• T. Fukui, T. Kii, K. Kusukame, K. Masuda, Y. Nakai, H. Ohgaki, T. Yamazaki, K. Yoshikawa, H. Zen
  Kyoto IAE, Kyoto

We have constructed system for 3~20μmeter FEL oscillation for bio/chemical energy researches which consists of a thermionic RF gun and a 3-meter accelerating tube at the Institute of Advanced Energy, Kyoto University. [1] A 30 MeV electron beam has been successfully accelerated and studies on the beam characterization have also been carried out. [2] In parallel, evaluation of an FEL oscillation in KU-FEL has been performed. We will report our present expectations and future visions about undulators in KU-FEL. The former is the numerical studies on the expected FEL gain of the existing Halbach type undulator in KU-FEL based on experimental measurements of the undulator and beam parameters. The latter is the upgrade plan of the undulator and the design of a variable polarization undulator to obtain higher gain or circular polarized FEL. These undulator parameters are calculated by simulation code Radia and TDA3D.

[1] T. Kii, et al., Proceedings of the 2004 FEL conference, (2004) 447 [2] K. Masuda et al., Proceedings of the 2004 FEL conference, (2004) 450

• T. Kii, T. Fukui, K. Kusukame, K. Masuda, Y. Nakai, H. Ohgaki, T. Yamazaki, K. Yoshikawa, H. Zen
  Kyoto IAE, Kyoto

We have studied on improvement of electron beam macropulse properties from a thermionic RF gun. Though a thermionic RF gun has many salient features, there is a serious problem that back-bombardment effect worsens quality of the beam. To reduce beam energy degradation by this effect, we tried to feed non-flat RF power into the gun. As a result, we successfully obtained about 1.5 times longer macropulse and two times larger total charge per macropulse. On the other hand, we calculated transient evolution of RF power considering non-constant beam loading. The beam loading is evaluated from time evolution of cathode temperature, by use of one dimensional heat conduction model and electron trajectories' calculations by a particle simulation code. Then we found good agreement between the experimental and calculation results. Furthermore, with the same way, we studied the electron beam output dependence on the cathode radius.

• K. Kusukame, T. Fukui, T. Kii, K. Masuda, Y. Nakai, H. Ohgaki, T. Yamazaki, K. Yoshikawa, H. Zen
  Kyoto IAE, Kyoto

Funding: Kyoto University,Institute of Advanced Energy

Thermionic RF guns show advantageous features compared with photocathode ones such as easy operation and much higher repetition rate of micropulses, both of which are suitable for their application to high average power FELs. They however suffer from the back-bombardment effect [1], i.e., in conventional RF guns, electrons are extracted from cathode also in the latter half of accelerating phase and tend to back-stream to hit the cathode, and as a result the macropulse duration is limited down to severalμsec Against this adverse effect in thermionic RF guns, introduction of the triode structure has been proposed [2], where the accelerating phase and amplitude nearby the cathode can be controlled regardless of the phase of the first accelerating cell in the conventional RF gun. Our one-dimensional particle simulation results predict that the back-bombardment power can be reduced by 99 % only with 30-40 kW RF power fed to the grid in the present triode structure with an optimal phase difference from the RF induced in the main accelerating cavities. We also carried out two-dimensional particle simulations to evaluate emittance of the refined RF gun.

[1] T.Kii et al.,Nuclear Instruments and Methods in Physics Research A 507 (2003) 340-344. [2] E. Tanabe et al., Proc. of 27th Linear Accelerator Meeting in Japan, Aug 7-9, 2002, Kyoto, Japan (in Japanese).

• H. Zen, T. Fukui, T. Kii, K. Kusukame, K. Masuda, Y. Nakai, H. Ohgaki, T. Yamazaki, K. Yoshikawa
  Kyoto IAE, Kyoto

Transverse phase space tomography [1] using a quadrupole magnet and a beam profile monitor is very useful for emittance measurements especially for non-Gaussian beams, since this method directly gives transverse phase space distributions. We have tried to apply the method to measure the beam emittance of our FEL driver Linac [2]. We found, however, this method suffers from both the energy spread of the beam and the reconstruction noise which deeply depends on the reconstruction algorithm. To obtain reliable results, numerical evaluation using PARMELA which simulates the beam profile in each rotation angle has been carried out. Several image reconstruction method, such as FBP method, ART method, and Ordered Subsets - Expectation Maximization (OS-EM) algorithm [3], have been applied to reconstruct the phase space distribution. We also have introduced a noise cut procedure, and evaluation of a tolerable energy spread where this method can be applied.

[1] C.B. McKee, et al., NIM A 358 (1995) 264. [2] K. Masuda, et al., Proceedings of the 2004 FEL Conference 450. [3] H.M. Hudson and R.S. Larkin, IEEE Trans. Med. Imaging, 13:601 (1994).