Laser Photocathode RF Research Articles

Development of a coherent THz radiation source based on the ultra-short electron beam and its applications

At the National Institute of Advanced Industrial Science and Technology (AIST), a coherent terahertz (THz) radiation source has been developed based on an ultra-short electron beam using an S-band compact electron linac. The designed THz pulse has a high peak power of more than 1 kW in the frequency range 0.1–2 THz. The entire system is located in one research room of about 10 m square. The linac consists of a laser photocathode rf gun (BNL type) with a Cs 2Te photocathode load-lock system and two 1.5-m-long S-band accelerator tubes. The electron beam can be accelerated up to approximately 42 MeV. The electron bunch was compressed to less than 1 ps (rms) with a magnetic bunch compressor. The coherent synchrotron radiation (CSR) of the THz region was generated from the ultra-short electron bunch at the 90° bending magnet, and it was extracted from a z-cut quartz window for THz applications. In this work, the THz scanning transmission imaging was successfully demonstrated for measuring the freshness of a vegetable leaf over a period of time.

Ultra-fast pulse radiolysis system combined with a laser photocathode RF gun and a femtosecond laser

In order to study the early events in radiation physics and chemistry, two kinds of new pulse radiolysis systems with higher time resolution based on pump-and-probe method have been developed at the Nuclear Engineering Research Laboratory, the University of Tokyo. The first one, a few picosecond (2 ps at FWHM) electron beam (pump) from an 18 MeV S-band Linac using a laser photocathode RF gun (BNL/KEK/SHI type: GUN IV) was operated with a femtosecond laser pulse (100 fs at FWHM), which also acted as the analyzing light (probe). The synchronization precision between the pump and the probe was 1.7 ps (rms). In a 1.0 cm sample cell, a time resolution of 12 ps was achieved. The second one, a picosecond (4 ps at FWHM) electron pulse from a 35 MeV S-band Linac employing a conventional thermionic gun with a sub-harmonic buncher, was synchronized with the femtosecond laser pulse, with a synchronization jitter of 2.8 ps (rms). A time resolution of 22 ps was obtained with 2 cm cell. This makes it possible to do the pulse radiolysis experiments in the time range from picosecond to sub-microsecond.

High-quality beam generation using an RF gun and a 150 MeV microtron

Low-emittance sub-picosecond electron pulses are expected to be used in a wide field, such as free electron laser, laser acceleration, femtosecond X-ray generation by Inverse Compton scattering, pulse radiolysis, etc. In order to produce the low-emittance sub-picosecond electron pulse, we are developing a compact Racetrack Microtron (RTM) with a new 5 MeV injection system adopting a laser photo cathode RF gun (Washio et al., Seventh China–Japan Bilateral Symposium on Radiation Chemistry, October 28, Cengdu, China, 1996). The operation of RTM has been kept under a steady state of beam loading for long pulse mode so far (Washio et al., J. Surf. Sci. Soc. Jpn. 19 (2) (1998) 23). In earlier work (Washio et al., PAC99, March 31, New York, USA, 1999), we have succeeded in the numerical simulation for the case of single short pulse acceleration. Finally, the modified RTM was demonstrated as a useful accelerator for a picosecond electron pulse generation under a transient state of beam loading. In the simulation, a picosecond electron pulse was accelerated to 149.6 MeV in RTM for the injection of 5 MeV electron bunch with a pulse length of 10 ps (FWHM), a charge of 1 nC per pulse, and an emittance of 3 πmm mrad.

フェムト秒電子シングルバンチの生成・計測・利用

A new S-band laser photocathode RF gun and chicane-type magnetic pulse compressor were installed in the twin linac system of Nuclear Engineering Research Laboratory, University of Tokyo. Low emittance (6πmm. mrad) femtosecond (240-290fs at FWHM) electron single bunch of 16MeV, 2% energy spread and 350pC/bunch was obtained. Femtosecond streak camera (200fs resolution) with dispersionless reflective optics, coherent transition radiation interferometer and far-infrared polychromator were used. We conclude that the streak camera and polychromator are the best for longer than 200fs (FWHM) bunch and for shorter, respectively. Detailed performances of the RF gun including the linac-laser synchronization and the reduction of dark currents were precisely evaluated. Further, we tried a new femtosecond quantum beam based pump-and-probe analysis, which is the time-resolved X-ray diffraction to visualize atomic motions. X-ray diffraction patterns of CuKα 1.2 from several monocrystal such as GaAs, KCl, CaF 2 were obtained using 10ps X-ray pulses. The visualization is under way. Finally, we introduce the new femtosecond ultrafast quantum phenomena research facility.

Experimental verification of laser photocathode RF gun as an injector for a laser plasma accelerator

The feasibility of the laser photocathode RF gun, BNL/GUN-IV, as an injector for a laser plasma accelerator was investigated at the subpicosecond S-band twin linac system of the Nuclear Engineering Research Laboratory, University of Tokyo. Electron beam energy of 16 MeV, emittance of 6/spl pi/ mm mrad, bunch length of 240 fs (FWHM), and charge per bunch of 350 pC were confirmed at 10 Hz. As for diagnosis of the femtosecond electron bunch, the quantitative comparison of performance of the femtosecond streak camera, the coherent transition radiation (CTR) Michelson interferometer, and the far-infrared polychromator was carried out. We concluded that the streak camera is the most reliable up to 200 fs and that the polychromator is the best for the shorter electron bunch. The 3.5-ps (rms) resolved synchronization between the YLF laser driver for the gun and the electron bunch was achieved. Based on the above experiences, we have designed and installed a much better laser-electron synchronization system using the Kerr-lens mode-locked Ti:Sapphire laser with the min harmonics synchrolocker and the stable 15-MW klystron. The timing jitter is expected to be suppressed down to 320 fs (rms).

Femtosecond electron beam generation and measurement for laser synchrotron radiation

One of the S-band twin linacs (18L linac) of Nuclear Engineering Research Laboratory of University of Tokyo is modified in order to produce femtosecond electron single bunch for femtosecond X-ray generation via Thomson backward scattering, namely laser synchrotron radiation. Laser photocathode RF gun and chicane-type magnetic pulse compressor are installed at the S-band linac. 10 ps (FWHM) laser pulse generates 5 MeV, 10 ps (FWHM), 1 nC electron single bunch, which is accelerated up to 20 MeV in the S-band accelerating tube and compressed to 200 fs (FWHM) by the chicane. Design study has been performed by using the code of PARMELA and the installation has been finished. For precise and reliable measurement of the compressed pulse length, the comparison of measurement between the femtosecond streak camera and coherent transition radiation interferometry was carried out. Good agreement between them for 1–10 ps (FWHM) pulses was achieved. A new Michelson interferometer for the 200 fs pulse is now under construction.

Fabrication of a high-field short-period superconducting undulator

Abstract The visible free electron laser (FEL) experiment at the accelerator test facility (ATF) at Brookhaven National Laboratory is designed to utilize the high brightness beam from a laser-photocathode rf gun injected into a 50 MeV S-band linac in conjunction with a 8.8 mm period undulator. The undulator is designed to provide a 0.5 T field on axis which is reached at a current 15% below the quench current. It consists of 3 contiguous sections. We discuss the field errors by evaluating the trajectory wander and the optical phase error. Whereas undulator sections with low errors can be built as machined without any shimming or trimming, special care has to be taken during the assembly of a full length device.

Proposed UV-FEL user facility at BNL

The NSLS at Brookhaven National Laboratory is proposing the construction of a UV-FEL operating in the wavelength range from visible to 750 Å. Nanocoulomb electron pulses will be generated at a laser photocathode rf gun at a repetition rate of 10 kHz. The 6 ps pulses will be accelerated to 250 MeV in a superconducting linac. The FEL output will serve four stations with independent wavelength tuning, using two wigglers and two rotating mirror beam switches. Seed radiation for the FEL amplifiers will be provided by conventional tunable lasers, and the final frequency multiplication from the visible or near UV to the VUV will be carried out in the FEL itself. Each FEL will be comprised of an initial wiggler resonant to the seed wavelength, a dispersion section, and a second wiggler resonant to the output wavelength. The facility will provide pump-probe capability, FEL on FEL, and FEL on synchrotron light from an insertion device on the NSLS X-ray ring.

Proposed UV FEL user facility at BNL

The NSLS at Brookhaven National Laboratory is proposing the construction of an UV FEL operating in the wavelength range from visible to 1000 Å. Nanocoulomb electron pulses will be generated at a laser photocathode rf gun at a repetition rate of 10 kHz. The 6 ps pulses will be accelerated to 250 MeV in a superconducting linac. The FEL consists of an exponential growth section followed by a tapered section. The amplifier input is a harmonic of a tunable visible laser generated either by nonlinear optical material or the nonlinearity of the FEL itself. The FEL output in 10 −4 bandwidth is 1 mJ per pulse, resulting in an average power of 10 W. The availability of radiation with these characteristics would open up new opportunities in photochemistry, biology and nonlinear optics, as discussed in a recent workshop held at BNL.