• Y.A. Kot
  DESY, Hamburg

The overall bunch compression in FLASH is limited on the one hand by the rf tolerances and on the other hand by linearization of the particle distribution in the longitudinal phase space. While the last one has been significantly improved after the installation of the third harmonic system during the upgrade 2009-2010, the constraint given by rf tolerances cannot be mitigated significantly. To avoid this limitation one has to operate with shorter bunches already at the injector. Since the bunch length is dominated there by the longitudinal space charge one has to go to lower bunch charges. Working points for the operation of the FLASH injector with 20-500pC bunches have been found by means of the optimization procedure based on ASTRA code. The expected bunch parameters are reported in this paper and compared with the experimental results. Further the discussion on advantages and drawbacks of the injector operation in low charge regime is given.

• T. Denis, K.-J. Boller, P.J.M. van der Slot
  Mesa+, Enschede

Compact, slow-wave, low energy electron beam radiation sources, like Cerenkov free-electron lasers (FELs), emit high power microwaves. However, they seriously degrade in output power, when scaled towards the THz range (0.1-10 THz). This prevents industry from applying THz radiation, although it would allow many new applications, like chemical selective security surveillance. The photonic free-electron laser (pFEL) is a promising concept for a handheld, tunable and Watt-level THz laser. In a pFEL several electron beams stream through a photonic crystal (PhC) leading to the emission of coherent Cerenkov radiation. The beams emit phase-locked due to the transverse scattering inside the PhC, which allows increasing the output power by increasing the number of beams streaming through the PhC. Therefore, scaling the pFEL’s operating frequency towards THz frequencies can be done without loss in output power. Furthermore, compact, low energy electron sources (< 15 keV) can drive the laser, due to the strong deceleration of the light by PhC’s. As a proof of principle, we developed the setup for a pFEL operating at 20 GHz to study the interaction between a single electron beam and the PhC.

• T. Kii, M. A. Bakr, Y.W. Choi, K. Ishida, N. Kimura, R. Kinjo, K. Masuda, H. Ohgaki, T. Sonobe, M. Takasaki, S. Ueda, K. Yoshida
  Kyoto IAE, Kyoto

The bulk high temperature superconductor staggered array undulator (Bulk HTSC SAU) has potential to generate strong periodic magnetic field in short period and to control K value without a mechanical gap control structure.* However, availability of the bulk HTSC magnets having matched performance of critical current density is a problem to be solved. In this study, we have numerically and experimentally estimated influence of variation of critical density upon field error. It was numerically found that the field error was naturally compressed, because the difference in critical current density was compensated by natural variation of the region where the supercurrent flows. In the conference, the experimental results of the field error compression and principle of the compression will be discussed.

* R. Kinjo, et al., “BULK HIGH-TC SUPERCONDUCTOR STAGGERED ARRAY UNDULATOR”, Proceedings of FEL 2008, 473 (2009).

• R. Kinjo, M. A. Bakr, Y.W. Choi, K. Ishida, T. Kii, N. Kimura, K. Masuda, K. Nagasaki, H. Ohgaki, T. Sonobe, M. Takasaki, S. Ueda, K. Yoshida
  Kyoto IAE, Kyoto

The bulk high temperature superconductor staggered array undulator (Bulk HTSC SAU) has several advantages: such as strong magnetic field, potential of short period undulator, K value variability without gap control. In addition to these advantages, the Bulk HTSC SAU can be used near the electron beam because the undulator is expected to show good performance at 20 – 30 K. In the conference, we will report the expected performance of the undulator at low temperature through magnetic measurement by using a superconducting quantum interference device (SQUID) magnetometer. Also we will report the results of the first operation at 4 – 77 K of new prototype undulator consisting of a helium cooling system and a 2 T superconducting solenoid.