Initial measurements of the UCLA rf photoinjector

Transcription

1 Nuclear Instruments and Methods in Physics Research A 341 (1994) North-Holland NUCLEAR INSTRUMENTS &METHODS IN PHYSICS RESEARCH Section A Initial measurements of the UCLA rf photoinjector J Rosenzweig a **, N Barov a, S Hartman a, M Hogan a, S Park a, C Pellegrini a, G Travish a, R Zhang a, P Davis ', G Hairapetian b, C Joshi 6 Department of Physics, Unu ersity of' Califorma, Los Angeles, CA 924, USA n Electrical Engineering Dept, Unwersity of California, Los Angeles, CA 924, USA An rf photocathode gun which, along with a compact linac, forms the injection system for a planned 1 mm free-electron laser amplifier experiment, has been commissioned m the Particle Beam Physics Laboratory at UCLA This high-gradient gun, based on the Brookhaven design, has emitted several picosecond, > 1 A electron beams of up to 4 MeV in energy These beams have been characterized by a variety of diagnostics The quantum efficiency of the copper cathode used has been measured at normal incidence, and at 7 incidence, where the polarization dependence was also examined Limits on laser intensity due to surface damage, and to longitudinal space charge suppression of photoemission have been explored The energy and energy spread of the beam were characterized using a dipole spectrometer, while the time structure was examined using a picosecond resolution streak camera Both energy spread and pulse length were found to be adversely affected by longitudinal space charge forces The emittance of the beam was measured using the pepper pot technique, and its dependence on space charge and rf phase were found The impact of these results on improving the design and operation of high brightness photoinjectors is discussed, in particular with respect to SASE FEL amplifiers such as the UCLA 1 mm FEL, and the proposed SLAC X-ray FEL 1 Introduction We report the initial results of the operation of the UCLA 45 MeV photocathode rf gun [1,2] This electron source is the injector portion of a 16 MeV compact electron linac which will be used for studies of the high-brightness beam dynamics, the beam-plasma interaction, and the generation of coherent radiation Our initial work has been dedicated to a characterization of the photocathode rf gun To obtain a clear understanding of the workings of this system, we have measured the emittance, pulse-length and peak current of the photo-emitted electron beam, as well as the quantum efficiency of a copper cathode for differing angles of laser incidence and polarization We have run our initial tests in a regime where space-charge plays a large role in both transverse and longitudinal dynamics of the beam We discuss the impact of our results on future plans for our IR FEL experiment and the SLAC X-ray linac coherent light source * Work Supported by SDIO/IST through ONR Grant No N14-9-J-1952 and US DOE Grant DE-FG3-92ER * * Corresponding author 2 Photoinjector system The 1 5 cell, 2856 MHz standing wave rf photocathode gun is of the Brookhaven type, and is excited by a SLAC XK5 type klystron producing up to 24 MW of rf power in a 4 Ir,s pulse Accelerating gradients of up to 1 MV/m are achieved, with a peak output energy of 46 MeV The rf system is phase locked to the photocathode drive laser, with feedback provided by a Lightwave 1 timing stabilizer, producing a jitter in laser pulse arrival with respect to rf phase of less than 4 ps (an upper limit deduced from beam energy observations) The photoinjector drive laser has been designed to produce ultrashort laser pulses at 266 rim (4 66 ev) with up to 3 [,J/pulse This is accomplished using chirped pulse amplification and compression of the mode-locked Nd-YAG oscillator (A = 1 64 p,m) along with a regenerative amplifier stage, followed by frequency quadrupling The laser pulse length has been measured using a streak camera, and has been determined to be shorter than 2 ps FWHM The laser energy per pulse has large shot-to-shot fluctuations of approximately 3% The electron beam is focused by a 25 cm long solenoid positioned after the exit of the gun, with /94/$ Elsevier Science BV All rights reserved SSDI (93)E839-K VII ACCELERATORS

2 38 J Rosenzweig et al /Nucl Instr and Meth in Phys Res A 341 (1994) additional quadrupole focusing used to transport the beam through long sections The electron beam diagnostics consist of CCTV-viewed phosphor screens to monitor the beam size and position, an integrating current transformer (ICT), which is used along with Faraday cups to measure the electron beam charge per pulse, a streak camera to measure the electron beam pulse length, and a dipole magnet is used to measure the energy and energy spread of the beam In addition, a pepper-pot based emittance measurement system is used to examine the phase space of the electron beams os oa 3 oz ot 88 : Electron beam energy and energy spread Due to accidental motion of the micrometer which controls the longitudinal position of the cathode in the initial half cell, the rf fields were out of balance in our present measurements The full cell accelerating field amplitude was 1 8 times larger than in the half cell, causing the measured energy to be smaller than design Typically, the gun was run with a peak output energy of 35 MeV The dependence of the beam energy on laser injection phase shown in Fig 1, and allowed calibration of the laser-rf phase by comparison with numerical solutions of the equations of motion which included the field imbalance of the gun and the measured (by bead-pull technique) spatial harmonics of the rf field Fluctuations in the energy of the beam caused by both timing jitter of the laser pulse and the variations in the rf amplitude are of deep concern for future FEL experiments In order to get a measure of the rf induced pulse-to-pulse variations we measured the fluctuations in the peak of the dark current electron spectrum From this, we have determined the rms W n il ~aoooa o ~ ~ O o ȯ o o a Data e o Theo, y e ti k o zz 5 +; 67 ; Ixt RF Phase (Degrees) r_1 Fig 1 Measured average beam energy (data) and numerical solution of equations of motion (theory), as a function of injection phase Phase constant chosen to give the best fit of theory to experiment so 1 Q (PC) Fig 2 Rms momentum spread as a function of bunch charge The scatter in the data is mainly due to the uncertainty in the Q measurement at small charge variation in energy due to the rf amplitude to he 67% When the energy fluctuations due to laser timing are included, by measurement of the peak in the photo-electron spectrum, the resulting rms energy fluctuation is 25% The beam energy spread measured at low current (bunch charge less than 5 pc) is less than 2% However, due to the large longitudinal space charge effects it increases rapidly with charge, as is shown in Fig 2 The increased energy spread is caused by both the direct energy spread from space charge forces and from bunch lengthening which causes an increase in phase The phenomenon of pulse lengthening is discussed further below 4 Pulse length measurements The electron beam pulse length was measured approximately 9 cm after the gun exit by focusing the beam onto a 25 win thick fused silica etalon (9) which served as the Cherenkov radiator, which provided photons which could be observed with a streak camera This particular geometry of Cherenkov radiator, along with the inherent performance limitations of the streak camera (Hadland Imacon 5), allows 36 ps FWHM time resolution in the streak images This resolution was partly verified by streaking the laser pulses, using the 532 nm light available after the first doubling crystal, producing an image which had a pulse width of 39 ps FWHM Taking into account the resolution of the streak camera, the actual laser pulse width should be 1 7 ps FWHM This is consistent with autocorrelation measurements of the laser, which resulted in pulse widths of 2 ps FWHM The pulse length was measured with both 2 and 7 laser infection onto the cathode The streak image

3 J Rosenzweig et al /Nucl Instr and Meth to Phys Res A 341 (1994) P, FWHNI Time (p') 8 Fig 3 Typcal streak image of beam emitted from undamaged copper cathode - ua na ux i 12 Q(nQ measured in the 7 injection case showed a strong asymmetry (2 ps) in time of arrival of differing sides of the bunch as would be expected from the glancing incidence of the laser During illumination at 7, the copper cathode surface was damaged by the intense laser light This region, which when observed by a transmission electron microscope, displayed intricate structures which are evidence of melting and resolidification This region, which due to the constraints of the laser optics entering the 7 injection port was only approximately 1 mm 2, displayed enhanced emission (see the discussion of quantum efficiency measurement below) A streak image of the beam obtained from the undamaged cathode at 2 laser injection is depicted in Fig 3, which shows significant bunch lengthening compared to the laser pulse length, to 12 ps FWHM Streaks from the damaged portion of the cathode resulted in slightly longer bunches due to an elongated tail, as is shown in Fig 4 The source for bunch lengthening is longitudinal space charge, which is strong due to the small laser illumination area, and may be aggravated by the presence of microemitters in the damaged portions of the cathode To illustrate this effect, PARMELA simulations were performed for various output bunch charge levels from 1 pc to 1 3 Fig 4 Typcal streak image of beam emitted from damaged copper cathode Fig 5 Rms bunch length at bunch measurement position, as calculated with PARMELA nc, as shown in Fig 5 These simulations show a bunch lengthening as output charge is increased For typical charge levels between 4 nc and 1 nc PARMELA predicts bunch lengths from 9 ps to 15 ps These bunch length predictions are within the scatter of the measured bunch lengths which did not, however, display a strong correlation with charge level 5 Quantum efficiency measurements The quantum efficiency 77 obtained from the copper cathode was measured by comparing the total energy in the laser pulse (measured using a calibrated photodiode) with the electron bunch charge as measured by the ICT At 7 injection, the laser polarization angle (h, the angle the electric field makes with respect to the plane of incidence, was varied continuously by rotation of the laser polarization through a full 36 by use of a 2 wave plate By this definition, = corresponds to p-polarized light and (h = 9 to s-polarized light The laser damaged spot is centered on the cathode and therefore all quantum efficiency measurements were made for photoemission emanating at least partially from the damaged area Measurements of collected charge vs laser energy for three representative cases are shown in Fig 6 From these measurements it is clear that saturation of the charge output due to longitudinal space charge occurs at laser energies above 5 l-j Therefore, values of quantum efficiency 77 are taken from the slope of the data in low charge limit Linear fits for laser energies below 25 wj are presented and labeled with the quantum efficiency (71) corresponding to the slope of the line From these fits, an enhancement in quantum efficiency of 5% is observed for 7 p-polarized over 7 s-polarized light The functional form of this enhancement fits a cos26 dependence, and implies VII ACCELERATORS

4 382 Û q v U J Rosenzweig et al /Nucl Instr and Meth in Phys Res A 341 (1994) complete discussion of the theoretical expectations concerning the quantum efficiency, and comparison to previous measurements by other groups, see ref [8] The saturation can be explained by space charge effects near the cathode surface When the electrons are produced by the laser pulse, they are emitted as a thin disk from the cathode The space charge electric field between the electron bunch and the cathode can be approximated by a surface charge density and its image charge in the cathode To explore this model further, consider a radial Gaussian emission region which produces a surface charge density of the form Laser Energy (W) Fig 6 Quantum efficiency measurements of copper cathode with 466 ev photons Saturation of photoem)ssion is due to longitudinal space charge at cathode that the enhancement is dependent on the square of the normal electric field For 2 injection, changing the polarization angle did not affect the quantum efficiency, as expected For small laser spot sizes, it was possible to inject the laser pulses at 2 incidence without impinging directly on the damaged area of the cathode Careful measurements were not taken under these conditions, however, a factor of approximately 3 decrease in the quantum efficiency was observed from the undamaged portions of the cathode For a more Y(r) - Qo e (,=/=,r, ), 2aro-,'` where or is the effective rms spot size Now, an elementary calculation of an infinitesimally thin charge layers' field limits the maximum surface charge density which can exist before electrons are repelled back to the cathode, 2: ro=e()e() sin(w()) To find the total charge emitted, we must add up those emitted in this saturated region and those in the unsaturated tail The total emitted in the saturated region is Q ( =Trai, where the saturated radius is simply Qo ( 2,rro-,2 m ) )/2 ) 5 U C G) bl) 4 cà U 5 Fig Expected Charge Cathode 7 Comparison of the quantum efficiency data with PARMELA simulations and theoretical model, for an rms bunch spot size or, = 39 mm 3

5 J Rosenzweig et al INucl Instr and Meth in Phys Res A 341 (1994) We can easily integrate the charge emitted in the tails, to yield Q2 - Qo et-' /2 ' The total charge off the cathode is that given off in the saturated region plus that given off in the tail, jm Qtot = Q1 + Q2 = Qii 4 where!() = Qo /(217o",2) is the peak surface charge density which would be emitted without the presence of space charge forces Note that this expression is valid only when the emission is already in saturation (:() Fig 7 shows the calculated curve from the above expression, plotted with the 2 experimental data and PARMELA simulations The only free parameter used to create the theoretical curve was the laser spot size (the accelerating field in the half-cell was determined to be about 39 MV/m at the laser injection time), which was determined by a best fit to the experimental data in the saturated region to be 39 mm It should be noted that the derivation above can easily be generalized to elliptical Gaussian distributions, with the results being identical upon substitution of o-,2 o-r cr, 6 Emittance measurements The UCLA photoinjector [7] is designed to produce a very high peak current beam, up to 25 A, at relatively low energy, 45 MeV The consequence of this is that electron beam transport is in the space charge dominated regime Since the beam is space charge dominated, linear beam transport formalisms such as matrix transformations do not apply This means that the usual quadrupole scan technique is not valid for the UCLA photo-electron beam To overcome this problem a pepper pot emittance measurement apparatus has been designed and implemented The pepper pot is designed such that upon the electron beams passage through the pepper pot it is transformed from a space charge dominated beam to an emittance dominated beam Once the electron beam is in the emittance dominated regime one can use linear transform theory to calculate the emittance Because of pulse to pulse fluctuations in the beam charge the measurements were performed in a single shot The pepper pot used in these experiments is actually an array of eight slits Using the envelope equation as a guide for determining the degree of space charge contribution to the transverse beam dynamics, we chose a slit size of 5 llm After the electron beam traverses the slits it drifts approximately 2 cm onto a phosphor screen The image then gives the full phase space b a Charge (nc) Fig 8 Normalized beam emittance as a function of bunch charge, Eh = 3 5 MeV, at a constant laser injection phase information, the emittance, and other Courant-Snyder parameters To have an unambiguous definition of emittance, we work within the rms formalism, in which Erms _ ( ~ x2xx '2) - ( xx')2)1/2 where the normalized emittance is given by E = ßyEr,ns and the envelope parameters ~ _ Cx 2 ) (xx') F'rms - and arms - Erm, Erras The emittance and the envelope factors arc determined on-line, at a rate up to the maximum repetition rate of the photoinjector system (5 Hz) The electron beam emittance in all of the following was measured at average beam momentum of 35 MeV/c The first step in our measurement was to find the linear portion of the dependence of Erms on charge Q The laser energy delivered to the cathode was varied and the emittance was measured as a function of charge transported through the ICT and to the slits For all other parameters fixed one expects the emittance to be linear in charge as per Kim's arguments [3] This is in fact what is observed, as is shown in Fig 8 To measure the rf induced emittance versus laser injection phase many emittance as a function of charge data runs were taken with varying laser injection phases The procedure for extracting the rf induced emittance is as follows For each laser injection phase the data is plotted and fit to a line The slope and intercept of this line are then extracted The zero charge intercept then gives the experimentally determined rf induced emittance Fig 9 displays the rf induced emittance as a function of phase, derived from experiment and from PARMELA simulation Also, to check the experiment versus the computational model for a set of relevant cases, we take the emittance versus phase expected at 1 nc (design VII ACCELERATORS

6 384 J Rosenzweig et al /Nuel Instr and Meth in Phys Res A 341 (1994) Laser Injection Phase (Degrees) Fig 9 The rf contribution to the normalized emittance, determined by the intercept at zero charge and compared to PARMELA simulations, as a function of the injection phase charge), as derived from the experimental data, and compare it to PARMELA simulations The results of this comparison are shown in Fig 1 The agreement is quite good, considering the uncertainty introduced by the damage to the cathode To explore the effect of the variation of the peak electric field the rf power into the photoinjector was varied, as is shown in Fig 11, which displays the normalized emittance as a function of the final beam energy, which is roughly proportional to the peak accelerating field in the cavity We obtain the expected decreasing of the emittance with rf field, as predicted by Kim's theory [3] More details and analysis of the emittance measurements are presently available in ref [4] Additional data and a more complete discussion will be presented in a subsequent paper 7 Conclusions Fig 1 The normalized emittance predicted by linear extrap olation of the data to l nc, compared to PARMELA simulations, as a function of the injection phase 2 16 Laser Injection Phase (Degrees) s Beam Energy (MeV) Fig 11 The normalized emittance at constant charge and injection phase, as a function of final energy (rf field amplitude) The UCLA photoinjector has been operated with performance as expected, considering deviations from the ideal design conditions that prevailed Our diagnostics have been commissioned successfully, and our measurements have been compared with good agreement to PARMELA modeling and theoretical calculations These tools will allow us to move forward with the commissioning of the full compact linac system, which is to be used in our experimental program The full commissioning of our system includes the building of a dedicated radiation shielding bunker to deal with the neutron flux generated by electron beams higher in energy than 1 MeV This beam will be obtained by injecting the beam from the rf gun into a plane-wave transformer (PWT) linac [9] In addition, we will install a more compact solenoid, which, given an appropriate choice of linac placement and accelerating gradient, will allow us to obtain compensation of the space charge derived emittance [1] The beam line will addditionally be instrumented non-destructive beam position monitors and a high energy emittance measurement system With our improved understanding of these beams produced from this photoinjector system, and the upgrading of the beam line design, we are confident that we will obtain beam that is of sufficient brightness to drive a SASE FEL amplifier at 1 win wavelength This is important to future projects based on SASE, such as the SLAC LCLS [I I], as we can explore the effects of fluctuations, superradiance, and slippage [12] in our longer wavelength device In addition, many future short wavelength FELs will be driven by photoinjector systems similar to ours ; it is important that this technology be proven viable in planning for the next stage in FEL development

7 J Rosenzweig et al /Nucl Instr and Meth in Phys Res A 341 (1994) References [1] SC Hartman et al, Proc 1991 Particle Accelerator Conf (IEEE, 1991) p 2967 [2] C Pellegrini et al, Initial operation and beam characteristics of the UCLA S-band rf photo-injector, m: Proc Particle Accelerator Conf, Washington, DC, 1993, to be published [3] K -J Kim, Nucl Instr and Meth A 275 (1989) 21 [4] S C Hartman et al, Emittance measurements of the 4 5 MeV UCLA rf photoinjector, in ref [2] [5] C Lejeune and J Aubert (eds ), Emittance and Brightness Definitions and Measurements (Academic Press, New York, 198) [6] G Hairapetian et al, Streak camera measurements of electron bunch length from a copper photocathode m an rf gun, in ref [2] [7] S Hartman, PhD thesis, University of California, Los Angeles (1993) [81 P Davis et al, Quantum efficiency measurements of a copper photocathode in an rf electron gun, m ref [2] [91 D Swenson, Proc Europ Particle Accelerator Conf, ed S Tazzari (1988) p 1418 [11 B E Carlsten, Nucl Instr and Meth A 285 (1989) 313 [111 C Pellegrmi et al, Nucl Instr and Meth A 331 (1993) 223 [121 R Bonifacio et al, Riv Nuovo Cimento 13 (9) (199) VII ACCELERATORS