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Initial Operation and Beam Characteristics of the UCLAS-band RF Photo-Injector*C. Pellegrini, N. Barov. S. C. Hartman, S. Park,J. Rosenzweig, G. Travish, R. ZhangDepartmentof Physics, University of California, Los Angeles 90024

P. Davis, C. Joshi, G. HairapetianElectrical Engineering Depcartment, niversity of California, Los Angeles 90024-aday Cup

Figure 1.AbstractThe UCLA rf photo-injector system has beencommissioned(l). All of the sub-components such as thehigh power rf, pica-second laser, rf photo-injector cavity,diagnostics, and supporting hardwarehave been testedand are

operational. W e briefly discuss he performanceof the variouscomponents since the details of each subsystem are verylengthy. The laser delivers a sub 4 ps pulse containing O-3OOpJof energy per pulse. The photo-injector produces O-3nC per bunch with an rf induced emittance of 1.5 7c(mm-mrad).I. INTRODUCTIONWe report the initial results of the operation of the UCLA 4.5MeV PhotocathodeRF gun. This electron source s part of a20 MeV compact electron linac described before(1). It will beused for studies of the interaction of relativistic beams,plasmas and the generation of coherent radiation. All the

components of this system have been built and tested. Fullassembly will be completed during the fall of 1993. Ourinitial work has been dedicated to a characterization of thephotocathode rf gun, which is the electron source for thesystem. As p,art of this work we have measured he electronbeamemittanceand the quantum efficiency of a coppercathodeunder different conditions. Detailed descriptions of the resultsof thesemeasurements nd the techniques used are eported nother papers presented at this conference. We will limit this*Work Supprted by SDIO/IST through ONR Grant No. NOOO14-90-J-1952 and US DOE Grant DE-FG03-92ER-40493

paper to a summnrizntion of the overall performance of thegun and our plans for its future development. Thephotocathode f gun has also been used o study a thin plasmalens(2). This experiment demonstratedelectron beam ocusingandconfiied theoretical expectations.II. ACCELERATOR SYSTEM DESCRIPTIONA. RF SystemThe gun is powered by a SLAC XK5 type klystronproducing 24 MWatts of rf power with a pulse duration of 4vs. The rf system s driven by a signal produced by a masteroscillator clock at a frequency of 38 .08 MHz. This signal ismultiplied 75 times to produce the klystron operatingfrequency of 2.856 GHz and is then sent to a 1 kwatt solidstate amplifier. The amplifier signal in turn feeds theklystron. This ensures the timing and a feed back loopstabilizes the laser pulse to rf jitter to less than 4 ps.

B. RF Photo-InjectorThe photocathode rf gun is based on the Brookhavendesign(3). It consists of a one and a half cell standing waveaccelerator producing a beam with an energy up to 4.5 MeV.Producing accelerating gradients of up to 100 MV/m areachievedC. Laser SystemThe drive laser is a mode locked Nd-YAG oscillatorcavity. To compress the pulse the laser is matched into aSOOmiber to produce a frequency chirp. The chirped pulse isthen amplified a million times by a regenerative amplifier and

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8003.si 600S3.g 400285 200

0

10 mm dia.FWFM spot size532nm laser pulse

measure the energy and energy spreadof the beam.IT. QUANTUM EFFICIENCY

A. Quantum Efficiency ResultsThe quantum efficiency measurementsare described indetail in (4). The quantum efficiency has been measuredas afunction of input laser energy, illumination angle, andpolarization. In the first series of measurements he cathodewas initially damaged by over focusing the laser to a smallspot size. This produced ocal melting and damaged he coppercathode surface. The damage limited the possibility ofproducing a beam with the design spot size of about 3 mm,since most electrons where produced by the damagedarea.Theresulting effective spot size was about 0.3 mm. This smallspot size led to huge spacecharge effects at small charge, so inthis series of measurements space charge was always adominant effect.

0 4 8 12 16 20Time (ps) Angle of Poharization QuantumFigure 2. Incidence,Degrees Efficiency

2 S 9x10-5sent to a grating pair where it is compressed. Once it leaves 2 P 9x10-5the compressionstage he laser beam s frequency upconverted 70 Sto green by a KD*P crystal. Then it is doubled again to UV, 9x10-5.266 nm, by a second frequency doubling crystal. The up 70 P 1.3x10-4conversion efficiency is typically 10%. The laser was Table 1.measuredwith a streakcameraand is shown in figure 1. The main results for the damaged cathode quantumefficiency aregiven in Tnblc 1. One can see hat the quantumD. Diagnostics efficiency depends on the incidence angle and the lightThe electron beam diagnostics are as follows. The main polarization. The largest quantum efficiency, 10s4, s obtaineddiagnostics are the phosphor screens which monitor the spot for 70 degrees incident angle and P polarized light. Thesize. An Integrating Current Transformer, ICT, is used to dependence f the quantum efficiency on the poharizationan lemeasure he electron beam charge along with faraday cups at I$ is shown in F ig. 3 and can be fitted with a cosL!4various locations. Some of the phosphor screensare floated dependencewhich implies single photon emission.so that they can double as faraday cups. A dipole is used to

2.12

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3 mm dia. FWFM spot size266nm laser pulse 9 -r

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b 4k 9-O 155 doPolarization Angle $ (deg)

III. ENERGY and ENERGY SPREADDark Current Peak Momentum Fluctuations

0.150.1

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-02-O.bO 5.00 10.00 15.00 20.00 25.00Figure 3. Figure 4.

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of the laser pulse and the fluctuations in the rfof deep concern for future FEL experiments. Inwe show the fluctuations in the dark current energy endhowing the level of the rf system fluctuations. In Fig. we show the jitter in peak momentum of the photoelectrons,includes also the layer pulse jitter. One can see hat theting rms momentum fluctuation is 0.25% and is dueresidual laser pulse itter. The dependence f theon the initial rf phase is shown elsewhere(5).shows that the accelerating field in the twowith a larger field in the full cell.this unbalance the data can be fitted using thetheory of Kim . From the beam energywe can also determine the gun shunt impedanceis evaluated to be 36 MRIm, smaller than the valuefrom the Superfish calculations. The beam energymeasured at low current, SO PC, is less than 0.2%.due to the lnrge space charge effects it increaseswith charge, when using the damaged athode.IV. EMIITANCE

The emittance has been measured(5)using a pepper pot.

rad rms . At larger charge the emittance is dominatedharge ffects, n the caseof the damaged aihodeandgun. These results as well as the results for theare compatible with the assumptionof an initialsize of 0.3mm radius, rms. The rf photoinjector iselectron source, capable of producingwith time structures determined by the incident laserMetal cathodes are very robust but put strongImportance of reducing amplitude fluctuations andter is also noted. The potoinjector produces a beam ofas exemplified by the successful plasma lens

6es2Tki.S2B.Y-ziEz2 Charge (nC)

Shot Number (in order of ascending Q)

Figure 5.experimentscompleted at UCLA.

V CONCLUSIONThe UCLA photo-injector has been operatedsuccessfully.The measurements how that the emittance scalesas expected.The important thing to note is that for these set of emittanceruns the rf photo-injector had a field imbalance. The filled inthe full cell was 1.8 times that in the half ceil. Thiscontributed to emittance blowup as did the phosphor screensused to measure he emittance. The phosphor screens wereplaced onto the beamline at a 45 angle. This created

broadening of the line widthsReferences1. S. C. Hartman et al., Photocathode Driven Linac atUCLA for FEL and Plasma Wakefield AccelerationExperiments, Particle Accelerator Conference Sanfransisco,CA., 1991), pp. 2967.2. G. Haimpetian et al, Experimental Demonstration ofPlasma Lens Fucusing, Particle Accelerator ConferenceWashington, DC., 1993),3. K. Batchelor et al, European Particle ConferenceAccelerator ConferenceRome, Italy, June 7-12, 1988),4. P. Davis et al, Quantum Efficiency Measurementsofa Copper Photocathode in an rf Electron Gun, ParticleAccelerator ConferenceWashington, DC., 1993),5. S. C. Hartman et al., Emittance Measurementsof the4.5 MeV UCLA rf Photoinjector, Particle AcceleratorConferenceWashinton, DC., 1993),

Figure 6.

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