Development of a High Brightness Photoinjector for Light Source by yettfz

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									                      Development of a High Brightness
Facility Highlights




                      Photoinjector for Light Source Research


                              Laser driven photo-cathode rf gun is the brightest electron source to date. It is a flexible system such that its
                              beam properties and time structure are controllable by the laser. To build up the expertise of advanced linear
                              accelerator technology, a 150 MeV high brightness injector system equipped with such electron gun is being
                              installed at NSRRC for light source research. The main part of the photo-cathode rf gun is a 1.6-cell -mode cavity
                              structure with dimensions optimized for operation at 2998 MHz. Copper cathode at the end-wall of the shorter
                              cell is illuminated by a 266 nm pico-second UV laser, intense photo-electrons emitted from the cathode surface
                              are and abruptly accelerated to relativistic energy by the high gradient accelerating rf field in the cavity. Since
                              emittance degradation effect due to space charge force is overcome by strong acceleration field near the cathode
                              surface, intense low emittance relativistic electron beam can be produced. Emittance compensation and bunch
                              compression techniques will also be developed for higher beam brightness. The possibilities of using such injector
                              system for ultrafast X-ray and single pass high gain free electron laser research are being investigated.”


                                                                                            Free electron lasers (FEL) attract much attention in the
                                                                                      light source community with their capability to produce
                                                                                      high power coherent radiations at short wavelengths.
                                                                                      Without using optical cavity, single-pass high gain FEL
                                                                                      suggests a feasible path towards intense coherent UV
                                                                                      radiation or even X-ray. One Angstrom SASE X-ray FEL
                                                                                      facilities under construction worldwide use 8-20 GeV rf
                                                                                      linacs as drive accelerators and are aiming at photon beam
                                                                                      peak brilliance that is billion times larger than conventional
                                                                                      third generation machine. These high energy driver linac
                                                                                      systems have lengths ranging from 400 m to 2 km. It is well
                                                                                      known that radiation wavelength of an FEL is proportional
                                                                                      to its undulator period and inversely proportional to the
                                                                                      square of beam Lorentz factor. Lower energy linac can in
                                                                                      principle be used if shorter period undulator is considered.
                                                                                      In order to reduce construction cost, micro-undulator X-ray
                                                                                      FEL facilities driven by a lower energy (e.g. few GeV) room
                                                                                      temperature linac are perhaps the future trend for next
                                                                                      generation light source. However, beam emittance increases
                                                                                      linearly as beam energy reduces. It is important to have
                                                                  Beamline            electron beam with emittance lower than or comparable
                                                                                      to the radiation wavelengths for a reasonable gain SASE
                                                              Not applicable          FEL length.

                                                                                           Unlike circular machines, beam quality of linacs de-
                                                                    Authors           pends very much on the brightness of their electron
                                                                                      sources. Various high brightness electron sources are being
                        W. K. Lau, Y.C. Huang, J.Y. Hwang, A.P. Lee, G.H. Luo,        developed for different demanding applications. Laser
                                                              and D.J. Wang           driven photo-cathode rf gun is still the brightest electron
                            National Synchrotron Radiation Research Center,           source so far. One attractive feature of such device is that
                                                             Hsinchu, Taiwan          its beam properties and time structure are controllable by



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                                                                                                                                       Facility Highlights
the laser. The main part of a photo-cathode rf gun is the
gun cavity with dimensions optimized for low emittance
operation. When the copper cathode at the end-wall of the
shorter cell is illuminated by a high energy pico-second UV
laser, intense photo-electrons escaped from the cathode
surface and are accelerated to relativistic energy by the high
gradient accelerating rf field in the gun cavity such that
low emittance but intense relativistic electron beam can be
produced because emittance degradation effect due space
charge force can be minimized. Emittance compensation
solenoid and laser pulse shaping techniques are often used
to improve beam brightness of the electron gun.                        Fig. 2: 3D-drawing of the photo-cathode rf gun test stand.

     To build up the expertise for advanced linear acc-                Mira 900-F Ti:Saphire laser. The laser system is synchronized
elerator technology in NSRRC, a 150 MeV high brightness                to the phase of the 2998 MHz microwave system with rms
photoinjector system equipped with a laser driven rf                   jitters as low as 250 fsec. The UV output power of the system
gun is being constructed. This system will serve also as a             is adjustable by an IR optical attenuator (a half-wave plate
testing ground for high brightness accelerator and novel               plus a polarizer) before the THG. Output pulse duration of
light sources in the future. In such injector system, a low            the laser system will be adjusted by a UV stretcher after
emittance 6 MeV beam is generated from the rf gun                      the THG. To avoid emittance degradation due to nonlinear
system (equipped with emittance compensation solenoid)                 space charge forces, transverse profile of laser beam is
and accelerated to the desired energy by the travelling-               flattened by a Newport UV beam shaper with deviation
wave rf linac structures. The rf gun cavity is a 1.6-cell              from flattop at less than 15%. Flattoping the laser pulse
  -mode standing wave structure operating at 140 MV/m                  for further reduction of beam emittance is under study.
accelerating gradient. Our first goal is to produce 1 nC, ~10           Two 35 MW, 2998 MHz klystron systems will be built as
psec electron bunches at 1 mm-mrad normalized transverse               the microwave sources for the rf gun and linac sections.
emittance. A UV drive laser system at 1 to 15 pico-second              In the first phase of the project, the photo-cathode rf gun
pulse duration will be setup to drive the Cu-cathode. In the           system will be setup in the gun test site as shown in Fig. 2.
laser system (Fig. 1), a 3 mJ, 798 nm Coherent Legend-Elite            Beam properties such as beam energy, bunch charge and
amplifier is employed to pump a third harmonic generator                transverse beam emittance will then be measured.
(THG) crystal for generation of the 300 J, 266 nm UV light.
For high stability and reliability, EVOLUTION-30 will be used              We can expect the installation of this 150 MeV high
to pump the amplifier which is seeded by a 0.65 W Coherent              brightness photoinjector system will be completed by
                                                                                          the end of 2009 when proper test
                                                                                          site is available. Bunch compression
                                                                                          techniques to improve beam
                                                                                          brightness and the possibilities of
                                                                                          using this high brightness photo-
                                                                                          injector for Thomson back scattering
                                                                                          and single pass high gain FEL research
                                                                                          are being investigated recently. In
                                                                                          the meantime, a collaborative effort
                                                                                          between NSRRC and the relativistic
                                                                                          photonic laboratory at National Tsing
                                                                                          Hua University (NTHU) has been
                                                                                          developed for THz radiation source
                                                                                          R&D, a 2856 MHz photo-cathode
                                                                                          r f gun and a SSRL-t ype 1.5-cell
                                                                                          thermionic cathode rf gun are being
                                                                                          fabricated in house at NSRRC and
                                                                                          delivered to the NTHU for a laser pre-
                                                                                          bunched THz FEL experiment. First
Fig. 1: The 300 J, 266nm UV laser system for the NSRRC high brightness injector.



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                                                                                                                                Activity Report
                                                                                                                                           2007/2008
Facility Highlights




                      operation of a recently built thermionic cathode rf gun has          Experimental Station
                      been tested at high rf power up to ~100 MV/m accelerating
                      gradient (Fig. 3).                                                   Not applicable



                      Experimental Station
                                                                                           Publications

                                                                                            W. K. Lau, S. S. Chang, S. Y. Hsu, C. S. Hwang, J. Y. Hwang, Y.
                                                                                            C. Wang, and T. T. Yang, International Journal of Modern
                                                                                            Physics B 21, Nos. 3-4, 422 (2007).
                                                                                            A. P. Lee, S. S. Chang, J. Y. Hwang, W. K. Lau, C. C. Liang, G.
                                                                                            H. Luo, and T. T. Yang, Proceedings of the 2007 Particle
                                                                                            Accelerator Conference, 1088 (2007).
                                                                                            W. K. Lau, B. J. Chan, L. H. Chang, C. W. Chen, H. Y. Chen,
                                                                                            S. Y. Hsu, K. T. Hsu, J. Y. Hwang, Y. C. Wang, and T. T. Yang,
                                                                                            Proceedings of the 2005 Particle Accelerator Conference,
                                                                                            1748 (2005).
                                                                                            Y. C. Wang, S. Y. Hsu, C. S. Hwang, J. Y. Hwang, W. K. Lau, F.
                                                                                            Y. Lin, T. . Yang, and S. Rimjaem, International Journal of
                                                                                            Modern Physics B 21, Nos. 3-4, 430 (2007).



                                                                                           Contact E-mail

                                                                                           wklau@nsrrc.org.tw



                      Fig. 3: The 2856 MHz thermionic cathode rf gun fabricated for
                              NTHU.




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