STATE-OF-THE-ART ELECTRON GUNS AND INJECTOR DESIGNS FOR ENERGY RECOVERY LINACS (ERL) Alan Todd Advanced Energy Systems P.O. Box 7455, Princeton, NJ 08543-7455, U.S.A. Corresponding author: Alan Todd Phone: 609-514-0316 ; FAX: 609-514-0318 ; e-mail: alan_todd@mail.aesys.net ABSTRACT A key technology issue of energy recovery linac (ERL) high-power free-electron laser (FEL) and fourth generation light sources is the demonstration of reliable, high-brightness, highpower injector operation. Ongoing programs that target up to 0.5 Ampere injector performance at emittance values consistent with the requirements of these applications, are described. There are three approaches that could deliver the specified performance. These are DC photocathode guns with superconducting RF (SRF) booster cryomodules, high-current normal-conducting RF (NCRF) photoinjectors that may also use SRF boosters, and SRF photocathode guns and boosters. The achieved performance at existing ERL facilities, the status of ongoing source development programs, and the proposed parameters of the injectors for planned ERL facilities are described and compared. As examples, we concentrate on three high-current injectors being developed by Advanced Energy Systems (AES) with collaborators at the Thomas Jefferson National Accelerator Facility (JLAB), Los Alamos (LANL) and Brookhaven (BNL) National Laboratories. PACS: 29.17.+w, 29.27.AC, 41.85.Ja, 85.60.Ha Keywords: Photoinjector, Electron Gun, DC Gun, High-Average-Current, Superconducting Radio Frequency (SRF), Energy Recovery Linac (ERL).
1. INTRODUCTION Although the ERL concept had been in existence for many years [1,2,3,4], the spark that spawned many of the ERL devices discussed below was the success of the JLAB ERL IR FEL [5]. Today, there are three active ERL facilities: the JLAB IR FEL Upgrade [6], the ERL FEL at the Japan Atomic Energy Research Institute (JAERI) [7] and the Budker Insitute for Nuclear Physics (BINP) Recuperator at Novosibirsk [8]. Near term ERL facilities under construction include the NSF-funded Cornell [9] and UK Daresbury ERL-Prototype [10] devices. Table 1 lists the principle injector specifications for these and other relevant devices, past, present and future, from which, in Table 2, we will later construct nominal parameter requirements and associated ranges for ERL injectors. The three columns in yellow are the injectors in development at AES that will be discussed in greater detail below. The devices on the left of the table all use DC guns as indicated by Gun Type. These include the active JLAB [6], JAERI [7] and BINP [8] FELs. For the JLAB column, the parameters in parentheses are the values at the wiggler, while the first number in each case is the inferred value after the injector. For BINP and JAERI, the achieved numbers are shown first with projected near-term upgrade values in parentheses. The Cornell ERL [9] injector targets 100 ma average current while the Daresbury ERLP [10] is very similar to the current JLAB device. Both these devices are presently under construction. KEK [11] are developing an ERL concept that is similar to that of Cornell and is not separately listed here. JAERI and BINP use thermionic guns as opposed to the photocathode guns of the other injectors, though JAERI are actively developing a photocathode gun to improve the achievable beam brightness, a key to high-performance for all ERL FELs or light sources. In each case, Gallium Arsenide is the photocathode material of choice for these DC guns. The other listed DC gun and booster consists of a JLAB DC gun and an AES SRF booster that is 1
presently being assembled for testing at the JLAB injector test stand beginning in 2006, and which is described in greater detail below. The next three injectors utilize normal-conduction RF (NCRF) guns. It is interesting to appreciate that at 32 ma, the Boeing gun [12,13] shown in Fig. 1 and now retired, which operated at 25% duty factor with a macropulse current of 132 ma, is still the demonstrated high-average-beam-current state-of-the-art despite the recent focus on DC guns. Below, we describe the LANL/AES 700 MHz NCRF Glidcop gun that is in fabrication for thermal testing at LANL in early 2006. The nominal operating current for this gun is 100 ma with a 33.3 MHz laser pulse repetition rate (PRF). However, as shown by the figures in parentheses, the same gun, operating at 350 MHz PRF, could, in principle, deliver more than 1 A of average current with the identical thermal load and stress. The LUX project [14] at Lawrence Berkley National Laboratory (LBNL) optimized the cell shaping to design an NCRF gun that delivers improved impedance to lower the thermal loads and stress, thereby permitting the use of OFE copper in place of Glidcop, which can be difficult to braze. The Boeing gun used a Cesium Potassium Antimonide multi-alkaline cathode material. Multi-alkalis are the cathodes of choice for these guns because, as compared to DC guns, the operating vacuum cannot be maintained high enough to permit the use of GaAs. Additionally, for any ERL injector, it is almost mandatory that one use a cathode that responds to green, as opposed to UV, illumination to make the photocathode drive laser tractable, reliable and affordable. The specifics of cathode issues for ERL injectors are addressed in the parallel Reference [15]. Finally, there is growing interest in SRF injectors following the pioneering work of Forschungszentrum Rossendorf, (FZR) [16], albeit at lower current than desired for proposed ERL devices. However, AES is fabricating a 703.75 MHz half-cell SRF gun, in collaboration with BNL, that is being designed to deliver 0.5 A. In addition to multi-alkalis, 2
the novel diamond cathode concept [17] is being considered for this gun. The figures in parenthesis show the significant improvement in the longitudinal beam quality that results from linearizing the longitudinal phase space with a harmonic RF cavity. While this technique can be usefully applied to all injector technologies, none of the other columns includes such optimization. This is one of the three injectors described in more detail below. The final column describes the plans for 4GLS proper as opposed to the ERLP. Here, the desire is to use an SRF gun with a diamond cathode for high-average current operation and most likely an NCRF gun for the alternate high-brightness operation shown in parentheses [18]. The table also indicates what type of booster is planned for each injector unless it is not applicable (N/A). The geometry of the booster in terms of cells per cavity, the power coupling technique and the power level per feed used or planned, whether coaxial (CX) or waveguide (WG), is also shown. In addition to FELs and light sources, there are other ERL and high-current injector applications that have not been specifically included in Table 1. The AES/BNL SRF gun operates at a harmonic of the RHIC frequency and will be a key element of a 0.5 A highcurrent ERL test at BNL in 2007. There are two goals for this work [19]: firstly to develop a gun for electron cooling of hadron storage rings such as the Relativistic Heavy Ion Collider (RHIC); and secondly to provide high-intensity electron beams for high-luminosity colliders such as e-rhic and ELIC. Because of the existing RHIC parameters, electron cooling requires 20 nc bunches at 9.4 MHz for an average current of ~200 ma. Hence the ideal injector for this application is not the above 0.5 cell SRF gun, which will however demonstrate the viability of the technology, but rather a 1.5 cell SRF gun without a booster accelerator that accelerates the beam to 5 MeV. The e-rhic collider calls for up to 16 nc 3
bunches at 28.15 MHz for an average current up to 450 ma, and would utilize a similar 1.5 cell gun for the injector. 2. INJECTOR OPTION COMPARISON In looking towards the future, we have combined the Table 1 specification for all the photocathode injectors that plan to operate near or above 100 ma. Table 2 shows the resultant nominal requirements that are being requested for the principal ERL injector parameters, together with an associated range. This table shows that the proposed highcurrent ERL facilities have much in common. It is also clear that there are three injector technologies being seriously considered for ERL facilities, namely: DC, NCRF and SRF guns that will likely be followed by SRF boosters. We do not discuss injector variants and hybrids [20] that utilize coaxial coupling [21,22] or doubly-resonant guns [23,24] that may some day find application in ERL devices. Of the three candidate technologies, the DC gun is the most mature with 10 ma CW already demonstrated at JLAB and 100 ma injectors in fabrication at Cornell and AES/JLAB. Recent Cornell simulations for optimized DC gun configurations show exceptional performance with transverse emittance values significantly lower than 1 µm at the nominal 77 pc charge level [25]. The achievable gradient and voltage with respect to field emission and breakdown may be limited to around 7 MV/m and 500 kv though Cornell plan to stretch these limits experimentally. There will also be a gradient limit set by dark current in these guns. The very high-vacuum capability of the DC gun does lead to a viable cathode option in GaAs, whose performance and lifetime today are limited by ion backbombardment. The 400 C drawn from such a cathode between recesiations [5] corresponds to over an hour of operation at 100 ma, which is not sufficient for ERL facility operation at 4
high availability, but is still very promising. Consequently, it appears likely that DC guns coupled with SRF boosters will be capable of delivering the required ERL injector performance. The maximum achievable gradient in CW NCRF guns is limited to around 10 MV/m due to thermal stress limits. There is also an efficiency penalty and associated cost due to the impedance and resistive losses. Achievable vacuum conditions limit visible cathode selection to multi-alkaline materials or diamond amplifiers and raise issues of achievable performance and lifetime. However, we noted that the Boeing injector was still the state-ofthe-art for high-average current gun operation and it serves as a proof-of-concept for NCRF injectors at current levels approaching 100 ma, and with higher beam-loading and PRF, ampere-level operation at the same engineering stress level and beam dynamics performance. The SRF gun, which can, in principle, deliver RF gun performance with DC gun efficiency is the least mature but also the most desirable ERL injector option. The maximum achievable accelerating gradient is likely around 20 MV/m with respect to peak gun fields. It is necessary to demonstrate viable high-average-current choke joint designs and cathode compatibility with the SRF environment, together with a lack of contamination of that environment. As with the NCRF gun, cathode selection is an issue with the diamond amplifier as the preferred approach and multi-alkalis as a backup. However, in contrast to the normal-conducting gun, the SRF gun is expected to have excellent vacuum properties at the cathode surface. At this point we must await experimental demonstration of this option before it can be adopted as a serious candidate for near term facilities. Finally, all the injector technology options must address field limits set by dark current and high-power RF delivery to the accelerating cavities. While high-order modes (HOM), wakefields and beam break up (BBU) instabilities are more issues for the ERL ring accelerating cavities than for the injector cavities, they must still be given serious 5
consideration in the design process at high beam power. Emittance growth due to space charge and coherent synchrotron radiation (CSR) in mergers and other beam transport elements is also an aspect of the injector design and selection. In the following sections, we describe the design and status of one example of each principle injector option. 3. DC GUN AND SRF BOOSTER The present leading candidate for high-average-current ERL injectors is a DC photocathode gun closely coupled to an SRF booster accelerator. One such device has been designed and fabricated by AES and is presently being assembled at JLAB for testing on their injector test stand. As shown in Fig.2, the device begins with a 500 kv DC gun [26] followed by an emittance compensation solenoid [27]. The electron beam then enters the 748.5 MHz SRF cryomodule which consists of three single cell fundamental cavities and one 2245.5 MHz third harmonic cavity that accelerate the beam to 7 MeV. The third harmonic cavity is the second cell in the string, whose function is to linearize the longitudinal phase space on exit from the cryostat, as shown by the longitudinal phase space plots in the figure. As was demonstrated with the Boeing injector [28], we have shown that the addition of the longitudinal phase space correction cavity significantly improves the beam quality in these injectors. Specifically at 1 nc or 0.75 A, this configuration can deliver 5.1 microns transverse and 43 kev-psec longitudinal rms emittance at 7 MeV, which is more than adequate for very high-power IR FEL operation. However, due to the recent addition of the harmonic cavity to the actual hardware, we do not list the effect of this correction for 133 pc bunches in Tables 1 and 3, and hence better performance is expected from this device than 6
indicated. In contrast, the Cornell approach uses five 1300 MHz double-cell booster cavities for their DC gun injector. When every RF bucket is filled with 133 pc, the single cell device provides an average current of 100 ma and an average electron beam power of 700 kw at the 7 MeV injector output. The injector and power couplers have been designed to handle these power levels. The sequence of single cell cavities provides latitude for adjusting the longitudinal phase space through different cavity phasing and ameliorates HOM and wakefield effects driven by high current. The use of 748.5 MHz means that the injector is compatible with the existing JLAB ERL FEL ring although there are no present plans to install it on that device. Fig. 3 shows four completed single-cell SRF cavities, while Fig. 4 shows the 3 rd harmonic cavity, all minus their helium vessels. Cavity cleaning has begun at JLAB with assembly scheduled for completion by the end of 2005. The three fundamental RF power couplers, one per cavity, are designed to deliver 350 kw each. Testing will begin at JLAB in 2006, following the completion of the injector test stand. Initial tests will demonstrate bunch charges up to 1 nc at low PRF, due to initial RF power limitations. Later RF power upgrades should enable 100 ma characterization in 2008. 4. NORMAL CONDUCTING RF GUN The second high-power injector approach that is under development at AES is the normal-conducting, CW, photocathode RF electron gun that is shown in Fig. 5. Los Alamos performed the physics and RF design for this 700 MHz, 2½-cell, 3 nc device that delivers 100 ma at a 35 MHz PRF. Higher currents could be delivered without impacting the gun thermal or beam properties, simply by increasing the PRF. A third full cell does not support acceleration but rather serves as a vacuum manifold to permit very high pumping on the gun 7
during operation. The crucial issue for this concept is the extremely high-average-power and the peak power densities that result from the resistive losses in the copper of the gun. Two dimensional and three dimensional thermal, stress, and displacement analyses were conducted on the gun design where the effects of temperature and pressure coupled with the RF response were all taken into account. The thermal stresses are such that the gun is manufactured from Glidcop. The total thermal load of the gun is about 720 kw, but analysis indicates the design coolant flow will adequately cool the device. Fig. 6 shows two of the gun cells plated and ready for brazing. One of the ridge loaded waveguide penetrations is visible on cell 3. It also shows the dogbone iris of the power coupler which is a critical thermal and braze point. This gun represents one of the most complex brazing cycles that has ever been attempted. Currently, we are 25% complete on the braze cycle. The gun is scheduled for delivery to Los Alamos for testing in late 2005. An existing Los Alamos 1 MW RF test stand will be used to perform a thermal test of the gun in early 2006. Beam tests up to about 200 ma CW could be performed in 2006 at reduced accelerating gradient with 1 nc bunches and 200 MHz PRF using the existing LANL RF power, once a preparation chamber and load lock are added to the gun to permit multi-alkali cathode insertion. Addition of a booster, probably SRF and similar to that described above, would enable achieving typical ERL injection energy and performance requirements. 5. SUPERCONDUCTING RF GUN The final option under development at AES in collaboration with BNL, JLAB and FZR is a high-current SRF gun. This gun is being designed and fabricated to deliver 0.5 A to the ERL test ring that is being built at BNL [19]. This high current requires a substantial 8
departure from the present FZR 3½-cell design at 1.3 GHz [16]. Firstly, we have adopted a ½-cell gun configuration at the lower frequency of 703.75 MHz, which is compatible with the RHIC RF frequency, in order to accommodate the higher currents, approaching 0.5 A, required for the RHIC electron cooling rings and the e-rhic collider. Secondly, primarily because of thermal issues, but also for simplicity of fabrication, we determined to utilize a quarter wave choke joint concept [29] that is different from the FZR approach. This is shown to the left in Fig. 7 together with the ½ cell cavity. The details of the output iris and beamtube are not finally set at this time and will change. Thermal and stress analyses of this SRF gun with the quarter wave choke joint have been completed. For 0.5 A operation, the thermal load on the cathode stalk, which is cooled with liquid nitrogen, is between 150 and 180 W, depending on whether a multi-alkali or diamond amplifier cathode is employed. Fig. 8 shows the SRF gun cryostat with a detail of the single cell in the upper left. The cathode preparation chamber is being designed for multi-alkali, diamond and dispenser cathodes. This injector is presently in the final design phase. Delivery to BNL is planned for early 2007. Initial gun performance testing will therefore be completed in 2007. 6. CONCLUSIONS High-current ERL facilities are being proposed and constructed all over the world. Present facilities operating at the 10 s of ma level will give way to 100 ma and higher current devices. Three technology options exist for the high-current electron injectors these ERL facilities will need. DC guns with SRF boosters are a relatively mature approach and suitable for nearterm deployment. They will likely deliver the required performance at the 100 ma current level and should permit extrapolation towards the Ampere-level. Further, the cathode issue 9
is largely solved for this technology at the 100 ma current level with the use of GaAs, although extending the time between recesiations is important. The achievable DC gun accelerating gradient, perhaps limited by dark current, needs to be determined. Normal-conducting RF injectors are the least attractive option for CW operation because of their inefficiency due to resistive wall losses. The achievable gradient is also limited by the resulting thermal constraints. Multi-alkalis are the cathode of choice but suitable lifetime and reliability have yet to be demonstrated. Nevertheless, the Boeing gun, at 32 ma CW, is still the state-of-the-art for high-average current injectors and serves as a proof-of-concept for 100 ma operation. If the cathode, thermal and power coupling issues are solved, these guns can deliver Ampere-level performance and they also remain attractive for lower PRF high-brightness applications as indicated in Table 1 for 4GLS. Superconducting RF injectors are the least mature option and unproven at highaverage current. However, they are the most desirable approach since, in principle, they deliver the better RF gun beam performance at DC gun efficiency levels. They also promise the highest accelerating gradient and thus the most compact option, but must demonstrate a compatible cathode technology and high RF power handling. The performance of three distinct high-current ERL injector technologies, which are under development at AES in partnership with National Laboratories, is listed in Table 3. The impact of harmonic correction, which is important for each technology, is shown in parentheses only for the SRF gun, and each requires further beam tailoring for ERL merging. The 100 ma DC gun with an SRF booster is presently being assembled at JLAB for testing in 2007. The normal-conducting gun is in fabrication for delivery to LANL in late 2005. It will undergo thermal testing equivalent to beam operation at 1 A in 2006. Finally, the SRF gun, which will be installed on the BNL test ERL is in final design. Operation testing of this gun to 0.5 A will occur in 2007. 10
ACKNOWLEDGEMENTS The AES injector work is supported by the Naval Sea Systems Command, the Office of Naval Research, the DOD Joint Technology Office and the Missile Defense Agency. We gratefully acknowledge the contributions of the following individuals to the material presented: A. Ambrosio, H. Bluem, V. Christina, M. D. Cole, M. Falletta, D. Holmes, E. Peterson, J. Rathke, T. Schultheiss and R. Wong of AES to the three AES injectors; S. Benson, E. Daly, D. Douglas, F. Dylla, W. Funk, C. Hernandez-Garcia, J. Hogan, P. Kneisel, J. Mammosser, G. R. Neil, L. Phillips, J. Preble, R. Rimmer, C. Rode, T. Siggins, T. Whitlach, M. Wiseman of JLAB, R. Campisi of ORNL, and J. Sekutowicz of DESY to the AES/JLAB DC Gun and Booster; I. Ben-Zvi, A. Burrill, R. Calaga, P. Cameron, X. Chang, H. Hahn, D. Kayran, J. Kewisch, V. Litvinenko, G. McIntyre, A. Nicoletti, J. Rank, J. Scaduto, T. Srinivasan-Rao, K. Wu, A. Zaltsman, Y. Zhao of BNL, D. Janssen of FZR, J. W. Lewellen of ANL, L. Phillips, J. Preble of JLAB, and V. Nguyen- Tuong of Tunnel Dust, to the AES/BNL SRF Gun; P. Colestock, J. P. Kelley, S. Kurennoy, D. Nguyen, S. Russell, W. Reass, D. Rees, D. Schrage, R. Wood of LANL and L. Young of TechSource to the LANL/AES NCRF Gun. Additionally, we wish to thank D. Dowell of LCLS and J. Adamski of Boeing (the Boeing injector and Figure 1), C. Sinclair of Cornell University (the Cornell ERL), R. Rimmer of JLAB (LUX), E. Seddon and M. Dykes of ASTeC, Daresbury, UK (4GLS), N. Vinokurov of BINP, Russia (BINP Recuperator) and E. Minehara of JAERI, Japan (JAERI FEL) for graciously providing information on their described ERL and injector projects. 11
FIGURE CAPTIONS Figure 1. 433 MHz, 32 ma Boeing NCRF injector. Figure 2. DC Gun and SRF booster (upper left) with fundamental cavity (upper right). Booster with 3 rd harmonic cavity showing evolution of longitudinal emittance (lower - the vertical scale varies for each phase space plot). Figure 3. Four 748.5 MHz SRF fundamental cavities. Figure 4. 2245.5 MHz third harmonic SRF cavity. Figure 5. Normal-conducting 2½-cell CW RF gun. Figure 6. NCRF gun cell 2 (upper left) and cell 3 (upper right) plated and ready for brazing. Waveguide irises machined and ready for plating (lower). Figure 7. SRF gun choke joint and ½ cell cavity. Figure 8. SRF gun cryostat with ½ cell cavity detail (upper left). TABLE CAPTIONS Table 1: Summary of parameters for high-current injectors and ERLs. Table 2: ERL injector nominal values and range. Table 3: AES ERL injector parameters. 12
REFERENCES [1] M. Tigner, A Possible Apparatus for Electron Clashing-Beam Experiments, Nuovo Cimento 37 (1965) 1228-1231. [2] S. O. Schreiber and E. A. Heighway, Double Pass Linear Accelerator Reflexotron, IEEE NS-22 3 (1975) 1060-1064. [3] T. I. Smith et al., Development of the SCA/FEL for use in Biomedical and Materials Science Research, NIM A259 (1987) 1-7. [4] D. W. Feldman et al, Energy Recovery in the LANL FEL, NIM A259 (1987) 26-30. [5] G. R. Neil et al., "Sustained Kilowatt Lasing in a Free-Electron Laser with Same-Cell Energy Recovery," Phys. Rev. Lett. 84 (4) (2000) 662-665. [6] G. R. Neil et al., The JLAB High-Power ERL Light Source, in this issue of NIM A. [7] E. J. Minehara, Development and Operation of the JAERI ERL (Energy Recovery Linac), ibid. [8] V. P. Bolotin, N. A. Vinokurov et al., ERL at Budker INP, ibid. [9] G. Hoffstaetter et al., ERL Upgrade of an Existing X-ray Facility: CHESS at CESR, Proc. EPAC 2004, ISBN 92-9083-231-2, Lucerne, Switzerland (2004) 497-499. [10] M. W. Poole and E. A. Seddon, 4GLS and the Prototype Energy Recovery Linac Project at Daresbury, Proc. EPAC 2004, ISBN 92-9083-231-2, Lucerne, Switzerland (2004) 455-457. [11] E.-S. Kim et al., Design Study for a 205 MeV Energy Recovery Linac Test Facility at the KEK, Proc. EPAC 2004, ISBN 92-9083-231-2, Lucerne, Switzerland (2004) 420-422. 13
[12] D. H. Dowell et al., Results of the Average Power Laser Experiment Photocathode Injector Test, NIM A356 (1995) 167-176. [13] D. H. Dowell et al., First Operation of a Photocathode Radio Frequency Gun Injector at High Duty Factor, App. Phys. Lett. 63 (15) (1993) 2035-2037. [14] R. A. Rimmer et al., A High-Gradient High-Duty-Factor RF Photo-Cathode Electron Gun, Proc. EPAC 2002, ISSN 1684-761X, Paris, France (2002) 1801-1803. [15] T. Rao et al., in this issue of NIM A. [16] D. Janssen et al., Status of the 3 1/2 Cell Rossendorf Superconducting RF Gun, Proc. FEL 2004, ISBN 88-87992-02-9, Trieste, Italy (2004) 359-362. [17] T. Rao et al., Diamond Amplifier for Photocathodes, AIP Conf. Proc. 737 (2004) 178. [18] E. A. Seddon and M. W. Poole, 4GLS and the Energy Recovery Linac Prototype Project at Daresbury Laboratory, to appear in Proc. PAC 2005, Knoxville, TN, USA, May 16-20, 2005. [19] I. Ben-Zvi and V. Litvinenko, ERLs in High Energy and Nuclear Physics, in this issue of NIM A. [20] S. L. Huang et al., Beam Loading Tests on DC-SC Photoinjector at Peking University, Proc. FEL 2004, ISBN 88-87992-02-9, Trieste, Italy (2004) 481-483. [21] M. J. de Loos et al., A High-Brightness Pre-accelerated RF-Photo Injector, Proc. EPAC 2002, ISSN 1684-761X, Paris, France (2002) 1831-1833. [22] D. Janssen et al., Axial Power Input in Photocathode Electron Guns, to appear in Proc. PAC 2005, Knoxville, TN, USA, May 16-20, 2005. [23] D.H. Dowell et al., A Two-Frequency RF Photocathode Gun, NIM A528 (2004) 316-320. 14
[24] J. W. Lewellen and J. Noonan, "Field-Emission Cathode Gating for RF Electron Guns," Phys. Rev. ST Accel. Beams 8, 033502 (2005) 1-9. [25] I. V. Bazarov and C. K. Sinclair, Multivariate Optimization of a High Brightness DC Gun Photoinjector, Phys. Rev. ST Accel. Beams 8, 034202 (2005) 1-14. [26] C. K. Sinclair, Nucl.Instr.Meth. A318 (1992) 410-414. [27] B. E. Carlsten, Photoelectric Injector Design Code, Proc. PAC 1989 Chicago, Il, USA, IEEE89CH2669-0 (1989) 313-315. [28] D.H. Dowell, T.D. Hayward and A.M. Vetter, Magnetic Pulse Compression Using a Third Harmonic RF Linearizer, Proc. PAC 1995, Dallas, TX, USA, IEEE95CH35843 (2002) 992-994. [29] V. Nguyen-Tuong, L. Phillips and J. Preble for Tunnel Dust, Inc., priv. comm. 15
Figure 1 16
2245.5 MHz 3 rd Harmonic SRF Cell With RF W/G Feed (not shown) Spaceframe DC Gun Cold Box Cold Box 750 MHz Fundamental SRF Cells RF Feed Helium Vessel 3.7 3.6 3.5 18 17.8 17.6 17.4 17.2 17 16.8 Emittance Compensation Solenoid 4.15 4.1 4.05 4 3.95 3.4 3.3 3.2 3.1 172 172.5 173 173.5 174 174.5 175 16.6 580.6 580.8 581 581.2 581.4 581.6 581.8 582 3.9 3.85 131 132 133 134 135 (βγ)z 1.75 1.73 1.71 1.69 1.67 1.65 1.63 1.61 46 47 48 49 50 51 z (cm) Figure 2 17
Figure 3 18
Figure 4 19
Vacuum Vacuum Pumps Pumps Vacuum Chamber with Pumps Bucking Solenoid Magnet Cathode Backplate Cooling Focusing Solenoid Magnet Ridge Loaded Waveguide Figure 5 20
Figure 6 21
SuperFish File Gun 6cm Iris F = 703.79036 MHz 18 18 16 14 12 Iris diameter = 120 mm Beampipe diameter = 190 mm Frequency = 703.79036 MHz 16 14 12 10 10 8 8 6 6 4 4 2 2 0 0 5 10 15 20 25 C:\DOCUMENTS AND SETTINGS\KAYRAN\MY DOCUMENTS\ERL\SCGUN_DESIGN\FROM_RAM\RGUN519.AM 4-28-2005 15:18:00 0 Figure 7 22
Internal Helium Dewar Top Cover with Facilities Feedthrough Cathode Isolation Valve Cathode Installation Assembly Magnetic and Thermal Shielding Adjustable Supports Vacuum Vessel Power Couplers Beam Line Isolation Valve Cavity Assembly Beam Tube with HOM RF Pickup Figure 8 23
DEVICE JLAB AES/JLAB Cornell Daresbury JAERI BINP Boeing LANL/AES LUX AES/BNL 4GLS PARAMETER ERL FEL Injector ERL ERLP ERL ERL FEL Injector Gun Gun Gun/ERL ERL Gun Type DC DC DC DC DC DC NCRF NCRF NCRF SRF SRF(NCRF) Injector and ERL RF Frequency (MHz) 1497 748.5 1300 1300 499.8 180 433 700 1300 703.75 1300 PRF (MHz) 74.85 748.5 1300 81.25 10.41 (83.3) 11.2 (90) 27 33.3 (350) 1300 351.88 1300 (0.001) Charge/Bunch (nc) 0.133 0.133 0.077 0.080 0.5 1.7 4.75 3.0 1.0 1.4 0.080 (1.0) Current (ma) 10 100 100 6.5 5 (40) 20 (150) 32(132 Peak) 100 (1050) 1300 500 100 (0.001) Injector Energy (MeV) 7 7 5-15 8.35 2.5 2 5 2.5 2 10 (150) Transverse rms Normalized Emittance (µm) < 7 (7) 1.2 < 1.0 1.5 30 32 (15) ~ 7 6 2.1 / 2.8 0.5 Longitudinal rms Emittance (kev-psec) 17 (80) 44 21 13.3 145 64 (19) RMS Bunch Length (psec) 3.2 (0.35) 6.3 2.0 4.0 50 15.4 RMS Energy Spread (%) 0.1 (0.13) 0.5 0.12 0.24 < 1 ~ 3 0.5 3.1 (2.1) ERLP Energy (MeV) 160 N/A 100 35 17 12.8 (14) N/A N/A N/A 20 ERL Energy Goal (MeV) 200 N/A 5000 35 40 N/A N/A N/A 40 600 (1000) Electron Gun DC Gun Voltage (kv) 350 500 500-750 350 230 300 N/A N/A N/A N/A N/A Gun Accelerating Field (MV/m) 4 7 8 4 1 7 / 7 / 5 20 / 13 / 13 20 25 (TBD) Cathode Material GaAs GaAs GaAs GaAs Thermionic Thermionic CsKSb Multi-Alkali TBD Dia./M-Alk. Dia./M-Alk. Drive Laser FWHM Pulse Length (psec) 44 44 30 20 N/A N/A 53 16 TBD 10 Laser Wavelength (nm) 527 527 527 527 N/A N/A 527 527 527 527 Laser Power at 5% QE (W) 0.5 5 5 0.325 N/A N/A 5 (53) 0.2 / 25 5 (~0) Booster (DC) or Gun (RF) Booster or Gun Type SRF SRF SRF SRF SRF NCRF N/A N/A N/A N/A N/A Geometry (Cavities x Cells) 2 x 5 4 x 1 5 x 2 2 x 9 2 x 1 3 x 1 1 x 1.5+1 x 3 1 x 2.5 1 x 2.5 1 x 0.5 1 x 3.5 (TBD) Couplers per Cavity / Type 1 / WG 1/CX:1/WG 2 / CX 2 / WG 1 / CX 2 / WG 2 / WG 3 / WG 2 / CX TBD Coupler Power (kw) 50 350 50 50 (200) 500 500 TBD Status Operational Assembly Fabrication Fabrication Operational Operational Retired Fabrication Analysis Design/Fab Analysis (Explanation) (At Wiggler) (Upgrade) (Upgrade) (Macropulse) (High PRF) (Correction) (Low PRF) Table 1 24
Parameter Value Nominal Range Output Energy (MeV) ~ 7 2 15 CW Average Current (ma) ~ 200 100 500 Bunch Charge (nc) ~ 0.5 0.075-3 Transverse rms Normalized Emittance (µm) ~ 1.5 < 1-6 Longitudinal rms Emittance (kev-psec) < 45 25-145 Bunch Length (psec) ~ 4 2-7 Energy spread (%) at Injection < 0.5 0.1-0.5 RF Frequency (MHz) ~ 700 500-1300 RF Feedthrough Power (kw) < 500 50-500 Photocathode Frequency Response Visible Visible Table 2 25
DEVICE AES/JLAB LANL/AES AES/BNL PARAMETER Injector Gun Gun/ERL Gun Type DC NCRF SRF RF Frequency (MHz) 748.5 700 703.75 PRF (MHz) 748.5 33.3 351.88 Charge/Bunch (nc) 0.133 3.0 1.4 Current (ma) 100 100 500 Injector Energy (MeV) 7 2.5 2 Mean Accelerating Gradient (MV/m) 7 / 13.5 7 23.5 Transverse rms Normalized Emittance (µm) 1.2 6 2.5 Longitudinal rms Emittance (kev-psec) 44 145 64 (19) RMS Bunch Length (psec) 6.3 15.4 RMS Energy Spread (%) 0.5 0.5 2.1 (1.7) Table 3 26