Initial commissioning results of the next generation photoinjector

See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/241468263 Initial commissioning results of the next generation photoinjector Article · March 1997 DOI: 10.1063/1.53071 CITATIONS READS 6 2 11 authors, including: Ilan Ben-Zvi C. Pellegrini 560 PUBLICATIONS 3,248 CITATIONS 389 PUBLICATIONS 3,767 CITATIONS Brookhaven National Laboratory SEE PROFILE University of California, Los Angeles SEE PROFILE Herman Winick Vitaly Yakimenko 122 PUBLICATIONS 1,548 CITATIONS 301 PUBLICATIONS 2,107 CITATIONS Stanford University SEE PROFILE Stanford University SEE PROFILE Some of the authors of this publication are also working on these related projects: Fresh bunch self-seeding View project TW X-ray FEL View project All content following this page was uploaded by C. Pellegrini on 27 January 2014. The user has requested enhancement of the downloaded file. SLAC{PUB{7392 Jan 1997 Initial Commissioning Results of Next Generation Photoinjector D. T. Palmer, X. J. Wangy , R. H. Miller M. Babzieny ,I. Ben-Zviy, C. Pellegrini, J. Sheehany , J. Skaritkay , H. Winick , M. Woodley , V. Yakimenkoy  Stanford Linear Accelerator Center Stanford University, Stanford, CA 94309 y Brookhaven National Laboratory Accelerator Test Facility Upton, NY 11973  University of California Los Angeles Department of Physics Los Angeles, CA 90095 Abstract The Next Generation Photoinjector(NGP) developed by the BNL/SLAC/UCLA collaboration has been installed at the Brookhaven Accelerator Test Facility(ATF). The initial commissioning results and performance of the photocathode injector are presented. The NGP consists of the symmetrized BNL/SLAC/UCLA 1.6 cell S-band photocathode RF gun and a single solenoidal magnet for transverse emittance compensation.1 The highest acceleration eld achieved on the cathode is 150 MV m , and the . The quantum eciency of the copper cathode was measured to normal operating eld is 130 MV m be 4.5x10,5. The transverse emittance and bunch length of the photoelectron beam were measured. The optimized rms normalized emittance for a charge of 300 pC is 0.7  mm-mrad. The bunch length dependency of photoelectron beam on the RF gun phase and acceleration elds were experimentally investigated. Contributed to 7th Advanced Accelerator Concepts Workshop Lake Tahoe, CA, USA Oct 12{Oct 18, 1996  Work supported by Department of Energy contract DE{AC03{76SF00515. 1 Introduction The Next Generation Photoinjector(NGP) has been installed at the Brookhaven Accelerator Test Facility(ATF) as the electron source for beam dynamics studies, laser acceleration and free electron laser experiments. The injector consists of the symmetrized BNL/SLAC/UCLA 1.6 cell S-band photocathode RF gun, powered by an XK-5 klystron, and is equipped with a single emittance compensation solenoidal magnet. There is a short drift space between the NGP and the input to the rst of two SLAC three meter travelling wave accelerating sections. This low energy drift space contains a copper mirror that can be used in either transition radiation studies or laser alignment. There is also a beam pro le monitor/faraday plate located 66.4 cm from the cathode plane. The ATF consists of two SLAC travelling wave linacs powered by a single XK-5 klystron. The high energy beam transport system consists of nine quadrupole magnets, an energy spectrometer, an energy selection slit and a high-energy faraday cup. Diagnostics located in the high energy transport consist of beam pro le monitors and strip lines. The strip lines are used for an on line laser/RF phase stability monitor. The drive laser is a Nd:YAG master oscillator/power ampli er system. A diode pumper oscillator mode locked at 81.6 MHz producing 21 psec FWHM pulses at 100 mW of average power. Gated pulses seed two ashlamp pumped multi-pass ampli ers and are subsequently frequency quadrupled. This nonlinear process leads to a factor of two reduction in the laser pulse length. The 266 nm beam is transported to the RF gun area via a 20 meter long evacuated pipe. The laser beam transport system near the injector includes an aperture, a set of telescoping lenses and a limiting aperture. This limiting aperture is imaged onto the cathode with a spherical lens and a pair of Littrow prisms which compensate for the anamorphic magni cation introduced by the 72o incidence on the cathode. The time slew across the cathode caused by this oblique incidence is corrected by using a di raction gradient. The relay imaging technique used throughout the optical transport improves the beam pointing stability. Since the laser beam over lls the limiting aperture, the transverse pro le of the beam a truncated gaussian. The spot size of the laser beam on the cathode is 2 mm diameter edge to edge. 2 Injector Design The NGP consists of the symmetrized BNL/SLAC/UCLA 1.6 cell S-band photocathode RF gun mated to a single emittance compensation solenoidal magnet. The 1.6 cell RF gun di ers from the original BNL 1.5 cell RF gun2 in that the half cell has been lengthened to decrease the RF eld levels on the cell to cell coupling iris and also to provide more RF focusing in the iris region. The 1.6 cell RF gun is not a side coupled 0-mode suppressed RF gun, as in the previous BNL type RF guns. High power RF is coupled only into the full cell. The enlarged beam iris diameter increases the cell to cell coupling, which provides a mode separation between the  and 0-modes of 3.225 MHz for a balanced eld con guration. The half cell is fully symmetrized with two 72o oblique incidence laser ports. The cathode plate is removable using a single helico ex seal for both the vacuum and RF seals. This removable cathode plate eliminates the multipacktering problem common to choke joint cathodes. The removable cathode allows for easy replacement of di erent cathode materials such as Cu and Mg. The Cu cathode results are presented in this paper. The full cell has two symmetrized plunger type tuners with a tuning range for both tuners of 2 MHz. The RF coupling slot is symmetrized by an identical coupling slot that provides additional vacuum pumping.3 The 1.6 cell gun uses resistive heating to maintain the resonant frequency. The single emittance compensation solenoidal magnet, that was speci cally designed to be used with the 1.6 cell gun, utilizing POISSON4 eld maps into PARMELA5 to study the beam dynamics consideration of di erent magnet designs. In previous emittance compensation system designs, a bucking coil is positioned upstream of the cathode plane to null the magnetic eld at the cathode. This is unnecessary in the present design since the Figure 1: Next Generation Photoinjector single solenoid magnet produces less than 9 gauss at the cathode when the peak solenoidal eld is 3 Kgauss. After rf conditioning the 1.6 cell RF gun operates at 5x10,9 torr with a eld gradient of 125 MV m and in the quiescent state the vacuum is 1x10,9 torr. 3 Gun Energy / Dark Current The BNL/SLAC/UCLA 1.6 cell S-band photocathode RF gun is designed to attain a eld level at the cathode and at the middle of the full cell of up to 150 MV m , and to operate with RF pulse widths up to 3.5 s. Calibration of the eld levels in the gun were veri ed by measurements of the beam energy using a cos() de ection magnet located inside the bore of the emittance compensation solenoidal magnet. The results of these energy measurements are shown in gure 2 where  is the laser injection phase.  = 0 and  = 90 is the zero crossing and crest of the rf respectively. Fowler−Nordheim Plot −19 6 10 I/E2 E (MeV) 5.5 5 4.5 4 0 −20 10 127 MV/m 115 MV/m 110 MV/m 20 40 Φ 60 80 Figure 2: Energy versus Field Level 100 −21 10 7.5 8 8.5 1/E 9 9.5 Figure 3: Fowler-Nordheim Plot Standard machining and Cu wool polishing techniques were used to manufacture both the full and half cells. The cathode plate was prepared using the procedures detailed in reference.6 These techniques, when combined together, produce a eld enhancement factor = 58 as can be seen in the Fowler-Nordheim Plot of gure 3. 4 Multi-Pole Fields Multi-pole eld e ects were studied by decreasing the laser spot size to 400 m and setting the laser injection phase to the Schottky peak. This injection phase causes an e ective electron bunch lengthening and a noticeable energy spread tail was observed. By adjusting the laser spot position we were able to eliminate this energy spread tail. This alignment minimizes the integrated higher order mode contribution to the beam distortion. Analysis indicates that the symmetrized BNL/SLAC/UCLA 1.6 cell photocathode RF guns electrical and geometric center are within 170 m of each other, which is within the laser spot alignment error. Compared to similar experimental results of the 1.5 cell BNL gun whose electrical and geometric centers di er by 1.0 mm,7 the 1.6 cell gun has ful lled the symmetrization criteria. Future work with custom laser masks to study the eld patterns at larger diameters are planned for the future.8 5 Quantum Eciency versus Polarization The laser time slew correction has the draw back of decreasing the available laser energy at the cathode by 50%. The available charge was measured as a function of polarization. In gure 4 it can be seen that the charge is maximized at a polarizer angle of 56o which correspondence to P polarized light on the cathode. 0.4 0.5 0.35 0.4 0.3 Q (nC) Q (nC) 0.25 0.2 0.15 0.3 0.2 0.1 0.1 0.05 0 0 50 100 Ω 150 200 Figure 4: Electron Bunch Charge versus Polarizer Angle 0 0 10 20 30 40 Laser Energy (µJ) 50 60 Figure 5: Electron Bunch Charge versus Laser Power The measured value of the Cu cathode's quantum eciency is QE = 4.4x10,5 which is calculated from gure 5. These studies were conducted at a laser injection phase of 90o which utilized the Schottky e ect to increase the available charge. n;rms n;rms Quad Scan 2.29  mm-mrad Two Screen 2.42  mm-mrad Table 1: Quad Scan and Two Screen Method Results Transverse Phase Space 0.5 3 0.4 2.5 ε n,rms (π mm mrad) Spot Size (mm) 6 0.3 2 1.5 0.2 1 0.1 0 4 emittance rms spot size 0.5 95 4.5 5 5.5 6 HQ5 Current (AMPS) 6.5 7 100 105 110 Solenoid current (A) 115 120 Figure 7: Emittance and RMS Beam Size versus Solenoidal Field The normalized rms emittance, n;rms, measurements were taken using a variation of the three screen method. Two screens were utilized while insuring that a beam waist was located at the down stream pro le screen. The two screen method is compared to the standard quadrupole scan technique in gure 6 and the results from these two methods are compared in table 1 Figure 6: Quadrupole Scan RMS Emittance Results PARMELA was used to simulate the emittance compensation process and the subsequent acceleration to 40 MeV.9 A correlation of the minimum spot size with an emittance minimum was noted during these simulations. This was experimentally veri ed during the commissioning of the 1.6 cell RF gun, using the beam pro le monitor located at the output of the second linac section, as can be seen in gure 7. These results are consistent with similar results of the BNL 1.5 cell RF gun.7 3.5 1.3 1.2 3 ε n,rms (π mm mrad) ε n,rms (π mm mrad) 1.1 2.5 1 0.9 0.8 1.5 0.7 1 0.6 0.5 100 2 200 300 Q (pC) 400 500 Figure 8: Emittance versus Electron Bunch Charge 0.5 20 30 40 Φ 50 60 70 Figure 9: Emittance versus Electron Bunch Charge There are four emittance terms that contribute to the total n;rms these are the space charge, rf, thermal and magnetic terms. The last term is due to the small but nite magnetic eld at the cathode. Studying the dependence of transverse emittance on the bunch charge in gure 8, we have noted that at Q = 0 charge there is a residual emittance term of 0.2  mm-mrad. This term is due to rf , thermal , mag and measurement errors. If we neglect the magnetic term, which is reasonable due to the initial cathode spot size and the small magnetic eld at the cathode we can estimate the thermal and RF emittance terms to be less than 0.2  mm-mrad. Since the measured n;rms is a factor of three less than the sc that Kim's theory10 predicated, we are con dent that we have produced an emittance compensated beam. Due to laser power limitation, RF gun bunch compression and the Schottky e ect it is not possible to keep the peak current constant for di erent laser injection phases. Therefore in gure 9 the plot is not for a constant current but for a decreasing charge from a maximum of 400 pC to a minimum of 178 pC. The functional dependence of this plot has been veri ed by comparison with Kim's theory. 7 Longitudinal Phase Space When measuring the bunch length and energy spread of the electron bunch the RF system is adjusted such that the bunch initially has a minimum energy spread. This is accomplished by adjusting the overall linac phase with respect to the laser injection phase by means of the low level RF system. The  between the two linac section is adjusted by means of a high power RF phase shifter, such that the energy spread of the beam is minimized. The beams energy is set to 40 MeV by adjusting the low level RF drive to the linac klystrons. The energy spread is estimated by measuring the beam size on a phosphor screen in the dispersion region. The dispersion in this region is 5.4 mm % . Figure 10 is a plot of the energy spread of the electron bunch as a function of the phase di erence between the linacs. 20 3 2.5 15 δγ/γ (%) τ (psec) 2 10 1.5 1 5 0.5 0 −20 −10 0 δϕ 10 20 0 20 30 40 Φ 50 60 Figure 11: Bunch Length versus Laser Injection Phase Electron bunch length was measured by dephasing the second linac section such that a linear energy chirp is produced along the bunch. This allows the bunch length to be correlated to the energy spread. Using the technique discussed previously to measure the energy spread, the bunch length is measured as a function of laser injection phase. Figure 11 is experimental veri cation of bunch compression in the 1.6 cell RF gun. Bunch compression in RF guns have been experimentally demonstrated previously.11 Figure 10: Energy Spread versus Linac Phase 8 Conclusions We have experimentally studied the six dimensional phase space of the electron beam that is produced by the BNL/SLAC/UCLA 1.6 cell S-band photocathode RF gun. We have experimentally veri ed longitudinal bunch compression, electron bunch energy and transverse emittance as a function of injection phase, solenoidal eld and charge for peak elds in the RF gun of 127 MV m . The optimized rms normalized emittance for a charge of 300 pC is 0.7  mm-mrad. Future work includes studies of the multi-pole elds that this gun is designed to suppress, cathode magnetic eld e ects, along with slice emittance and inverse RADON transforms that will elucidate the electron beams transverse phase space. Emittance measurements for a bunch charge of 1 nC are also planned. 9 Acknowledgments The authors would like to thank the technical sta at UCLA, the Stanford Linear Accelerator Center and at Brookhaven Accelerator Test Facility for all their dedicated work on this project. We would also like to thank Mr. James N. Weaver from SSRL for all the technical discussions and help that he has provided. 10 [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] REFERENCES B. E. Carlsten, NIM, A285, 313 (1989) K. Batchelor et al., Proc. of 1990 EPAC p. 541 D. T. Palmer et al., Proc. 1995 Part. Accel. Conf. (1995) p. 982 K. Halbach and R. F. Holsinger, Particle Accelerators 7, 213 (1976) L. M. Young, private communications T. Srinivasan-Rao et al., BNL-62626 X. J. Wang et al., Proc. 1995 Part. Accel. Conf. (1995) p. 890 Z. Li, private communications D. T. Palmer et al., Proc. 1995 Part. Accel. Conf. (1995) p. 2432 K. J. Kim, NIM, A275, 201 (1989) X. J. Wang et al., Phys. Rev. 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