• I. Yamane
  KEK, Ibaraki
• M. Nakamura, H. Okuno
  RIKEN Nishina Center, Wako

An asymmetrical con-focal cavity is composed of tow concave mirrors with different focal length, placed face to face, and their axes and focal points coincide. When a laser beam is injected in parallel with the mirror axis, from backward of and just outside of the mirror with the smaller focal length, the laser beam is trapped in the cavity and repeats reflection by mirrors. Then, the beam reflected by the mirror with the larger focal length passes every time the focal point and the period by which pulses return to the focal point is constant. Therefore, if the repetition period of the injected laser pulse is equal to the repetition period in the cavity, all laser pulses comes to the focal point at the same time and the beam intensity is stacked up. Calculation on the performance of an asymmetrical con-focal cavity shows that a laser pulse can be recycled more than a few tens turns and the beam intensity can be stacked to more than a few tens times of the original beam intensity when the laser beam is a Gaussian beam and the reflectance of the mirrors is 100%. Results of calculation is examined using a He-Ne laser and a pair of high reflection mirrors.

• Z.G. He, Q.K. Jia, X.E. Wang
  USTC/NSRL, Hefei, Anhui

A Photo-Cathode RF Gun is under development at NSRL. In this paper, the drive laser system is introduced and performance parameters are presented. We adopt a BNL type gun with laser illuminating the cathode at oblique incidence. To orrect 'time slew' and 'elliptical spot' problems arisen on the cathode, an adjustable optical transport line is designed.

• I. Jovanovic, Y. Yin
  Purdue University, West Lafayette, Indiana
• S. Boucher, R. Tikhoplav
  RadiaBeam, Marina del Rey
• G. Travish
  UCLA, Los Angeles, California

One approach for detecting special nuclear material (SNM) at a distance is to use highly penetrating gamma-rays (>6 MeV) to produce photofission. We are investigating inverse gamma-ray sources (IGS), based on inverse Compton scattering (ICS) of a laser pulse on a relativistic electron bunch. Nearly monochromatic gamma rays with high brightness, very small source size and divergence can be produced in IGS. For the interaction drive laser recirculation it is necessary to meet the repetition rate requirements. Three implementations of laser recirculation are proposed for the interaction drive laser, which can significantly reduce the requirements on the interaction drive laser average power. It is found that the recently demonstrated recirculation injection by nonlinear gating (RING) technique offers unique advantages for beam recirculation in IGS.

• M. Shverdin, F. Albert, S.G. Anderson, C.P.J. Barty, A.J. Bayramian, M. Betts, T.S. Chu, C.A. Ebbers, D.J. Gibson, F.V. Hartemann, R.A. Marsh, D.P. McNabb, M. J. Messerly, H.H. Phan, M.A. Prantil, C. Siders, S.S.Q. Wu
  LLNL, Livermore, California

Generation of mono-energetic, high brightness gamma-rays requires state of the art lasers to both produce a low emittance electron beam in the linac and high intensity, narrow linewidth laser photons for scattering with the relativistic electrons. Here, we overview the laser systems for the 3rd generation Monoenergetic Gamma-ray Source (MEGa-ray) currently under construction at Lawrence Livermore National Lab. We also describe a method for increasing the efficiency of laser Compton scattering through laser pulse recirculation. The fiber-based photoinjector laser will produce 50 uJ temporally and spatially shaped UV pulses at 120 Hz to generate a low emmittance electron beam in the X-band RF photoinjector. The interaction laser generates high intensity photons that focus into the interaction region and scatter off the accelerated electrons. This system utilizes chirped pulse amplification and commercial diode pumped solid state Nd:YAG amplifiers to produce 0.5 J, 10 ps, 120 Hz pulses at 10⁶⁴ nm and up to 0.2 J after frequency doubling. A single passively mode-locked Ytterbium fiber oscillator seeds both laser systems and provides a timing synch with the linac.