• A. Kanareykin, C.-J. Jing, A.L. Kustov, P. Schoessow
  Euclid TechLabs, LLC, Solon, Ohio
• A. Altmark
  LETI, Saint-Petersburg
• W. Gai, J.G. Power
  ANL, Argonne

Beam breakup (BBU) effects resulting from parasitic wakefields are a serious limitation to the performance of dielectric structure based accelerators. We report here on numerical studies and experimental investigations of BBU and its mitigation. An experimental program is underway at the Argonne Wakefield Accelerator facility that will focus on BBU measurements in dielectric wakefield devices. We examine the use of external FODO channels for control of the beam in the presence of strong transverse wakefields. We present calculations based on a particle-Green's function beam dynamics code (BBU-3000) that we are developing. We will report on new features of the code including the ability to treat space charge. The BBU code is being incorporated into a software  framework that will significantly increase its utility (Beam Dynamics Simulation Platform). The platform is based on the very flexible Boinc software environment developed originally at Berkeley for the SETI@home project. The package can handle both task farming on a heterogeneous cluster of networked computers and computing on a local grid. User access to the platform is through a web browser.

• W. Gai, M.E. Conde, J.G. Power
  ANL, Argonne
• C.-J. Jing
  Euclid TechLabs, LLC, Solon, Ohio

In this paper, we present a linear collider concept based on drive beam generation from an RF photoinjector, and employing dielectric structures for power extraction and acceleration. The collider is based on a modular design with each module providing 100 GeV net acceleration. A high current drive beam is produced using a low frequency RF gun (~ 1GHz), and subsequently accelerated to ~1 GeV using conventional standing wave cavities. High frequency (20 GHz) RF power, extracted from the drive beam using a low impedance dielectric structure, is used to power the main linacs, which are based on high impedance high gradient dielectric loaded accelerating structures. We envision this scheme will produce high gradients (300 MeV/m), leading to a very compact design. The modularity of the design will allow a staged construction that will enable extension to multi-TeV energies.

• S.P. Antipov, W. Gai, O. Poluektov
  ANL, Argonne
• C.-J. Jing, A. Kanareykin, P. Schoessow
  Euclid TechLabs, LLC, Solon, Ohio

As the dimensions of accelerating structures become smaller and beam intensities higher, the transverse wakefields driven by the beam become quite large with even a slight misalignment of the beam. These deflection modes can cause inter-bunch beam breakup and intra-bunch head-tail instabilities along the beam path. We propose a built-in tunable absorption mechanism for damping the parasitic transverse modes without affecting the operational modes in dielectric loaded accelerating (DLA) structures and wakefield power extractors. The new principle for HOM absorption is based on electron paramagnetic resonance. The dielectric tube of the DLA has to be doped with a material exhibiting high EPR, for example ruby, Al2O3 overdoped ~1% with Cr³⁺. The absorption frequency can be tuned by an external DC magnetic field to match the frequency of the transverse mode. At the resonance imaginary part of permeability becomes significant and the dielectric tube acts as an absorber for the transverse modes. The external DC magnetic field is solenoidal and has to have a magnitude of about 3 kG. This configuration in fact is desirable to focus the beam and provide additional control of beam break up.

• 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.

• J.G. Power, M.E. Conde, W. Gai
  ANL, Argonne
• Z. Li
  SLAC, Menlo Park, California
• D. Mihalcea
  Northern Illinois University, DeKalb, Illinois

We report on the design of a 7 cell, standing wave, 1.3 GHz LINAC cavity and the associated beam dynamics studies for the upgrade of the drive beamline for the Argonne Wakefield Accelerator (AWA) facility. The LINAC design is a compromise between single bunch operation (100 nC @ 75 MeV) and minimizing the energy droop due to beam loadning along the bunch train during bunch train operation. The 1.3 GHz drive bunch train target parameters are: 75 MeV, 10-20 ns macropulse duration, 16x60nC microbunches; this is equivalent to a macropulse current and beam power of 80 Amps and 6 GW, respectively. Each LINAC structure accelerates approximately 1000 nC in 10 ns by a voltage of 11 MV at an RF power of 10 MW. Due to the short bunch train duration desired (~10 ns) and the existing frequency (1.3 GHz), compensation of the energy droop along the bunch train is difficult to accomplish with the two standard techniques: time-domain or frequency-domain beam loading compensation. Therefore, to minimize the energy droop, our design is based on a large stored energy LINACs. In this paper, we present our LINAC optimization method, detailed LINAC design, and beam dynamics studies of the drive beamline.

• 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.

• C.-J. Jing
  Euclid TechLabs, LLC, Solon, Ohio
• S.P. Antipov, M.E. Conde, W. Gai, F. Gao, J.G. Power, Z.M. Yusof
  ANL, Argonne
• H. Chen, C.-X. Tang, S.X. Zheng
  TUB, Beijing
• P. Xu
  Tsinghua University, Beijing

Wakefield excitation has been experimentally studied in a 3-cell X-band standing wave Photonic Band Gap (PBG) accelerating structure. Major monopole (TM01- and TM02-like) and dipole (TM11- and TM12-like) modes were indentified and characterized by precisely controlling the position of beam injection. The quality factor Q of the dipole modes was measured to be ~10 times smaller than that of the accelerating mode. A charge sweep, up to 80 nC, has been performed, equivalent to ~30 MV/m accelerating field on axis. A variable delay low charge witness bunch following a high charge drive bunch was used to calibrate the gradient in the PBG structure by measuring its maximum energy gain and loss. Experimental results agree well with numerical simulations.

• C.-J. Jing, F. Gao, A. Kanareykin, P. Schoessow
  Euclid TechLabs, LLC, Solon, Ohio
• M.E. Conde, W. Gai, R. Konecny, J.G. Power
  ANL, Argonne

High frequency, high power rf sources are needed for many applications in particle accelerators, communications, radar, etc. We have developed a 26GHz high power rf source based on the extraction of wakefields from a relativistic electron beam. The extractor is designed to couple out rf power generated from a high charge electron bunch train traversing a dielectric loaded waveguide. The first high beam experiment has been performed at Argonne Wakefield Accelerator facility. The experimental results successfully demonstrate the 15ns 26GHz rf pulse generated from the wakefield extractor with a bunch train of 16 bunches. Meanwhile, ~ 30MW short rf pulse has been achieved with a bunch train of 4 bunches. Beam Breakup has prevented charge transport through the power extractor beyond 10nC. We are doing simulations and developing methods to alleviate the BBU effect.

• C.-J. Jing, A. Kanareykin, P. Schoessow
  Euclid TechLabs, LLC, Solon, Ohio
• M.E. Conde, W. Gai, J.G. Power
  ANL, Argonne
• E. Nenasheva
  Ceramics Ltd., St. Petersburg

Dielectric-Loaded Accelerating (DLA) structures are generally lack of approaches to tune frequency after the fabrication. A tunable DLA structure has been developed by using an extra nonlinear ferroelectric layer. Dielectric constant of the applied ferroelectric material is sensitive to temperature and DC voltage. Bench test shows the +14MHz/°C, and 6MHz frequency tuning range for a 25kV/cm of DC bias field. A beam test is planned at Argonne Wakefield Accelerator facility before the IPAC conference. Detailed results will be reported.