• K. Nanbu, H. Hama, F. Hinode, M. Kawai, F. Miyahara, T. Muto, Y. Tanaka
  Tohoku University, School of Scinece, Sendai
• S. Kashiwagi
  Tohoku University, Research Center for Electron Photon Science, Sendai

A thermionic RF gun has been developed to generate very short electron bunch for a THz light source at Tohoku University. Bunch compression scheme requires, in general, linear momentum distribution of the particles with respect to the longitudinal position, so that measurement of longitudinal phase space is significant for better bunch compression. However, such measurement for the low energy electrons is difficult because space charge effect is so strong that longer drift space should not be included. Consequently, we have performed deliberation for employing energy dependent angular distribution of Cherenkov radiation. Though the energy dependence of emission angle of Cherenkov radiation is rapidly getting small as increasing the beam energy, it is still 25 deg/MeV at an energy around 2.0 MeV when we use radiator having refractive index of 1.035. Thus the beam energy distribution can be measured if we observe Cherenkov ring with sufficient angular resolution. Since this method needs only thin radiator, the drift space length will be minimized. We will discuss limitation for resolutions of both the time and the energy as well.

• N.Y. Huang, S.S. Yang
  NTHU, Hsinchu
• H. Hama
  Tohoku University, School of Scinece, Sendai
• W.K. Lau
  NSRRC, Hsinchu

Design of an intense but tightly focused ultrashort electron beam for production of sub-hundred femtosecond x-ray pulses that based on head-on inverse Compton scattering (ICS) has been studied. The three dimensional (3D) space charge dynamics has been tracked and optimized throughout the whole beamline. It is found that the focusing ultrashort electron pulses as short as 67 fs can be produced by compressing the energy-chirped beam from a thermionic cathode rf gun with an alpha magnet and linac operating at injection phase near zero crossing. This multi-bunch electron beam has an intensity of 30 pC per bunch and is accelerated to 27 MeV with an S-band linac structure. The compressed electron beam is focused to 64 μm for scattering with an 800 nm, 3.75 mJ laser in the laser-beam interaction chamber. With this method, total peak flux of back-scattered x-ray photons exceeds 10¹⁸ photons/sec is achievable with the shortest wavelength of 0.7 Å.

• F. Hinode, H. Hama, M. Kawai, F. Miyahara, T. Muto, K. Nanbu, H. Oohara, Y. Tanaka
  Tohoku University, School of Scinece, Sendai
• S. Kashiwagi
  Tohoku University, Research Center for Electron Photon Science, Sendai

A test accelerator for the coherent terahertz source (t-ACTS) has been under development at Laboratory of Nuclear Science, Tohoku University*. Intense coherent terahertz radiation will be generated by the very short electron bunch less than 100 fs using a thermionic RF gun (ITC RF-gun). ITC RF-gun is designed to have two cells uncoupled with each other, so that it can be operated at various combinations of different rf-power level and phase difference so as to optimize the longitudinal phase space distribution of the electron beam for bunch compression**. The gun employs single-crystal LaB6 cathode with small diameter of 1.8 mm to obtain a very small initial emittance with sufficiently high current density. The RF gun has been already manufactured and the measurement of RF characteristics is now in progress. We will present the results of low-power measurement and also discuss the effect of the cathode misalignment on the beam parameters such as transverse emittance and longitudinal phase space distribution.

* H. Hama et al., New J. Phys. 8 (2006) 292.
** H. Hama et al., Nucl. Instr. and Meth., A 528, (2004) 371.

• F. Miyahara, H. Hama, F. Hinode, M. Kawai, T. Muto, K. Nanbu, H. Oohara, Y. Tanaka
  Tohoku University, School of Scinece, Sendai

We have studied production of the very short-bunch electron beam to generate intense coherent THz radiation*. The bunch length of 100 fs is required to produce CSR around 1 THz. The beam from the thermionic RF-gun is introduced into the bunch compression system consist of an alpha magnet and a linac. The alpha magnet is often used as a bunch compressor for electron energy of several MeV. However, for our system, the alpha magnet plays a role of the longitudinal phase space rotator and energy filter. The bunch compression is done in the linac employing velocity bunching. The beam is injected on near the zero-cross phase of the RF field in the linac, and then the beam phase slip toward the crest. The longitudinal phase space and beam phase with respect to RF field at the entrance of the linac are optimized so that the bunch length would be minimum. In current analysis using numerical simulation based on the GPT code**, an rms bunch length of 30 fs has been obtained for a bunch charge of 20 pC. We will discuss the bunch compression scheme and the beam dynamics in the system. Prospect of the coherent radiation from the beam will be also reported.

* H. Hama et al, Proc. Ultrashort Electron & Photon Beam Techniques and Applications, Xian, China (2009)
** General Particle Tracer (GPT), URL: http://www.pulsar.nl/gpt

• H. Hama
  Tohoku University, School of Scinece, Sendai
• N.Y. Huang
  NTHU, Hsinchu

In compact linac system, alpha magnet seems to be a useful device to manipulate the longitudinal phase space. Particularly combined use with thermionic RF gun has been regarded as a convenient system for bunch compression. The alpha magnet simply acts to rotate the longitudinal phase space of the beam, besides energy selection by an aperture in it. However, by using the alpha magnet, if we like to produce high brilliant electron beam with considerable charge, space charge force has to be carefully taken into account to evaluate the beam property for not only the longitudinal but also the transverse. Since the both transverse motions and the longitudinal one are coupled with each other in the alpha magnet, it is mostly impossible to evaluate the space charge effect analytically. Meanwhile, because energies of the electrons from the thermionic RF gun are ranging from zero to the maximum, a conventional way to count Coulomb force in the rest frame may be not satisfactorily valid in numerical simulations. We will discuss space charge dominated phase spaces derived from 3-D tracking simulations* for the alpha magnet. *GPT (General Particle Tracer) and an FDTD code developed ourselves.