Performance of a DC GaAs photocathode gun for the Jefferson lab FEL

Transcription

1 Nuclear Instruments and Methods in Physics Research A 475 (2001) Performance of a DC GaAs photocathode gun for the Jefferson lab FEL T. Siggins a, *, C. Sinclair a, C. Bohn b, D. Bullard a, D. Douglas a, A. Grippo a, J. Gubeli a, G.A. Krafft a, B. Yunn a a Thomas Jefferson National Accelerator Facility, Jefferson Avenue, Newport News, VA 23606, USA b Fermi National Accelerator Laboratory, P.O. Box 500, Batavia, IL60510, USA Abstract The performance of the 320 kv DC photocathode gun has met the design specifications for the 1 kw IR Demo FEL at Jefferson Lab. This gun has shown the ability to deliver high average current beam with outstanding lifetimes. The GaAs photocathode has delivered 135 pc per bunch, at a bunch repetition rate of MHz, corresponding to 5 ma average CW current. In a recent cathode lifetime measurement, 20 h of CW beam was delivered with an average current of 3.1 ma and 211 C of total charge from a cm 2 illuminated spot. The cathode showed a 1=e lifetime of 58 h and a1=e extracted charge lifetime of 618 C. We have achieved quantum efficiencies of 5% from a GaAs wafer that has been in service for 13months delivering in excess 2400 C with only three activation cycles. r 2001 Elsevier Science B.V. All rights reserved. Keywords: Photocathode; FEL; Emittance 1. Gun description The DC photocathode high brightness source for the Jefferson Lab FEL was proposed [1], specified [2] and then developed through a University of Illinois thesis project [3]. The gun design is similar to the 100 kv polarized electron gun for the CEBAF accelerator [4], which uses a strained lattice GaAs wafer and runs an average of 50 ma with 70 75% polarization. The FEL gun design uses a planar stainless-steel cathode geometry (Fig. 1), with a cm gap operated at 320 kv, *Corresponding author. Tel: ; fax: address: siggins@jlab.org (T. Siggins). and incorporates a bulk GaAs wafer photocathode 3.18 cm in diameter. The exposed wafer surface (2.54 cm in diameter) is activated to negative electron affinity with cesium and nitrogen trifloride. A frequency-doubled, mode-locked Nd:YLF laser at 527 nm is used to drive the gun with a 6 mm spot illuminating the cathode. The design parameters for the FEL require the gun to deliver 5 ma at MHz corresponding to 135 pc per bunch for IR lasing [5]. We have achieved a performance increase by adjusting our GaAs cleaning methods and by using high voltage processing techniques for the electrode structure. The limiting factor in cathode lifetime is ion back bombardment [4]. Higher DC voltage operation is presently prevented by field emission. However, /01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S (01)

2 550 T. Siggins et al. / Nuclear Instruments and Methods in Physics Research A 475 (2001) simulations indicate that working at higher voltage would result in improved electron beam emittance. We currently have a program underway to study materials for improved high voltage standoff characteristics. 2. Operations Fig. 1. Mechanical layout of the FEL gun. During early operations, the cathode quantum efficiencies (QE) were lowf %. Intermittent arcs occurring on the high voltage electrode structure resulted in permanent cathode degradation and a need to replace the GaAs wafer after a typical 3month operations period. Several steps were taken to improve the QE and to minimize the occurrence of high voltage arcs. Changes were made in the hydrogen cleaning system [6] used to prepare the GaAs wafer. The original geometry of the hydrogen plasma source in the cleaning system was constricted allowing for recombination in the plasma and poor cleaning results. The wafer was moved away from the nominal position for cleaning to open up the conductance and enhance the atomic hydrogen flux. A pre-activation heat clean cycle for the GaAs photocathode was developed for use on the Jefferson Lab polarized source gun [4]. This heat cleaning procedure was transferred to and optimized for use with the FEL gun. The larger mass of the FEL gun required the heat cycle to be run for a longer period to obtain good cleaning results. From observations made during numerous high voltage conditioning and cesiation cycles, it was determined that a damaged region existed on the stainless-steel ball cathode that could not be removed by our normal mechanical polishing or high voltage processing techniques. The ball cathode was replaced and the electrode structure showed an immediate improvement in performance during and after high voltage conditioning. We determined that the QE of the photocathode could be damaged during the high voltage conditioning process. Consequently the wafer is now retracted into the ball cathode for protection during high voltage processing, thus preventing damage and minimizing contamination of the photocathode surface. These changes have resulted in QEs of 5% and a GaAs wafer that has been in service for 13months delivering in excess 2400 C over three activation cycles. The gun has delivered 5 ma of recirculated CW beam at MHz with 135 pc to meet the design parameters for the FEL. 3. Photocathode lifetime Twenty hours of CW beam at MHz were run to measure cathode lifetime. The run started with the cathode at a peak QE of 2.7%. A constant drive laser power was used and the resulting drop in beam current was measured. The FEL driver accelerator was set up to transport 3.60 ma of beam, dropping to 2.56 ma as the QE decayed, delivering a total charge of 211 C. The data in Fig. 2 shows a 1=e lifetime of 58 h at an average current of 3.1 ma and a 1=e extracted charge lifetime of 618 C.

3 T. Siggins et al. / Nuclear Instruments and Methods in Physics Research A 475 (2001) Fig. 2. Plot of beam current vs. time. Fig. 4. Cathode QE scan after the lifetime run. Fig. 3. Cathode QE scan prior to the lifetime run. Although we see some signs of chemical poisoning of the cathode, ion back bombardment is the limiting factor in cathode lifetime. Figs. 3 and 4 show quantum efficiency scans of the cathode surface before and after the previously described lifetime run. Ion back bombardment damage to the cathode is seen in Fig. 4 as a hole in the center of the cathode scan following delivery of 20 h of CW beam. The FEL gun vacuum system currently operates in the mid Torr range as measured by a RGA. Improving the anode cathode gap vacuum should increase cathode lifetimes substantially, as indicated by Jefferson Lab s experience with GaAs-based polarized electron sources [4]. These sources operate with at least an order of magnitude lower vacuum (t10 12 Torr).

4 552 T. Siggins et al. / Nuclear Instruments and Methods in Physics Research A 475 (2001) Emittance measurements Electron beam emittance data was taken at an energy of 10 MeV using a multi-slit diagnostic located down stream of the FEL gun and a 10 MeV super-conducting RF accelerator module. Measurements were taken from 20 to 135 pc per bunch as shown in Fig 5. These data show that the FEL upgrade design specifications (p30 pmm mrad at 135 pc) was exceeded [7]. The measured emittance values in Fig. 5 are larger than those quoted in Ref. [5] because the injector set up was changed to support high charge running at 135 pc. 5. Field emission The FEL gun is currently operated at 320 kv DC with a gradient of 3. 9 MV/m at the cathode surface. The stainless-steel electrode structure is routinely conditioned to 420 kv prior to cathode activation. This has provided a very stable environment for the cathode. As the voltage is increased above our operating level, we start to see unacceptable levels of field emission from the cesiated surfaces. We are currently working on a system that will allow us to activate the cathode without exposing the electrode surface to cesium. One configuration employs a focused cesium source mounted below the anode plate delivering a 18 mm spot of cesium to the photocathode. Another configuration mounts cesium channel sources inside the ball cathode structure and the cesiation of the photocathode is performed there. Different materials for the electrode structure are currently being studied for their field emission properties in a large area electrode test system. This Field Emission Test System (Fig. 6) uses 15 cm diameter electrodes with Rowgowski profiles and a 125 kv DC power supply. The test cathode is rigidly mounted while the anode is mounted on three micrometer adjustments so the electrode gap and electric field may be varied. Results obtained from this materials survey should allow us to increase the operating voltage of an improved gun design with subsequent improvement of the emittance of the electron beam. Fig. 6. Field Emission Test System. Fig. 5. Plot of emittance vs. charge per bunch.

5 T. Siggins et al. / Nuclear Instruments and Methods in Physics Research A 475 (2001) Conclusions The DC GaAs photocathode gun for the Jefferson Lab FEL is operating at higher voltage and delivering more average current than any bulk GaAs photocathode gun in operation. The good QEs and long cathode lifetimes have been obtained using hydrogen cleaning, an ultra-high vacuum environment and shielding of the wafer during high voltage processing. The gun has met the original design specifications for the Jefferson Lab IR Demo FEL and can routinely deliver 5 ma of CW beam for extended periods. Work being done on improving the gun s vacuum system and raising the operating voltage should allow us to further increase cathode lifetimes and improve the quality of beam delivered. References [1] G.R. Neil, et al., Nucl. Instr. and Meth. A 318 (1992) 212. [2] C.K. Sinclair, Nucl. Instr. and Meth. A 318 (1992) 410. [3] D. Engwall, et al., Proceedings of the 1997 Particle Accelerator Conference, Vancouver, BC, IEEE, Piscataway, NJ, 1998, p [4] M. Poelker, et al., Proceedings of the Workshop on Polarized Electron Sources Nagoya, Japan, AIP, New York. to be published. [5] G.R. Neil, et al., Phys. Rev. Lett. 84 (4) (2000) 662. [6] C.K. Sinclair, et al., Proceedings of the 1997 Particle Accelerator Conference, Vancouver, BC, IEEE, Piscataway, NJ, 1998, p [7] D. Douglas, et al., Driver accelerator design for the 10 kw upgrade of the Jefferson Lab IR FEL, Proc. of the XXth International Linac Conference, Monterey, CA, August 21 25, Acknowledgements Work supported by U.S. DOE under contract # DE-AC05-84-ER40150, the office of Naval Research and the Commonwealth of Virginia.