• W. Liu, W. Gai
  ANL, Argonne
• L. Rinolfi
  CERN, Geneva
• J. Sheppard
  SLAC, Menlo Park, California

We propose a viable positron source scheme that uses circularly polarized gamma rays generated from the main 250 GeV electron beam. The beam passes through a helical superconducting undulator with a magnetic field of ~ 1 Tesla and a period of a few centimeters. The gamma-rays produced in the undulator in the energy range between ~ 3 MeV ~ 100 MeV will be directed to a titanium target and produces polarized positrons. The positrons are then captured, accelerated and transported to a damping ring. Detailed parameter studies of this scheme including positron yield, undulator parameter dependence and target composition and geometry will be presented. Effects on the 250 GeV drive beam, including emittance, energy spread and energy loss from the beam passing through the undulator will also be discussed.

• W. Liu, W. Gai
  ANL, Argonne

We present an update on the ANL ILC positron source study. We examined the impact of different drive beam energies on the positron yield and polarization for the ILC RDR baseline undulator. The e⁺ yield is found to drop rapidly as the drive beam energy is reduced. We studied different undulator parameters for their effect on the positron yield and polarization when working at lower drive beam energies. Using a lower K (B field level) can increase the photon energy, but it is still very difficult to bring the yield up for low drive beam energies. For 250 GeV drive beam options, we studied the RDR undulator performance as a function of K. Instead of powering off some sections of the undulator, one can also consider lowering the B field to bring the positron yield back to the desired 1.5 e+/e-. We also studied the liquid lead target option for ILC positron source and the energy deposition in the reference design Ti target wheel.

• M.E. Conde, S.P. Antipov, W. Gai, R. Konecny, W. Liu, J.G. Power, Z.M. Yusof
  ANL, Argonne
• C.-J. Jing
  Euclid TechLabs, LLC, Solon, Ohio

The AWA Facility is dedicated to the study of advanced accelerator concepts based on electron beam driven wakefields. The facility employs an L-band photocathode RF gun to generate high charge short electron bunches, which are used to drive wakefields in dielectric loaded structures, as well as in metallic structures. Accelerating gradients as high as 100 MV/m have been reached in dielectric structures, and RF pulses of up to 44 MW have been generated at 7.8 GHz. In order to reach higher accelerating gradients and higher RF power levels, several upgrades are underway: (a) a new RF gun with higher QE photocathode will replace the present drive gun; (b) the existing RF gun will generate a witness beam to probe the wakefields; (c) three new 25 MW L-band RF power stations will be added to the facility; (d) five additional linac structures will bring the beam energy up from 15 MeV to 75 MeV. The drive beam will consist of bunch trains of up to 32 bunches, with up to 60 nC per bunch. The goal of future experiments is to reach accelerating gradients of several hundred MV/m and to extract RF pulses with GW power level.

• D. Wu, W. Liu, C.-X. Tang
  TUB, Beijing

The coherent Smith-Purcell radiation (CSPR) has been demonstrated as an efficient technique for measuring the longitudinal profile of beam bunches. To measure the ultrashort beam bunches, the simulations for the measurements using CSPR are anlyzed with tools of three dimensional particle-in-cell simulations and Kramer-Kronig reconstruction. Different parameters such as rms length of beam bunch and profiles of grating are studied. Furthermore, the measurement device based on a Martin-Puplett Interferometer is introduced, in which noises and attenuation can be reduced.

• W. Liu, W.-H. Huang, C.-X. Tang, D. Wu
  TUB, Beijing

Smith-Purcell radiation (SPR) is emitted when an electron passes near the surface of a periodic metallic grating. The radiation wavelength λ observed at the angle θ measured from a direction of surface grating is determined by λ=D/|n|(1/β cosθ), Where D is the grating period, βc is the electron velocity, c is the speed of light, and the integer n is the spectral order. This radiation mechanism is widely applicated to THz radiation source, for which can be developed into tunable and compact one. In this paper, the radiation characteristics of terahertz (THz) SPR generated from the ultrashort electron beam are analyzed with the three-dimensional particle-in-cell simulation. For obtaining the intense THz radiation, the grating parameters and that of ultrashort electron beam are optimum. The radiation power and energy are obtained by the PIC simulation. The band width of train bunches is compared with that of single bunch. The formation factors including the longitudinal and transverse are calculated. Through this study, we observe that the radiation power is enhanced and the band width can be adjusted.