• I. Pavlishin, M. Babzien, I. Pogorelsky, D. Stolyarov, V. Yakimenko
  BNL, Upton, Long Island, New York
• M. S. Zolotorev
  LBNL, Berkeley, California

Optical stochastic cooling for the Relativistic Heavy Ion Collider (RHIC) based on optical parametric amplification was proposed by M. Babzien et al., Phys. Rev. ST Accel. Beams v.7, 012801, (2004). According to this proposal a CdGeAs2 nonlinear crystal is used as an active medium for the optical parametric amplifier because of extremely large nonlinear coefficient, wide transparency range, and possibility to be phase matched over the required spectral range. We discuss experimental results of the parametric amplifier gain and coherency for the conditions applicable to optical stochastic cooling for RHIC.

• E. Kallos, T. C. Katsouleas, P. Muggli
  USC, Los Angeles, California
• W. D. Kimura
  STI, Washington
• I. Pavlishin, I. Pogorelsky, D. Stolyarov, V. Yakimenko
  BNL, Upton, Long Island, New York

We investigate various plasma wakefield accelerator schemes that rely on multiple electron bunches to drive a large amplitude plasma wave, which are followed by a witness bunch at a phase where it will sample the high acceleration gradient and gain energy. Experimental verifications of various two bunch schemes are available in the literature; here we provide analytical calculations and numerical simulations of the wakefield dependency and the transformer ratio when M drive bunches and one witness bunch are fed into a high density plasma, where M is between 2 and 10. This is a favorable setup since the bunches can be adjusted such that the transformer ratio and the efficiency of the accelerator are enhanced compared to single bunch schemes. The possibility of a five bunch ILC afterburner to accelerate a witness bunch from 100 GeV to 500 GeV is also examined.

• P. Muggli, E. Kallos, T. C. Katsouleas
  USC, Los Angeles, California
• W. D. Kimura
  STI, Washington
• K. Kusche, I. Pavlishin, D. Stolyarov, V. Yakimenko
  BNL, Upton, Long Island, New York

Two ~100 fs electron bunches, separated in energy by approximately 1.8 MeV and in time by 0.5-1 ps, were sent through a capillary discharge plasma. The plasma density was varied from ~1·10¹⁴/cc to ~1·10¹⁷/cc. A 2-D PWFA model indicates the net wakefield produced by the bunches will depend on their relative charge, temporal separation, and the plasma density. This will affect the amount of energy gain or loss of the second bunch. During measurements of the energy spectrum of the second bunch, we observed a difference in the amount of gain or loss depending on the plasma density, which is consistent with the model prediction.