EVALUATION OF THE BACK-BOMBARDMENT EFFECT IN THE ITC-RF GUN FOR T-ACTS PROJECT AT TOHOKU UNIVERSITY

Abstract An ITC (independently tunable cells) RF gun is currently used to produce sub-picosecond electron pulses as part of the injector for coherent terahertz radiation at Tohoku University. Experiments and simulations of particle tracing by GPT show that the back-bombardment effect on the LaB6 cathode's surface is serious and should be controlled carefully. To evaluate the temperature increase due to backbombardment a 2D model is created for heat transfer inside the cathode. In the 2D model, the back-streaming electrons are treated as external heat source as well as the cathode heater that heats the cathode from its side along with thermal radiation from its surface. The energy deposit of backbombardment inside the cathode is calculated by EGS5 or Geant4 by use of the information of back-streaming electrons derived from GPT simulation. In addition, we will also compare the simulating results with experimental data on the increase of emission current density of cathode due to back-bombardment

Light source based on a 100 mm-long monolithic undulator magnet with a very short 4 mm-period length

A novel method to fabricate undulator magnets of a-few-millimetre-periodlength is being explored. Plate-type magnets, 100 mm-long with 4 mm-periodlength, have been successfully fabricated. They produce an undulator field ofapproximately 3 kG at a gap of 1.6 mm. Prototype undulators based on thistechnology have been constructed. Field measurements and characterizationshow that the quality of the undulator field of these plate magnets is sufficientfor an undulator light source, and the calculated spectrum shows that thefundamental radiation emitted from this field is quite satisfactory. Testexperiments for light generation using a real electron beam have been carriedout at a test accelerator at the Research Center for Electron Photon Science(ELPH), Tohoku University, Japan, which is able to realize optics conditions toaccept a very short gap of 1.6 mm. First observation and characterization ofblue light was successfully accomplished

Properties of the Coherent Radiation Emitted from Photonic Crystal in the Millimeter Wave Region I(III. Accelerator, Synchrotron Radiation, and Instrumentation)

Using a short-bunched beam of electrons of a linear accelerator, sharp directionality of the coherent radiation emitted from a photonic crystal (PhC) was observed in the millimeter wave region. When the PhC was rotated around an axis perpendicular to the surface of the PhC, the radiation intensity decreased drastically. On the other hand, the dispersion relation of the radiation changed little with the rotation. The decrease of the intensity was conspicuous in the high frequency region, in comparison with that of the low frequency region

Study of Coherent Smith–Purcell Radiation in the Terahertz Region Using Ultra-Short Electron Bunches

Smith–Purcell radiation (SPR) can be generated nondestructively, providing valuable applications in light sources and beam monitors. Coherent SPR is expected to enable single-shot measurements of very short bunch lengths on the fs scale. Since the reconstruction of the longitudinal bunch shape from the coherent SPR is based on the reliable SPR spectrum, a more detailed understanding of the properties of the radiation is important in this context. Employing a 100 fs ultrashort electron bunch at the t-ACTS test accelerator, the spectrum, angular distribution, and polarization of the produced coherent SPR were measured in the terahertz frequency region and compared with a model calculation. In addition to the widely known surface current model evaluation, the effect of the geometrical shading effect on induced currents on metal surfaces was evaluated using 3D numerical calculations. The obtained SPR characteristics are also presented. In the evaluation of the grating with a shallow blaze angle, it was found that the shading effect has a non-negligible effect on the generated SPR intensity; the measured angular distribution and polarization results were in good agreement with this result

Amplitude and Phase Control of RF Pulse Using IQ Modulator to Improve Electron Beam Quality

A test-Accelerator as Coherent Terahertz Source (t-ACTS) has been under development at Tohoku University, in which an intense coherent terahertz radiation is generated from the short electron bunches. Velocity bunching scheme in a traveling wave accelerating structure is employed to generate the short electron bunches. The in-phase and quadrature (IQ) modulator and demodulator were installed to the low-level RF systems of t-ACTS linac to control and measure the amplitude and phase of RF power. The amplitude and phase of the RF power applied to an RF electron gun cavities and the accelerating structure are controlled to produce the electron bunches with a uniform and small momentum spread suitable for the velocity bunching. By installing the feed-forward control system using IQ modulators for the beam conditioning, we have successfully generated flat RF pulses and improved beam quality, including the energy spectrum of the beam. The details of feed-forward control system of the amplitude and phase using the IQ modulator and the beam experiments are presented in this paper