Working Group VI Abstracts

High Peak- and Average-Power Ultrafast Lasers and Their Application to Femtosecond X-Ray Generation
T. Ditmire1, T. E. Cowan2, G. LeSage3, I. P. Mercer1, K. B. Wharton1, M. D. Perry1, H. T. Powell1, J. B. Rosenzweig4

The recent rapid advances in high peak power, ultrafast lasers and the extension of such lasers to high average power opens a wide range of possibilities for developing unique, advanced femtosecond x-ray sources. A number of groups around the world are developing high repetition rate 100 TW to 1 PW class, sub-100 fs lasers. These lasers, when used with a number of x-ray generation schemes, including the production of laser-plasmas or the interaction of the laser pulses with high brightness relativistic electron bunches, can produce high peak brightness x-rays with photon energies ranging from 100 eV to 10 MeV. The future development of TW class lasers with average powers exceeding 100 W will make possible high average flux x-ray sources as well. We will discuss the current trends in compact, high peak power laser technology as well as the range of x-ray generation techniques that are made possible with this technology. We will also discuss potential avenues toward high average flux laser-based x-ray sources.

1Laser Program, L-477, Lawrence Livermore National Laboratory, Livermore, CA 94550
2Physics and Space Technology, Lawrence Livermore National Laboratory, Livermore, CA 94550
3Engineering Program, Lawrence Livermore National Laboratory, Livermore, CA 94550
4Dept. of Physics, University of California, Los Angeles, CA

Gas Laser Technology for Future Laser Synchrotron Sources
I. V. Pogorelsky
ATF/NSLS, BNL, 725C, Upton, NY 11973-5000 (e-mail:

We briefly review the present status of the high-power excimer and molecular gas lasers that are considered as potential pumping devices for future laser synchrotron light sources (LSLS). The analysis shows that gas lasers permit to achieve an attractive combination of a high peak power, the picosecond pulse duration, and a high repetition rate. We outline the advantages of CO2 lasers for the LSLS application that are based on their long wavelength and the technology capabilities. These considerations drive the ongoing development of the first terawatt picosecond CO2 laser at the ATF/BNL and its application for the Compton scattering experiment.

Short-Pulse X-Ray Production by Relativistic Thomson Scattering
Tom Cowan

Thomson scattering of intense laser pulses by relativistic, low-emittance electron beams may provide a very bright source of ultrafast pulses of x-rays. Such laser-electron beam Thomson scattering sources could be an important component of next generation x-ray light sources for the dynamic study of physical, chemical and biological systems on the sub-ps time scale. The progress by several groups toward high-brightness Thomson sources will be reviewed.

Free-Electron Radiation Sources Based on High-Contrast Energy Modulation of Electron Beams
Roman Tatchyn
Stanford Synchrotron Radiation Laboratory, Stanford Linear Accelerator Center, Stanford University, Stanford, CA 94309 USA

High-contrast energy modulation (HCM) of electron beams by high-power IR/visible/UV lasers can be achieved when the vector potential of the laser field (Kr) becomes comparable to that of the insertion device (Ku). Detailed theoretical studies of this parameter regime, conducted over the last several years [1,2,3] indicate that practical HCM configurations could be implemented with currently-available terawatt lasers [4], high-brightness electron beams in the 100+ MeV range [5], and low-intensity magnetic field synthesizers [6]. In this talk the basic physics of HCM and bunching, including simulations of collective dynamics in warm beams, will be briefly summarized, and a general comparison of HCM with FEL and Optical Klystron (OK) systems, including their radiative properties, will be presented. Selected novel directions for HCM research, in particular future-generation light source development, as well as possibilities toward reducing the size and cost of short-pulse X-ray FELs, will be introduced for workshop evaluation.

[1] R. Tatchyn, "Quantum Limited Temporal Pulse Generation," Proceedings of the Workshop on Fourth Generation Light Sources, M. Cornacchia and H. Winick, eds., SSRL Report 92/02, p. 482.
[2] R. Tatchyn, "Particle beam modulation techniques for the generation of subfemtosecond photon pulses in the VUV/soft X-ray range," NIM A 358, 56(1995).
[3] R. Tatchyn, "Principles of High-Contrast Energy Modulation and Microbunching of Electron Beams," presented at the FEL'98 Conference, Aug. 17, 1989, Williamsburg, VA; to be published in Nucl. Instrum. Meth., 1999.
[4] C. Le Blanc, E. Baubeau, F. Salin, J. A. Squier, C. P. J. Barty, C. Spielmann, "Toward a terawatt-kilohertz repetition-rate laser," IEEE Journal of Selected Topics in Quantum Electronics 4(2), 407(1998).
[5] J. F. Schmerge, D. A. Reis, M. Hernandez, D. D. Meyerhofer, R. H. Miller, A. D. T. Palmer, J. N. Weaver, H. Winick, D. Yeremian, "High brightness photoinjector development at the SLAC gun test facility," Nucl. Instrum. Meth. A407, (1998).
[6] R. Tatchyn, "Fourth Generation Insertion Devices: New Conceptual Directions, Applications, and Technologies," Proceedings of the Workshop on Fourth Generation Light Sources, M. Cornacchia and H. Winick, eds., SSRL Report 92/02, p. 417.

Ultrafast Lasers and Laser-Based Coherent X-Ray Sources
Sterling Backus
University of Michigan

During the past decade, there has been a revolution in the field of ultrafast science. Visible light pulses of only a few optical cycles in duration can be generated from a simple laser, and new pulse measurement schemes analogous to optical oscilloscopes are now available. Moreover, using extreme nonlinear optics, we can directly convert visible laser pulses to shorter wavelengths, to generate coherent, soft-x-ray beams of only a few femtoseconds in duration. Finally, new frequency conversion techniques have allowed us to extend nonlinear optics into the x-ray region of the spectrum, to increase the conversion efficiency of the process.

Ultrashort Electron Beam Generation in Laser Plasma Accelerators
Eric Esarey, C.B. Schroeder, and W.P. Leemans
Ernest Orlando Lawrence Berkeley National Laboratory, University of California, Berkeley CA 94720

In a plasma-based accelerator [1], the accelerating gradient is very high E[V/m] ~ 100ne[cm-3], but the wavelength of the accelerating field is the plasma wavelength lambdap[cm] ~ 3 106ne-[cm-3], which is relatively short, lambdap ~ 100 µm for a plasma density of ne ~ 10 17cm-3. To maintain a small energy spread in the accelerated electrons, the length of the injected electron bunch should be small compared to lambdap. This makes direct injection by photocathode RF guns difficult, since these guns typically produce bunches with durations > 300 fs (> 100 µm). This talk will discuss alternative methods for electron injection and trapping. In the self-modulated laser wakefield accelerator (LWFA), self-trapping from the background plasma has resulted in the observation of accelerated electrons with energies as high as 100 MeV. Possible mechanisms for self-trapping include wavebreaking [2] and coupling of the wake to Raman backscatter [3]. In the standard LWFA, optical injection of electrons into a single period of the wake can be achieved by an additional transversely directed injection pulse [4] or by two collinear colliding injection pulses [5]. Simulations of the colliding pulse injector indicate the production of relativistic ultrashort (< 3 fs) bunches with low energy spread (<3 %) and low normalized emittance (<1 mm-mrad). Applications to future light sources will be discussed.

This work was support by the Department of Energy.

[1] For a review, see E. Esarey et al., IEEE Trans. Plasma Sci. PS-24, 252 (1996).
[2] K.C. Tzeng et al., Phys. Rev. Lett. 79, 4194 (1997).
[3] E. Esarey et al., Phys. Rev. Lett. 80, 5552 (1998).
[4] D. Umstadter et al, Phys. Rev. Lett. 76, 2073 (1996); R. Hemker et al., Phys. Rev. E 57, 5920 (1998).
[5] E. Esarey et al., Phys. Rev. Lett. 79, 2682 (1997); Phys. Plasmas, May (1999); C.B. Schroeder et al., Phys. Rev. E, May (1999).

Ultrashort-Duration X-Rays from Laser-Plasmas
D. Umstadter
University of Michigan

X-rays are now routinely produced via the acceleration of electrons in plasmas by intense-light beams, generated with table-top-scale lasers. Their attributes include high-peak brightness, micron-scale-source size and femtosecond-pulse duration. They have been employed recently as x-ray probes, synchronized with laser pumps, in experiments on either time-resolved diffraction, imaging or absorption spectroscopy. Current and future prospects will be discussed.

High Energy Electron Injection and Acceleration with Intense Short Pulse Lasers *
Antonio C. Ting +
Plasma Physics Division, Naval Research Laboratory, Washington, DC 20375

The recent advances in laser-driven, high-energy acceleration of electrons have demonstrated acceleration gradients in excess of 100 GeV/m, and final electron energies of 100 MeV. However, the energy spread of the accelerated electrons is almost 100 %. Part of the reason for the large energy spread is due to the inability to initialize the low energy electrons at the optimal phase of the acceleration cycle. It is difficult to generate femtosecond electron beam pulse from a photocathode RF gun injector. A more direct approach is to utilize an intense short pulse laser to generate the required synchronized short electron bunches for injection. Different laser generated electron injection schemes will be discussed. In particular, the Laser Ionization and Ponderomotive Acceleration (LIPA) [1] electron injector will be introduced and its recent experimental results will be presented.

* Supported by DoE and ONR
+ Collaborators: C.I. Moore, S. McNaught, R. Burris, and P.Sprangle

[1] C.I. Moore et al, PRL, v. 82, 1688 (1999).