At the Fast Neutron Research Facility (FNRF), Chiang Mai University (Thailand), the SURIYA project has been established aiming to produce femtosecond electron pulses utilizing a combination of an S-band thermionic rf gun and a magnetic bunch compressor ({alpha}-magnet). A specially designed rf-gun has been constructed to obtain optimum beam characteristics for the best bunch compression. Simulation results show that bunch lengths as short as about 50 fs rms can be expected at the experimental station. The electron bunch lengths will be determined using autocorrelation of coherent transition radiation (TR) through a Michelson interferometer. The paper discusses beam dynamics studies, design, fabrication and cold tests of the rf-gun as well as presents the project current status and forth-coming experiments

/SLAC, SSRL

Chumphongphan, S.

Jinamoon, V.

Kusoljariyakul, K.

Rhodes, M.W.

Rimjaem, S.

Saisut, J.

Thongbai, C.

U., /Chiang Mai

U., /Mae Fah Luang

Vilaithong, T.

Wichaisirimongkol, P.

Wiedemann, H.

UNT Digital Library

Work supported in part by Department of Energy contract DE-AC02-76SF00515.GENERATION OF FEMTOSECOND ELECTRON PULSES S. Rimjaem*, V. Jinamoon, K. Kusoljariyakul, J. Saisut, C. Thongbai, T. Vilaithong,  FNRF, Chiang Mai University, Chiang Mai, Thailand M.W. Rhodes, P. Wichaisirimongkol, IST, Chiang Mai University, Chiang Mai, Thailand  S. Chumphongphan, School of Science, Mae Fah Luang University, Chiang Rai, Thailand H. Wiedemann, SLAC/SSRL, Menlo Park, California, USA Abstract At the Fast Neutron Research Facility (FNRF), Chiang Mai University (Thailand), the SURIYA project has been established aiming to produce femtosecond electron pulses utilizing a combination of an S-band thermionic rf-gun and a magnetic bunch compressor (α-magnet). A specially designed rf-gun has been constructed to obtain optimum beam characteristics for the best bunch compression. Simulation results show that bunch lengths as short as about 50 fs rms can be expected at the experimental station. The electron bunch lengths will be determined using autocorrelation of coherent transition radiation (TR) through a Michelson interferometer. The paper discusses beam dynamics studies, design, fabrication and cold tests of the rf-gun as well as presents the project current status and forth-coming experiments. INTRODUCTION Femtosecond electron and photon pulses have become interesting tools for basic and applied applications, especially in time-resolved experiments [1,2,3]. Such short electron pulses can be used to generate intense coherent far-infrared (FIR) radiation, ultrashort X-ray pulses, and free electron laser (FEL). Although, photocathode rf-gun is a promising source to generate a high brightness ultra short electron pulses [4], it involves a complex laser injector and requires high financial cost. On the contrary, a thermionic cathode rf-gun with suitable compression system can play similar role to produce intense electron beam with pulses duration in order of femtosecond [5]. At SURIYA, femtosecond electron bunches will be produced from a combination of an S-band rf-gun with thermionic cathode and a magnetic bunch compressor in the form of an α-magnet [6]. The schematic layout of SURIYA beamline is presented in Fig. 1.           Figure 1: Schematic layout of SURIYA set up at FNRF. The rf-gun generates 2-3 MeV electron beams, which are then traveling through the α-magnet for bunch compression. A 30 MeV linac is used to achieve higher energy as well as more confine beams. At experimental stations, coherent FIR radiation will be generated as coherent TR either for beam diagnostics or other experimental uses. RF-GUN AND RF SIMULATIONS  Fig. 2 is illustrated the SURIYA rf-gun cross section and 3D-view showing the thermionic cathode attached at the end wall of the half-cell (HC) and electron beam exits out at the end of the full-cell (FC). The field in the two cells is coupled through an external coupling cavity.          Figure 2: Rf-gun cross-section and 3D-view. Numerical simulations for rf characteristics of the SURIYA rf-gun were performed using the EM field solver code SUPERFISH [7]. Based on 2D-simulation, SUPERFISH cannot be used to investigate 3D-shape include coupling cavity, rf-input and vacuum pumping ports. This leads to a deviation of the simulated results from the measured values. SUPERFISH results, however, give fair ideas regarding to the rf-gun electrical properties as well as the helpful scaling law concerning the rf-gun dimension and its resonant frequency (frf).  During fabrication processes, modifications of the rf-gun dimension based on SUPERFISH scaling law were performed together with the rf-measurements.  BEAM DYNAMICS STUDY A particle-in-cell computer code PARMELA was used to simulate and investigate particle dynamics through the EM fields inside the rf-cavities [8]. In the studies the 6-mm diameter thermionic cathode is set to emit a 2.9 A uniform distribution electron beam which is represented by 100,000 macroparticles per 2856 MHz rf-period with space-charge effects.  Longitudinal Particle Dynamics Due to a sinusoidal time-varying field inside the cavities a very concentration of particles are accumulated experimental station30 MeV LINACrf-gunalpha magnetrf inputthermionic cathodefull cell electron beamhalf cellrf-i tthermioni  cathode-ce l l-cell el t_________________________________________  *neung@fnrf.science.cmu.ac.th  SLAC-PUB-11174Presented at the 9th European Particle Accelerator Conference (EPAC 2004), Lucerne, Switzerland, 5-9 Jul 2004. in the head (defining as the field becomes acceleration) of the bunch leading to a small correlated particle distribution between energy and time as shown in Fig. 3. Ten percent of high-energy particles concentrate at the head of the bunch appears in the length of some 10-20 ps.           Figure 3: particle distribution in energy-time phase space for a single S-band electron bunch at the rf-gun exit. The concave shape of the particle distribution at the rf-gun exit in Fig. 3 matches well to bunch compression in the α-magnet. It will rotate clockwise as the beam travels through the α-magnet with α loop paths of the length (S) depending on the particle momentum (cp) and the magnetic gradient (g) as S≈[cp/g]1/2. Thus, lower momentum particles follow shorter path than the higher one, leading to simply magnetic bunch compression. The value of the α-magnet field gradient is chosen to compensate velocity dispersion so that the shortest bunches are generated at the desired experimental station. A computer code Bcompress is used to simulate particles dynamics from the gun exit to the experimental station. Locations of the experimental station where the shortest bunch length can be achieved as well as other parameters can be determined using the code. Among those, the choice of the electric accelerating field employed in each cell of the rf-gun highly affects the bunch compression. The simulation results suggested that the field ratio between HC and FC should be 1:2 for optimum bunch compression [9]. The temporal particle distribution after compression and acceleration with respect to the initial momentum at the rf-gun exit is presented in Fig. 4. The expected bunch length, including transverse path length dispersion, at an optimum experimental location is 53 fs rms with a total charge of 94 pCb.            Figure 4: Particle distribution in energy-time phase space after bunch compression and transport to the experimental station. Transverse Particle Dynamics  Since bunch lengthening depends quadratically on beam divergence, reducing the beam divergence facilitates ultrashort bunch generation. For this reason, the SURIYA rf-gun was therefore designed with a flat cathode plate and optimised the iris radii to reduce the radial rf-field effects. PARMELA calculation including the finite effect of the thermal cathode emittance was performed to simulate the transverse beam dynamics. The simulation results reveal that a normalized emittance of 3.8 mm-mrad can be achieved at the rf-gun exit [9]. Some of the beam parameters from PARMELA and Bcompress simulations are listed in Table 1. Table 1: Some parameters for the optimised rf-gun. Parameters Value Max. beam momentum, cp (MeV) 2.91 Avg./max. field in half-cell (MV/m) 23.9/29.9 Avg./max. field in full-cell (MV/m) 45.5/67.6 Cathode emission current (A) 2.9 Charge/bunch (pCb) 94 Peak current (A) 707 Bunch length, rms (fs) 52.8 εn,rms at  rf-gun exit (mm-mrad) 3.8 FABRICATION AND COLD TESTS The HC and the FC of the rf-gun have been fabricated precisely, based on the SUPERFISH results, out of OFE copper cylindrical blocks, at the Thai-German Institute (Thailand). The cavities were first machined to resonate at a slightly different frequency from the target frequency (2856 MHz) to leave some rooms for fine modification. Preliminary cold tests of the rf-gun cavities before brazing have been preformed using Network Analyser at the Stanford Synchrotron Research Laboratory (SSRL/ SLAC), USA, and at the National Synchrotron Research Centre (NSRC), Thailand. Cold test results of the rf-cavities ready for brazing show that the frf of the rf-gun is very close to 2856 MHz at the operating temperature of 45o. The rf-gun was then brazed at the National Synchrotron Radiation Centre (SSRC), Taiwan, in an O2-free high temperature furnace.  The rf-cold tests of the rf-gun were performed again after brazing at SSRL to measure the frf of individual rf-cells and determine the coupling between the two cells by adjusting the tuning rod of the coupling cavity while measuring longitudinal field profile. The measurement is done by Bead-Pull technique, which is performed by introducing a 2.36 mm diameter dielectric sphere bead at each position in the rf-gun. This results in a resonant frequency shift that reflects directly the longitudinal electric field magnitude at that location. Fig. 5 illustrates the consistency between normalized axial longitudinal time (fs)0 60 120 180 240 300 360histogram (particles/fs)01000200030004000kinetic energy (MeV)1.92.02.12.22.32.42.5time (ps)0 20 40 60 80 100klinetic energy (MeV)0.0.51.01.52.02.5histogram (particle/ps)01000200030004000fields from SUPERFISH and the bead-pull measurement. The ratio of the maximum field amplitude in the FC to the HC from SUPERFISH calculation and Bead-Pull measurement is 2.186 and 2.067, respectively.           Figure 5: Longitudinal electric field distribution on axial axis from SUPERFISH and Bead-Pull measurement. The coupled-cell resonant frequency and Q-factor were measured at the Rajamangala Institute of Technology (RIT), Thailand. Table 2 shows a comparison of the predicted rf-parameters obtained from SUPERFISH to those measured values in the rf-cold tests. Table 2: Rf-parameters of the rf-gun calculated using SUPERFISH comparing to the cold test results. Parameter SUPERFISH Cold Tests frf HC/FC Coupled-cell 2881/2862 2862 2855/2858 2855 Q factor HC/ FC Coupled-cell 15334/12980 13037 - 12979 PRESENT STATUS OF SURIYA The optimised rf-gun, the α-magnet and the 30 MeV SLAC type linac including 5 MW S-band klystrons for the rf-gun and the linac as well as the beam transport line have been installed and the system is now being commissioned The picture of SURIYA system is illustrated in Fig. 6.                 Figure 6: Beamline set up at SURIYA project. At an experimental station, coherent TR radiated at wavelength longer than the bunch length will be generated by introducing the Al-foil at 45o with respect to the electron path. The backward TR emitted at 90o can be used for bunch length measurements or FIR spectroscopy. For the electron bunch as short as we expected, the bunch length measurement by autocorrelation of coherent TR should be done in vacuum to avoid pulse spacing in the ambient air [10]. An in-vacuum Michelson interferometer as shown in Fig. 7 is ready to be installed for the measurement.         Figure 7: In-vacuum Michelson interferometer set up. ACKNOWLEDGEMENTS Authors are grateful to M. Nalls at SSRL for cathode installation and testing. We thank SSRL, NSRC and RIT for the use of Vector network Analyzer as well as SSRC for brazing process. We would like to acknowledge the support of the Thailand Research Fund, the National Research Council of Thailand, the Thai Royal Golden Jubilee Scholarship Program, the US Department of Energy, the Hansen Experimental Physics laboratory (HEPL) of Stanford University, the Faculty of Science, and the Graduated School of Chiang Mai University. REFERENCES [1] H. Ihee et. al., “Direct Imaging of Transient Molecular Structures with Ultra Fast Diffraction,” Science, 291(2001)458. [2] M. Bauer et. al., “Direct Observation of Surface Chemistry Using Ultra Fast Soft-X-Ray Pulses,” PRL  87(2001)025501-1. [3] D.A. Reis et al., “Probing Impulsive Strain Propagation with X-Ray Pulses,” PRL 86(2001)3072. [4] R. Sheffield et. al., “High-Brightness Electron Injectors: A Review,” Proc. 1989 Part. Accel. Conf., IEEE Catalog No. 89CH2669-0(1989)1089.  [5] P. Kung et. al., “Generation and Measurement of 50-fs (rms) Electron Pulses,” PRL, 73(1994)967. [6] T. Vilaithong et. al., “SURIYA, a Source of Femto-second Electron and Photon Pulses,” Proc. APAC01, Beijing, China, September 2001, p 91. [7] L.M. Young, “POISSON/SUPERFISH,” LANL Technical Note LA-UR-96-1834, 1999. [8] L.M. Young, “PARMELA,” LANL Technical Note LA-UR-96-1835, July 2002. [9] S. Rimjaem et. al., “Femto-second Electron Bunches from an Rf-gun,” accepted for publish in NIMA. [10] C. Settakorn, “Generation and Use of Coherent Transition Radiation from Short Electron Bunches,” PhD Thesis, Stanford University, August 2001. z (mm)0 2 4 6 8 10E-field (relative amplitude)0.0.2.4.6.81.0 SUPERFISHBEA D-PULL

Generation of Femtosecond Electron Pulses

Generation of Femtosecond Electron Pulses

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