Photoinjector design for the LCLs

SLAC-PUB-8962 LCLS-TN-01-05 17 August 2001 Photoinjector design for the LCLS∗ P.R. Boltona, J.E. Clendenina , D.H. Dowellb, M. Ferrarioc, A.S. Fishera, S.M. Giermana, R.E. Kirbya, P. Krejcika, C.G. Limborga, G.A. Mulhollana, D. Nguyend, D.T. Palmera, J.B. Rosenzweige, J.F. Schmergea, L. Serafinif, X.-J. Wangg a SLAC, Stanford, CA 95309, USA Boeing Physical Sciences Research Center, Seattle, WA 981124, USA c INFN-LNF, 00044 Frascati (Roma), IT d LANL, Los Alamos, NM 87545, USA e UCLA, Los Angeles, CA 90095, USA f INFN-MI, 20133 Milano, IT g BNL, Upton, NY 11973, USA b Corresponding author: J.E. Clendenin, P.O. Box 4349, Stanford, CA 94309, USA. Tel.: +650-926-2962; fax: +650-926-8533. E-mail address: clen@slac.stanford.edu. Abstract The design of the Linac Coherent Light Source assumes that a low-emittance, 1-nC, 10-ps beam will be available for injection into the 15-GeV linac. The proposed rf photocathode injector that will provide a 150-MeV beam with rms normalized emittances of 1 µm in both the transverse and longitudinal dimensions is based on a 1.6-cell S-band rf gun that is equipped with an emittance compensating solenoid. The booster accelerator is positioned at the beam waist coinciding with the first emittance maximum and is provided with an accelerating gradient of ~25 MeV/m, i.e., the "new working point." The uv pulses required for cathode excitation will be generated by tripling the output of a Ti:sapphire laser system consisting of a highly stable cw mode-locked oscillator and two bow-tie amplifiers pumped by a pair of Q-switched Nd:YAG lasers. The large bandwidth of the Ti:sapphire system accommodates the desired temporal pulse shaping. Details of the design and the supporting simulations are presented. PACS Codes: 29.25.Bx, 29.27.Ac, 41.60.Cr, 41.85.Ar Keywords: Photoinjectors; Low Emittance; Free Electron Lasers Contributed to The 23rd International Free Electron Laser Conference Darmstadt, Germany 20-24 August 2001 ∗ Work supported by Department of Energy contract DE-AC03-76SF00515. 1 1. Introduction The proposed Linac Coherent Light Source (LCLS) is an x-ray free electron laser (FEL) that will use the final third of the SLAC 3-km linac for the drive beam. The performance of the LCLS in the 1.5 Å regime is predicated on the availability of a 1-nC, 100-A beam at the 150-MeV point with normalised rms transverse emittance of 1 µm. A low energy spread, both integrated and slice, and low charge and timing jitters are also required. The initial injector design made use of a low-gradient booster as was then the standard [1]. An improved design that better meets the LCLS requirements is the subject of this paper. The basic layout of the injector is shown in Fig. 1. This layout is consistent with the concept of a “new working point” that was introduced in 1999 [2]. The basic difference seen in this design compared to the earlier version is that the drift distance between the gun and booster is greater and also the booster accelerating gradient is much higher. Note that a relatively weak solenoidal field is now used around the first booster section instead of discrete focusing coils between sections. 2. Gun and cathode The LCLS gun will be a modified version of the BNL/SLAC/UCLA symmetrized 1.6-cell rf photocathode gun [3]. The principal differences are the addition of a load-lock to better ensure cathode performance and improved cooling to allow operation at 120 Hz. In addition, the rf power for the LCLS gun will be fed symmetrically into the full cell in order to eliminate higher order modes. To increase its heat capacity, the standard 1.6-cell 2 gun will be modified by increasing the number and size of the water cooling channels and pushing them closer to the iris and cathode surface without compromising structural integrity [4]. A copper photocathode is chosen because the entire end plate of the half cell can be formed in the standard manner of rf cavities, permitting operation at the highest field values. The photoelectric response time of metal cathodes is on the sub-picosecond level, thus imposing no limitation on the desired temporal pulse shaping. Since the source is not required to produce multiple microbunches within each pulse, the lower quantum efficiency (QE) of metal cathodes compared to alkali and semiconductor photocathodes is not a major concern. The QE for Cu illuminated with uv light depends on surface preparation, but 10-5 for normal incidence at 266 nm (4.7 eV) in a non-load-locked gun is achievable [5,6,7]. Much better QE is available from copper installed through a loadlock. This is illustrated by the data of Fig. 2 for which a carefully prepared polycrystalline Cu photocathode was inserted into high vacuum surface diagnostic chamber using a load-lock (no baking) and the QE spectrum measured at low voltage [8]. The figure shows a QE of 2.3×10-4 at 266 nm. At 266 nm, an optical pulse of 500 µJ on the cathode is required to produce 1 nC of charge when the QE is 10–5. A laser system to meet this requirement is described in Section 3. Copper photocathodes made from single-crystal Cu have proven to have not a only high QE but also low dark current [9]. In addition, the QE uniformity across the cathode surface appears to be superior to polycrystalline Cu. Single-crystal copper boules with a 3 diameter sufficient to fabricate the back plane of an S-band gun are available, thus the use of a cathode plug can be avoided even for single-crystal Cu cathodes. 3. Laser The laser system for illuminating the cathode is essentially unchanged from that shown in reference [1]. A cw mode-locked Ti:sapphire oscillator, pumped by a diodepumped Nd:YVO4 laser, delivers a stable, continuous pulse train of 12-nJ, 100-fs pulses at a repetition rate of 79.33 MHz. This frequency, the 36th subharmonic of the linac’s 2856-MHz rf, locks the timing of the laser pulses to the phase of the rf in the linac and rf gun. The wavelength is tuned to 798 nm and tripled to 266 nm after amplification to provide a suitable wavelength for the photocathode. A Pockels cell and polarizer are used to gate single pulses, at 120 Hz, from the 79.33MHz pulse train. The energy of such selected pulses is increased in a two-stage Ti:sapphire rod amplifier system using a four-pass bow-tie configuration for each stage [10]. Both are pumped by a pair of Q-switched, doubled Nd:YAG lasers that fire in alternation, each producing a 60-Hz train of 3 to 10-ns pulses. Relay imaging is used to maintain a good transverse mode while efficiently filling the pumped volume of the Ti:sapphire rods. The large oscillator bandwidth (of order 1%) enables transformed-limited ultrashort pulse-width capability and use of the well-established chirped pulse amplification [11] technique to reduce peak power levels in the amplifier stages. With this scheme, pulse stretching to hundreds of picoseconds is accomplished by imposing a positive chirp on 4 the waveform of the pulse. Following amplification the stretched pulses are compressed by reversing the imposed chirp. The temporal pulse shape is determined by modifying (under computer control) the oscillator output spectrum. Additional pulse-width flexibility is afforded by partial compression of the stretched, amplified pulses. After the second amplifier, the transverse shape of the pulse is modified from Gaussian to uniform to better match the requirements for obtaining a low emittance from the gun. Next, two crystals triple the frequency of the light to a wavelength of 266 nm. The spatially flattened pulse also improves efficiency and uniformity in this harmonicgeneration process. Finally, the beam is transported through an evacuated tube to an optics platform next to the gun. Since the Fourier-relay image plane that follows the long transport tube has a spot size that is too small for the photocathode, the spot is magnified and imaged onto a circular aperture that slightly trims the edge of the beam. This aperture is in turn imaged onto the photocathode, so that the illuminated region of the photocathode is precisely defined without jitter. The imaging includes compensation for the temporal and spatial distortion that results if grazing incidence on the photocathode is used. The net energy transmission of the laser system is as follows: transmission through the spatial flattener is about 50%, through the compressor 50%, through the frequency tripling stage 25%, and through the optical transport to the gun 50%. If grazing incidence is used, the final steering optic will be a uv grating. Appropriate choice of coating, polarization, blazing angle and groove density of the grating can yield first order uv 5 diffraction efficiencies in excess of 90%. Consequently, starting with 18 mJ after the second amplifier, the required 500 µJ is delivered to the cathode. 4. Simulations Using the semi-analytic code HOMDYN [12], a wide range of injector parameters was investigated earlier under the constraints imposed by the invariant-envelope [13] matching condition: injection into the matched accelerating gradient of the booster at a laminar waist. As a result, it was found that by increasing the gun focusing solenoid strength so that the waist also occurs when the emittance has its relative maximum, the second emittance maximum can be shifted to higher energy with a lower final emittance value than previously achieved [2,14]. This new configuration is here referred to as the “new working point.” The LCLS injector has been designed using version 3 of the LANL-maintained code PARMELA to establish the details of the “new working point.” The electric field map of the gun was obtained with SUPERFISH and directly used in PARMELA. SUPERFISH was also used to simulate the fields in the traveling-wave accelerating sections, and space harmonics were calculated to be used in PARMELA. RF fields were assumed to be cylindrically symmetric. A magnetic field map for the emittance compensating solenoid at the gun was produced using POISSON and passed to PARMELA. The magnetic field for the air core solenoid around the first accelerating section was modeled in PARMELA using single coils each with appropriate strength to represent the field. 6 Using only the gun, solenoid, and the immediately following drift space (i.e., no booster), the first emittance minimum after the solenoid was optimized by varying the solenoidal field and the beam radius at the cathode. A value of Bz = 3.15 kG and hardedge radius of 1 mm was found to be optimum. The emittance minimum very nearly coincides with the “new working point.” A slightly larger value of BZ is found here than with HOMDYN, consistent with the solenoid being displaced somewhat downstream because of the physical interference of the gun structure. Next the position of the booster with an accelerating gradient of 25 MV/m was optimized followed by the position and field of the linac solenoid. The injector parameters determined in this manner are summarized in Table 1. A thermal emittance of 0.26 µm has been added to the PARMELA deck. For the parameters of Table 1, the emittance and beam size for a risetime of 0.5 ps is shown in Fig. 3 as a function of distance from the cathode. The normalized transverse phase space at the exit of the booster for 100K particles is shown in Fig. 4. The upper left plot is a normalized x-y scatter plot, with xn and yn amplitudes in units of rms beam size. The normalized xn − xn′ phase space is shown in the upper right with the rms emittance ellipse given by the circle of unity radius in the center. The density of particles in the center of the beam is not evident in these plots. The normalized rms slice emittance in x and y, as a function of axial distance along the bunch, is shown in the lower left. The projected value is shown by the horizontal line. On the lower right, the beta-mismatch amplitude, ζ, is shown as a function of ∆z. The beta-mismatch amplitude, which is normalized such that ζ• LQGLFDWHV WKH GHJUHH RI PLVPDWFK EHWZHHQ WKH VOLFH 7ZLVV 7 parameters and the projected Twiss parameters. Transporting the beam through a followon accelerating channel may be difficult if the variation in ζ is large [15]. The same PARMELA simulations yield an integrated (slice) energy spread at the booster exit that is within 0.1 (0.005)%. However, initial results of an ongoing comparison of PARMELA with PIC code simulations near the photocathode indicate that the energy spread predicted by PARMELA may be overly optimistic [16]. 5. Conclusions An rf photoinjector for the LCLS based on the “new working point” configuration has been described. The injector will utilize a 1.6-cell rf gun designed for 120 Hz and equipped with a Cu photocathode and load-lock. The cathode is illuminated with 500 µJ of uv light provided by a tripled-Ti:sapphire laser. At the output of the injector, the 150 MeV beam has a transverse emittance that is well below the 1-µm requirement and the slice emittance is about 25% (relative) lower except for the head and tail. The integrated (slice) energy spread is also very low. 8 References [1] R. Alley et al., Nucl. Instrum. and Meth. A 429 (1999) 324. [2] M. Ferrario et al., in The Physics of High Brightness Beams, eds. J. Rosenzweig, L. Serafini, World Scientific (2000), p. 534. [3] D.T. Palmer et al., SPIE 2522 (1995) 514. [4] X.J. Wang et al., “High-Rep Rate Photocathode Injector for LCLS,” contributed to the 2001 Particle Accelerator Conference, June 18-22, 2001, Chicago, IL. [5] T. Srinivasan-Rao et al., J. Appl. Phys. 69 (1991) 3291. [6] P. Davis et al., in Proc. of the 1993 Particle Accelerator Conference, p. 2976. [7] E. Chevallay et al., Nucl. Instrum. and Meth. A 340 (1994) 146. [8] G. Mulhollan, "Common Sense Copper and RF Guns," LCLSTN-99-9 (July 1999), unpublished. [9] P.R. Bolton et al., “Transverse and longitudinal emittance measurements on an S-band photocathode rf electron gun,” this conference. See also D.T. Palmer et al., in The Physics of High Brightness Beams, eds. J. Rosenzweig, L. Serafini, World Scientific (2000), p. 439. [10] The gain per pass in Ti:sapphire amplifiers is sufficient to get from 1 nJ to a few mJ in eight (8) passes. For example, see S. Backus et al., Opt. Lett. 20 (1995) 2000. [11] P. Maine et al., IEEE J. Quantum Electron. QE-24 (1988) 398. [12] M. Ferrario et al., Part. Acc. 52 (1996). [13] L. Serafini, J. Rosenzweig, Phys. Rev. E 55 (1997) 7565. [14] M. Ferrario et al., Proc. of the 7th European Particle Accelerator Conference, Vienna (2000) 1642. [15] P. Emma, private communication. [16] V. Ivanov, E. Colby, C. Limborg, private communication. 9 Table 1. PARMELA parameters and results. Parameter Value Bunch charge at cathode/at booster exit 1.0/1.0 nC Bunch shape at cathode spatial/temporal Uniform/uniform Bunch radius at cathode 0.71 mm rms Bunch length at cathode/at booster exit 2.9/2.9 ps rms Peak rf field at gun (extraction phase) 140 MV/m (32°) Gun solenoid axial field 3.15 kG Cathode to booster-entrance distance 1.4 m Booster accelerating gradient (phase L01/L02) 25 MV/m (-2.5/+2.6°) Linac solenoid axial field/length -1.5 kG/1.0 m Energy at booster exit 150 MeV Integrated (slice) energy spread, σ γ γ 0 0.10 (0.005) % rms Normalized transverse emittance: 0.5 (1.0) ps rise time, εn,th included, 100 K particles 10 0.80 (0.95) µm rms Figure Captions Fig. 1. Schematic layout (not to scale) showing only the principal beamline elements, the location of the diagnostics, and the rf distribution system. In the figure are shown the rf gun (G), the emittance compensating solenoid (S1), charge coupled devices (CCD), klystrons (K), the focusing solenoid (S2) around the first 3-m accelerating section (L0-1) of the booster. Fig. 2. QE of copper as a function of quantum energy measured with low (22 V) dc bias with the surface untreated after installation in the analysis system using a load-lock. Fig. 3. Transverse normalized rms emittance as a function of distance from the cathode for 100K particles. A rise time of 0.5 ps is assumed. A normalized rms thermal emittance of 0.26 µm is included. Fig. 4. Normalized transverse phase space at the exit of booster for 100K particles. (1) Distribution of particles in the beam (upper plots). The scales are derived from the right-hand figure in which the rms emittance ellipse in the x-x’ plane (only) is normalized to a circle having a radius of unity. (2) Transverse normalized slice emittances (lower left plot) in both planes and mismatch parameter, ζ, (lower right plot) in both planes along the bunch z-axis. The bunch head is at the right. 11 Figure 1 K K S1 G S2 K LO-1 e– LO-2 CCD Legend 8-2001 8612A1 12 Solenoid Toroid x and y Corrector Viewing Screen Faraday Cup BPM Figure 2 Quantum Yield (electrons/photon) 10–2 10–3 10–4 10–5 6.0 4-2001 8560A98 5.5 5.0 Energy (eV) 13 4.5 2.5 2.0 2.0 1.6 1.5 1.2 Emittance 1.0 0.5 0 0.8 0.4 Beam Size 0 200 σx,y (mm) γ εx,y (µm) Figure 3 400 z (cm) 14 600 800 0 3-2001 8560A76 Figure 4 10 x'n yn 10 0 –10 –10 0 xn –10 –10 10 0 xn 10 1.0 0.4 ζx,y (mm) 0.8 γεx,y (µm) 0 γεx 0.5 ζx γεy 0 3-2001 8560A88 –1 ζy 0 ∆z (mm) 0 1 15 –1 0 ∆z (mm) 1