Radiation Effects Studies at the Advanced Photon Source

M. Petra, P.K. Den Hartog, E.R. Moog, S. Sasaki, N. Sereno and I.B. Vasserman
Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439 USA

Abstract

At the Advanced Photon Source (APS) concern for radiation-induced demagnetization of the insertion devices (IDs) in the storage ring and in the free-electron laser has prompted systematic radiation effects studies towards the development of efficient techniques for ID protection. The studies include radiation dose monitoring, investigation of the factors that could lead to ID demagnetization at the APS, as well as, potentially, a dedicated radiation effects testbed at the APS providing GeV electron beams. Such studies could also be directly applicable to future generation facilities, such as the Linac Coherent Light Source. Results and discussion of the radiation damage studies at the APS are presented.

PACS codes: 42.88, 41.85.L, 75.50.W
Keywords: radiation effects, wiggler magnets, permanent magnets

1. Introduction

The insertion devices (IDs) at the Advanced Photon Source (APS) storage ring and the free-electron laser (FEL) are subjected to a harsh radiation environment that can cause radiation-induced demagnetization. This radiation environment is composed of a broad energy spectrum of gamma rays, x-rays, electrons and neutrons [1].

Radiation damage in materials is, in general, a function of the type and energy of the radiation, the integrated dose and dose rate, the irradiation temperature, the impurity content of the material and the irradiation history.

Radiation damage studies of permanent magnets have been performed under a variety of radiation fields, such as electron, proton, gamma, neutron and mixed electron-photon (bremsstrahlung) [2-6]. The studies have shown that charged particles and high-energy neutrons are effective at causing radiation damage in permanent magnets. In addition, the larger the coercivity, the higher the radiation resistance of the magnet. Magnets of SmCo have also been found to be more radiation resistant than NdFeB magnets, which could be attributed to their higher Curie temperature and superior thermal properties [7].

Most of the permanent magnets used in the APS undulators are Shin Etsu N38H NdFeB, while those used for the Linac Coherent Light Source (LCLS) prototype are Shin Etsu N39SH NdFeB.

Results from radiation damage studies are sometimes difficult to interpret because of unknown differences in factors that are known to affect the likelihood of damage, such as the irradiation temperature, the sample geometry, and the grade and manufacturer of the magnet material. Further study is needed in understanding the damage process so that efficient techniques can be developed for magnet protection.

Radiation-induced demagnetization has been observed in two IDs at ESRF caused by electron beam misteering onto the vacuum chamber wall of the ID [8].

2.  Radiation Effects Studies

2.1 Dosimetry

Until recently, dose monitoring at the APS has been performed with thermoluminescent dosimeters (i.e., TLD-700 and TLD-800) and radiachromic films [9-10]. In 2002, a new high-dose dosimetry technique has been added, i.e., alanine electron paramagnetic resonance (EPR) dosimetry.

Alanine is an amino acid in which disrupted molecular bonding due to ionizing radiation gives rise to free radicals. The radical concentration is a function of the absorbed dose and is then analyzed by EPR. This technique exhibits equivalent response for photons and electrons and an energy-independent response for photon energies above approximately 100 keV; however, dose underestimation occurs for photon energies below that value [11]. Absorbed doses up to ~20 Mrad can be measured with this technique.

Dose monitoring using alanine EPR dosimetry has been initiated on the IDs in the APS storage ring and the FEL. Other critical components around the accelerator (such as CCD cameras, encoders, motors and cables) are also being monitored. Integrated doses on these components provide information on their lifetime in the APS radiation environment.

Dose measurements on the IDs are performed on the upstream (US) and the downstream (DS) ends of the devices. The dosimeters are being replaced at the end of each run (i.e., approximately at three-month time intervals). The US doses are being affected by the gap-size history during the run and the presence or absence of shielding US of the IDs; a number of IDs in the storage ring are not shielded due to space constraints.

To date, the highest doses have been observed in 2001 in sector 3 of the APS storage ring (i.e., at least 10 Mrad during run 2001-3.) This is primarily due to increased beam losses caused by a recent reduction in the size of the ID vacuum chamber in that sector, that is, from an 8 mm aperture to a 5 mm aperture. The 5-mm-aperture ID vacuum chamber is the limiting aperture in the storage ring, and the first such chamber after the injection point is located in sector 3. In addition, the APS storage ring was run in top-up mode, with a new low-emittance and lower lifetime lattice, and the injection efficiency was not yet optimized for these new conditions.

Absorbed doses in the other sectors around the storage ring are lower than those in sector 3. Recorded doses with alanine dosimeters remained below 1 Mrad on most of the IDs on those sectors during 2002 (run 2002-2).

2.2 Radiation-Induced Damage to IDs at the APS Storage Ring

The IDs APS27#2 and U27#12 that are located in sector 3 suffered severe radiation damage. Abnormally high doses recorded by radiation dosimeters on those two devices during the run 2001-3 prompted removal of the devices from the storage ring and investigation for potential radiation damage. The device U27#12 was removed in December 2001 for repair and reinserted in the storage ring (Fig. 1a). At the end

 

 

Fig. 1. Magnetic field changes in the downstream undulator in sector 3, U27#12, at 10.5 mm gap. The rightmost side of the figure represents the US end of the ID. (The data for the weaker end poles are omitted.)

(a) The top panel shows the peak magnetic field under each pole as measured in June 1997 (before the device was installed) and in Dec. 2001. The bottom panel shows the difference in the magnetic field.

(b) The top panel shows the peak magnetic field under each pole as measured in Jan. 2002, after the device had been retuned, and in May 2002. The bottom panel shows the difference in the magnetic field of the next run, it was again removed and checked for potential radiation damage, and further damage was found (Fig. 1b).
The most severe radiation effects have been observed in sector 3; however, radiation damage has also been found in U33#15 located in sector 1.

Table 1 shows the rms phase errors measured for each of the demagnetized IDs initially, after the damage was observed, and after the ID was retuned. The amount of the mechanical taper introduced is also shown in the table. The third harmonic of the undulator radiation as calculated from the measured magnetic field is given in table 1 as a percentage of the ideal intensity that would be obtained from a perfect undulator. (The original undulator specification aimed at having the third-harmonic intensity be at least 70% of the ideal.) The radiation damage had a particularly strong effect on the intensity of the third harmonic. The performance degradation was most pronounced for U27#12 and least pronounced for U33#15 in accordance with the dosimetry results.

Magnetic tuning was not able to remove all the damage in U27#12. Instead, more extensive repair to this ID will be needed.

Table 1. Rms phase errors for the damaged IDs initially, after observed damage, and after retuning.

U27#12, Gap 10.5 mm, Sector 3 DS   

APS27#2, Gap 11.5 mm, Sector 3 US

U33#15, Gap 11.5 mm, Sector 1 DS

In 2002, temperature monitoring along the length of the IDs in sector 3, as well on the vacuum chamber and the ambient air in that sector, was initiated. Initial temperatures remained below 30^oC. In addition, dose monitoring along the length of the damaged IDs is being performed to allow comparisons between absorbed dose and demagnetization profiles. The radiation sensitivity of both APS and LCLS type magnets will be investigated in future experiments. In addition, neutron activation analysis (NAA) is being performed in both type sample magnets for impurity identification.

2.3 Proposed Radiation-Damage Testbed at the APS

A dedicated testbed for systematic study of radiation damage in magnets is proposed for the APS. Use of the testbed for radiation-damage studies of materials other than magnets could also be possible. The proposed testbed will be capable of delivering 325 MeV – 7 GeV electron beams at a maximum current of 5 nC/per shot and a cycle rate of 2 Hz. For that purpose, an existing stub line on the booster ring can be built out with a few new components (Fig. 2).

Fig. 2. Proposed radiation-damage testbed
beamline and associated beam sigmas.

3.  Summary

The radiation-induced demagnetization of IDs is a critical issue. Radiation damage has now been observed in a few IDs in the APS storage ring. Systematic dose monitoring provides the dose- distribution profiles around the APS storage ring and the FEL and identification of regions where the probability for damage is highest.

4.  Acknowledgements

Gratitude is expressed to Roger Dejus (APS) for analysis of the radiation-induced intensity degradation of the third harmonic. The authors also thank Marc Desrosiers (National Institute of Standards and Technology) for helpful discussions on the alanine EPR dosimetry. This work was supported by the U.S. Department of Energy, BES, under contract No. W-31-109-ENG-38. NAA was performed at the University of Wisconsin-Madison under the U.S. DOE reactor sharing program.

5.  References

[1] N. Ipe et al., Argonne National Laboratory report no. ANL/APS/TB-7 (1993).
[2] T. Bizen et al., Nucl. Instrum. Methods A 467-468 (2001) 185.
[3] O.P. Kähkönen et al., Europhys. Lett. 12(5) (1990) 413.
[4] S. Okuda et al., Nucl. Instrum. Methods B 94 (1994) 227.
[5] J.R. Cost et al., IEEE Trans. Magn. 24 (1988) 2016.
[6] H.B. Luna et al., Nucl. Instrum. Methods A 285 (1989) 349.
[7] A.F. Zeller, “Radiation Damage in Permanent Magnetic Materials,” NSCL internal report (1999).
[8] J. Chavanne et al., ESRF Machine Technical Note 1-1996/ID (1996).
[9] E.R. Moog et al., Proc. of Tenth U.S. National Conf. on Synch. Radiat. Inst. (1997) 219.
[10] J. Alderman et al., Argonne National Laboratory report no. LS-283 (2000).
[11] E.L. Florián et al., CERN report no. CERN/TIS-CFM/IR/93-03 (1993).