to 25 MV/m) will be installed this year. From recent cavity tests a gradient of 25 MV/m can be taken as state of the art. Cavity production, preparation, and installation was remarkably improved, a prototype linac (TESLA Test Facility) for a large linear collider shows good performance.

Classification Category: A03

CW RFQ Fabrication & Engineering *


The design and fabrication of a four-vane RFQ to deliver a 100 milli-amp CW proton beam at 6.7 MeV is described. This linac is an OFE copper structure 8 meters in length and was fabricated using hydrogen furnace brazing as the joining technology.

* Work supported by the U.S. Department of Energy.

Classification Category: T05

Real-Time Transverse Emittance Diagnostics*


With increasing interest in high-brightness beams and recent advances in photoemission guns capable of producing high-charge, low-emittance beams, measuring transverse emittance has become a primary concern especially in driver accelerators for free-electron lasers (FELs), where a degradation of this parameter could result in disastrous effects on the FEL gain. Hence, frequent and fast measurements are needed, particularly when detailed parametric studies are required. A commonly used method consists of sampling the beam in either one of the two transverse directions with a multislit mask. This technique allows on-line transverse measurement and phase-space reconstruction. Furthermore, it is well suited for measuring emittance of space-charge dominated beams since the transmitted beamlets are emittance-dominated and the transverse phase-space can be inferred from the drift behaviour of these beamlets. Jefferson Lab's FEL has been instrumented with two devices based on the multislit technique. The typical update rates achieved are 1 Hz for the calculation of emittance and Twiss-parameters, and approximately 3 sec for isocontour plot of the transverse phase space. In this paper we discuss methods that can be implemented for on-line emittance diagnostics, and present advantages of the multislit method along with the results obtained. We also survey results obtained with similar devices by other groups.

* Work performed under the auspices of the U.S. DOE, contract #DE-AC05-84ER40150, the Office of Naval Research, the Commonwealth of Virginia, and the Laser Processing Consortium.

Classification Category: T02

Periodic Permanent Magnet Development for Linear Collider X-Band Klystrons


The Stanford Linear Accelerator Center (SLAC) klystron group is currently designing, fabricating and testing 11.424 GHz klystrons with peak output powers from 50 to 75 MW at 1 to 2 microseconds rf pulsewidths as part of an effort to realize components necessary for the construction of the Next Linear Collider (NLC). In order to eliminate the projected 40 million dollar operational-year energy bill for klystron solenoids, Periodic Permanent Magnet (PPM) focusing has been employed on our latest X-band klystron designs. A PPM beam tester has operated at the repetition rate, voltage and average beam power required for a 75 MW NLC klystron. A prototype 50 MW PPM klystron was built and tested during 1996 which operates at 55 MW, 1.5 micrseconds, 60 Hz, and at an efficiency of 55%. A PPM klystron capable of 75 MW was fabricated in 1997 and some preliminary testing was completed using below-spec magnets. Stronger focusing magnets will arrive in Spring 1998 and testing will continue. This paper will discuss the design of these PPM klystrons and the results of testing to date along with future plans for the development of a low-cost Design for Manufacture (DFM)75 MW klystron.

Classification Category: T04

Review of Fabrication of SC Cavity Structures


The search for higher and higher accelerating fields with low rf losses, plus the need of know-how transfer from scientific laboratories to firms for industrial cavity production, have contributed to the establishment of fabrication technology standards. Over years of research both on high beta and on low beta superconducting resonators, various criteria improving resonator rf performances have been codified as guidelines or canons in the fabrication process. However, the simple transfer of the standard cavity fabrication technology developed so far is no longer sufficient for the new generation machines. Not only the progressive achievement of higher accelerating fields, but also the drastic reduction in resonator production time and costs (K$ per MV/m) is compulsory for the feasibility of more and more powerful accelerators. This is the motivation under the research toward simpler and cheaper fabrication techniques as for instance that of seamless cavities. Beside the standard fabrication technology the paper reviews also the status of those new forming techniques under development in several laboratories, as hydroforming, spinning, back extrusion or superconductor/normal metal coated cavities suitable for prototype fabrication and, at least in principle, considerable for mass scale production.

Classification Category: T05

The Department of Energy Grand Challenge in Computational Accelerator Physics*


Future particle accelerators will play a major role in solving problems of international importance, including the clean-up and destruction of nuclear waste, the production of tritium, and the production of energy. The next generation of accelerators will also be crucial to advances in basic and applied science, in areas such as high energy physics, materials science, and biological science, through the development of linear colliders and spallation neutron sources. The design of accelerators for all of these projects will require a major advance in numerical modeling capability, due to extremely stringent beam control and beam loss requirements, and the presence of highly complex three-dimensional accelerator components. In 1997 the DOE approved a "Grand Challenge" in Computational Accelerator Physics. The primary goal of this project is to develop an advanced, parallel modeling capability, based on High Performance Computing resources and state-of-the-art numerical methods and algorithms, that will enable the design, optimization, and numerical validation of future accelerators for the above-mentioned projects. In this paper we will report on our progress to date, including two specific examples: (1) parallel Particle-In-Cell simulations for the APT program with up to 100 million particles using a new 3D linac simulation code, and (2) parallel electromagnetics simulations of an entire 206-cell accelerating section for the ILC using a new finite element code.

*Work supported by the U.S. Department of Energy, Division of Mathematical, Information, and Computational Sciences, Division of High Energy Physics, and Office of Defense Programs.

Classification Category: D04

High-Intensity Linac Studies in France


Teams from different French research agencies are working on high-intensity high-duty factor proton, H- and deuteron linear accelerators for several applications (waste transmutation, spallation neutron sources, tritium production, materials irradiation facilities). The conceptual design of the TRISPAL project achieved by the CEA for tritium production is presented. A separate R&D program undertaken by a CEA-CNRS (IN2P3) collaboration is also discussed. This program includes both the construction of a high intensity (100 mA), cw, 10 MeV, prototype linac (IPHI) and the fabrication and test of v/c <1 superconducting cavities.

Classification Category: U03

Production and Use of keV Positron Beams*


Kiloelectron volt energy positron beams are widely used for experiments in materials science, fundamental physics and chemistry, and electron linacs are the most common mode for production of high current, keV positron beams. Positron beam production techniques were first developed at LLNL which has since been the site of the highest current beam. We will describe techniques for positron production, new developments in positron spectroscopy and advances in applications of keV positron beams and positron spectroscopy. These include new spectroscopies with highly focused beams and beams that are pulsed with fine time structure.

*This work was performed under the auspices of the U.S. Department of Energy by LLNL under contract No. W-7405-ENG-48.

Classification Category: A01

Commissioning of the KEKB Linac

Y. OGAWA, KEK, and Linac Commissioning Group

The injector linac for the KEKB ring has been commissioned step by step since last autumn, while continuing the construction of remaining parts as well as the ordinary operation for beam injection into the Photon Factory. The commissioning has given quite satisfactory results so far: (1) A single-bunched beam with a charge of about 1 nC for direct injection into the ring was accelerated to the end of the linac at an energy of about 7 GeV. The energy will be increased soon after the rf conditioning of several remaining klystrons and should reach to the design value of 8 GeV by the full commissioning target scheduled in May. (2) An almost single-bunched beam with a charge of several nano-Coulombs for positron production has been accelerated to the positron production target. The test of positron production is to be performed soon. (3) Various kinds of beam instrumentation have been employed and utilized for precise beam tuning and diagnosis-- for example the achromatic and isochronous features of the arc section were verified utilizing beam position monitors and a bunch monitor with a streak-camera system.

Classification Category: A01

A Review of Accelerator Concepts for the Advanced Hydrotest Facility*

A. J. TOEPFER, SAIC; AHF External Advisory Committee

The Advanced Hydrotest Facility (AHF) is a facility under consideration by the Department of Energy (DOE) for conducting explosively-driven hydrodynamic experiments. The major diagnostic tool at AHF will be a radiography accelerator having radiation output capable of penetrating very dense dynamic objects on multiple viewing axes with multiple pulses on each axis, each pulse having a time resolution capable of freezing object motion (~ 50-ns) and achieving a spatial resolution ~ 1 mm at the object. Three accelerator technologies have been proposed for AHF by the DOE national laboratories at Los Alamos (LANL), Livermore (LLNL), and Sandia (SNL). Two of these are electron accelerators which will produce intense x-ray pulses from a converter target yielding a dose ~ 1,000 - 2,000 Rads @ 1 meter. LLNL has proposed a 16 - 20-MeV, 3 - 6-kA linear induction accelerator (LIA) driven by FET-switched modulators driving metglas loaded cavities. SNL has proposed a 12-MeV, 40-kA Inductive Voltage Adder (IVA) accelerator based on HERMES III pulsed power technology. The third option is a 25 - 50-GeV proton accelerator capable of ~ 1011 protons/pulse proposed by LANL. This paper will review the current status of the three accelerator concepts for AHF.

* Work supported by Sandia National Laboratories under Subcontract No. AV-0973.

Classification Category: A07

Review of Beam Diagnostics in Ion Linacs


High quality beam diagnostics are very important both for the successful commissioning and operation of ion linacs, especially if the linac contains many separate RF cavities. Recently INR has worked in collaboration with many laboratories (Fermilab, SSC, CERN, DESY, TRIUMF, KEK, GSI) for the development of beam diagnostics tools and for beam measurements in operating linacs. Experience with commissioning of new beam diagnostics devices, their use for beam measurements and studies of the machine performance will be discussed. A description of the new tools for the time-of-flight, as well as bunch shape measurements and the results of beam studies will be reported.

Classification Category: T02

Microns and Submicron Long Electron Beam Generation and Their Applications*


Electron beam bunch length on the order of 100 femtoseconds has a great potential applications in femtosecond X-ray and IR productions; micro-bunched electron beam (bunch length on the order of the optical wave length) can be used as an injector for laser based electron beam accelerators, and UV High-Gain Harmonic-Generation (HGHG) FEL. Electron beam as short as a few hundred fs have been produced using photocathode RF gun [1], thermionic RF gun [2], and magnetic bunch compressor [3]. Femtosecond micro-bunched electron beam has been observed [4] using an Inverse Free Electron Laser (IFEL) buncher. For compact light source applications (both X-ray and IR), the phase space (6-D) density of the electron beam will determine the the brightness of the radiation it generates. We will present an analysis to show that emittance compensated photocathode RF gun injector and magnetic bunch compressor are uniquely suitable for this application. Experiment is now under way at the Brookhaven Accelerator Test Facility (ATF) to produce a 100 fs (FWHM) electron beam with a peak current on the order of 1 kA. We will also discuss the latest experimental results for micro-bunched electron beam using a 5-micron SASE FEL and a 10-micron IFEL.

* Work supported by U.S. Department of Energy contract DE-AC02-76CH00016.

[1] X.J. Wang et al., Phys. Rev. E 54, R3121 (1996).
[2] P. Kung et al., Phys. Rev. Lett. 73, 967 (1994).
[3] B.E. Carlsten et al., Phys. Rev. E 53, R2072 (1996).
[4] Y. Liu and X.J. Wang et al., Experimental Observation of Femtosecond Electron Beam Micro-bunching by IFEL Acceleration, submitted to Phy. Rev. Lett. (1997).

Classification Category: T01

Halo Formation in Intense Linacs*


Halo formation in high-power linear accelerators is of considerable concern. In the paper, we review recent theoretical, computational and experimental studies [1-5] of the mechanisms of halo formation in intense ion linacs. In the theoretical and computational studies, the important effect of a periodic focusing field on the beam dynamics is included, whereas the effects of the accelerating field and beam bunching are ignored. For root-mean-square matched beams, it is shown theoretically [1] that self-field nonlinearities associated with nonuniformities in charge densities can induce chaotic particle motion and halos when the vacuum phase advance exceeds sixty degrees and space-charge depression on the phase advance is sufficiently large. The parameter dependence of halo envelope is discussed. Experimental evidence supporting the theoretical prediction is presented [2,3]. For root-mean-square mismatched beams, the threshold for halo formation is obtained numerically in the uniform focusing approximation. The halo size relative to the core envelope is determined numerically over a wide range of system parameters [4]. Finally, a scaling law is presented for edge emittance growth induced by numerical discrete-particle effects in intense beam simulations [5].

*Work supported by U.S. DOE and in part by AFOSR.

[1] Q. Qian, R. C. Davidson and C. Chen, Phys. Plasmas 2, 2674 (1995); Y. Fink, C. Chen and W. P. Marable, Phys. Rev. E55, 7557 (1997).
[2] S. Yu et. al., AIP Conf. Proc. 377, 134 (1996).
[3] C. Chen et. al., Proc. BEAMS'96, p. 1018.
[4] C. Chen and R. A. Jameson, Phys. Rev. E52, 3074 (1995).
[5] R. Pakter and C. Chen, Proc. 1997 Part. Accel. Conf., in press.

Classification Category: D01

RF Pulse Compression for Linear Colliders


In the high gradient electron linacs such as an X to S-band pulsed operation scheme, a conventional klystron driver (an ordinary modulator) cannot supply short pulses that linear collider operation requires. Therefore, some RF pulse compression system that can compress the RF pulse length and at the same time heighten the RF pulse height while conserving the total pulse energy is necessarily required. Such an RF pulse compression system is already utilized successfully in the S-band electron linacs of several laboratories. In the studies of the X-band linear accelerators, such RF pulse compression systems, i.e. SLED (SLAC Linac Energy Doubler) or its direct successors such as SLED-2 and VPM (VLEPP Pulse Multiplier), were developed as an RF pulse compression system in future linear colliders. Instead of these SLED family, a new RF pulse compression equivalent system, named as DLDS (Delay Line Distribution System) was recently introduced. This DLDS and BPC (Binary pulse compression) are expected to achieve better energy efficiencies than any member of SLED family.

In this report, general aspects of these RF pulse compression systems and their R&D activity towards linear colliders are discussed.

Classification Category: T04

The Design of an Accelerator for Advanced Pulse Radiolysis Experiments


New accelerator techniques have made it possible to make shorter and shorter electron pulses. These pulses, if appropriate detection techniques are available, make it possible to measure many important sub-picosecond chemical processes. We are proposing an accelerator system that will make it possible to measure important sub-picosecond chemical processes, such as solvation, vibrational relaxation, excited state relaxations and molecular fragmentation.

Classification Category: A01

New Techniques for Emittance Tuning in the SLC


In the 1997-98 run, the luminosity of the SLAC Linear Collider (SLC) increased by about a factor of four compared to previous runs. A significant contribution to this improvement came from revised emittance tuning techinques throughout the accelerator. A new strategy was used to cancel wakefield and dispersion in the LINAC. Careful monitoring and control of the ARCs optics reduced the emittance growth due to coupling, wakefield and synchrotron radiation. New Final Focus optics and upgraded diagnostics improved the emittance measurement resolution and optimization. These and other new procedures resulted in an average 25% spurious residual emittance growth in both planes from the damping rings to the interaction point.

Classification Category: A03

Toward the Zero Beam Diagnostics


Beam diagnostics measurements are a topic and well-known item in all particle accelerators based facilities. Different solutions based on different methods were deeply investigated and realized to give the right answer to all the typical requirements. In the last years some new research projects have put a big challenge to the beam diagnostics experts: the necessity to develop new devices for quasi-zero-current and quasi-zero-energy beam diagnostics. Nuclear physics (radioactive ion beams), solid state physics (implantation, radiation damage) and medical physics (proton therapy, light ion therapy) as well, have the same problem to characterize very low intensity (lower than 1.E-6 pps) and, some time, low energy (lower than 1 MeV) ion beams. This means that standard techniques based on electrical or optical devices, well proved for high intensities and energies, must be drastically revised in order to improve their performances toward the lower limits. In the last years, the LNS received different requests for beams with such extreme characteristics. This was the reason to start a research activity on this subject. The obtained results are presented together with a general overview of the worldwide activity on this particular topic.

Classification Category: T02

Nonlinear Space Charge Effects and Emittance Growth in Linac*


The nonlinear space charge effect of bunched beam in linac is one of the important reasons that induce the emittance growth. In this paper, we present the general formulas for calculating the potential of space charge with nonuniform distribution in surrounding structure. The free energy of a cylinder model of space charge has been worked out for different distributions. For a bunched beam with nonuniform distributions in a waveguide of linac, following the procedure by Hofmann [1], we obtain a differential equation for emittance change. Finally, some numerical results are given to show the emittance growth caused by these nonuniformities.

* Work supported by National Natural Science Foundation of China.

[1] I. Hofmann and J. Struckmeier, Particle Accelerators 21, 69 (1987).

Classification Category: D02

A High Charge, High Duty Factor RF Photoinjector for the Next Generation Linear Collider


Testing of the prototype TESLA Test Facility (TTF) RF photoinjector has been completed, and fabrication of the high duty factor TTF injector is underway. Experimental results from the prototype tests conducted at Argonne National Laboratory will be presented. Engineering design work for the high power TTF gun will be discussed together with initial operating experience with a Cesium telluride photocathode in short (50 microbunch) pulse trains, and long RF pulses (1000 millisecond), conducted at Fermilab. An outline of future advanced accelerator R&D activities at Fermilab will also be presented.

Classification Category: T01

RF System Developments for CW and/or Long Pulse Proton Linacs


High Power Proton Linacs are under development or proposed for development at Los Alamos and elsewhere. By current standards these linacs all require very large amounts of RF power. The Accelerator for Production of Tritium (APT) is a CW accelerator with an output current and energy of 100 mA and 1700 MeV, respectively. The Spallation Neutron Source (SNS), in its ultimate configuration, is a pulsed accelerator with an output current, energy, and duty factor of 140 mA, 1000 MeV, and 6%, respectively. Other accelerators such as those which address transmutation and upgrades to LANSCE have similar requirements. For these high average power applications, the RF systems represent approximately half of the total cost of the linac and are thus key elements in the design and configuration of the accelerator. Los Alamos is fortunate to be actively working on both APT and SNS. For these programs we are pursuing a number of component developments which are aimed at one or more of the key issues for large RF systems: technical performance, capital cost, reliability, and operating efficiency. This paper provides the background and status on the key items under development.

Classification Category: T04

Linac R&D in Korea*


The Pohang Light Source (PLS), the first large-scale accelerator complex in Korea, is a national synchrotron radiation users' facility for basic and applied science research. It consists of a full-energy 2-GeV injector linac and a low-emittance storage ring, and has been serving users since September of 1995. With the success of this facility, there are strong demands on accelerator facilities for various applications, such as a neutron facility for transmutation and energy production called the KOMAC-project, a nuclear data center, a small cyclotron for radioisotope production. Most of the linac R&D programs are subsystems in the low-energy end or special components to improve the performance of the existing/PLS linac. This paper presents current R&D activities related to linear accelerators at PLS and proposed accelerator programs in Korea.

*Work supported by MOST and POSCO.

Classification Category: A05

Review of Negative Hydrogen Ion Sources High Brightness/High Current


Due to the development of reliable H- ion sources, charge-exchange injection into circular accelerators has become routine. This paper reviews recent developments in negative hydrogen ion sources. Various types of such sources with some basic physics, operating parameters and beam characteristics will be described.

Classification Category: T01

Emerging Industrial Applications of Linacs

A. M. M. TODD, Northrop Grumman Corp.

Medical applications, particularly clinical x-ray systems, continue to dominate the industrial use of linear accelerators. However, several developments suggest that significant new markets may soon emerge. Among these are food irradiation, long pushed within the technical community but never developing market pull to secure a commercial foothold, which should benefit from the recent FDA approval of the technique for the sterilization of meat. Another growth area, driven by significant FAA and DOD funding, is contraband detection, particularly for the inspection at airports, seaports, military bases and border crossings, of the large cargo volumes of train, truck, airline and shipping containers. Although the current market surge is dominated by low energy x-ray devices, linac systems appear to have a niche for large volumes. In a more speculative vein, ultraviolet and infrared material processing with high-power, linac-driven, free-electron lasers has the potential to evolve into a major and varied market. Here the many applications are proven and their commercialization is limited only by the economics of the available light sources. These specific applications, their performance requirements and economics will be discussed in terms of their impact on accelerator type, parameters, reliability and cost.

Classification Category: U01

The Challenge of Inertial Fusion Driven by Heavy Ion Accelerators

I. HOFMANN, GSI Darmstadt

Accelerators have a chance to participate in the enormously growing energy market of the 21st century if inertial confinement fusion can be shown to be competitive with fossil and fission alternatives. The general advantages of fusion energy with respect to safety, environment (CO2) and inexhaustable fuel supply are further enhanced by the relative "simplicity" of inertial fusion compared with magnetic confinement fusion. While it is expected that ignition and gain of fusion pellets will be demonstrated by the National Ignition Facility after 2005, the development of a suitable driver with high efficiency and rep-rate capability remains the next challenging issue. We review the status of the European HIDIF study group which has the goal of demonstrating the feasibility of the RF linac based heavy ion driver. The main issues of the envisaged funneled linac tree (400 mA of Bi+ at 10 GeV) and storage ring accelerator complex are to minimize emittance growth in six-dimensional phase space and to keep beam loss at an acceptable level. The progress of the high-intensity upgrade of the GSI heavy ion injector plays an important role in this development. The requirements of a fusion driver injector can be scaled from this concept using a MEVVA ion source and an RFQ and IH-structure injector designed to accelerate 15 mA of U4+ in 1999.

Classification Category: U03

Fourth Generation X-ray Source