• M.E. Conde, S.P. Antipov, W. Gai, C.-J. Jing, R. Konecny, W. Liu, J.G. Power, H. Wang, Z.M. Yusof
  ANL, Argonne, Illinois

The Argonne Wakefield Accelerator Facility (AWA) is dedicated to the study of electron beam physics and the development of accelerating structures based on electron beam driven wakefields. In order to carry out these studies, the facility employs a photocathode RF gun capable of generating electron beams with high bunch charges (up to 100 nC) and short bunch lengths. This high intensity beam is used to excite wakefields in the structures under investigation. The wakefield structures presently under development are dielectric loaded cylindrical waveguides with operating frequencies of 7.8 or 15.6 GHz. The facility is also used to investigate the generation and propagation of high brightness electron beams. Presently under investigation, is the use of photons with energies lower than the work function of the cathode surface (Schottky-enabled photoemission), aimed at generating electron beams with low thermal emittance. Novel electron beam diagnostics are also developed and tested at the facility. The AWA electron beam is also used in laboratory-based astrophysics experiments; namely, measurements of microwave Cherenkov radiation and fluorescence of air. We report on the current status of the facility and present recent results.

• W. Liu
  Illinois Institute of Technology, Chicago, Illinois
• W. Gai, J.G. Power, H. Wang
  ANL, Argonne, Illinois

In this paper, we consider the contribution of potential energy to beam dynamics as simulated by PARMELA at low energies (10 - 30MeV). We have developed a routine to calculate the potential energy of the relativistic electron beam using the static coulomb potential in the rest frame (first order approximation as in PARMELA). We found that the potential energy contribution to the beam dynamics could be very significant, particularly with high charge beams generated by an RF photocathode gun. Our results show that when the potential energy is counted correctly and added to the kinetic energy from PARMELA, the total energy is conserved. Simulation results of potential and kinetic energies for short beams (~1 mm) at various charges (1 - 100 nC) generated by a high current RF photocathode gun are presented.

• H. Wang, W. Gai, K.-J. Kim, W. Liu
  ANL, Argonne, Illinois

It is apparent that the gamma-ray based positron source components including positron linac and damping rings for ILC can not be easily commissioned until the electron beam is fully conditioned at high energies (> 150 GeV). In this paper, we discuss a scheme that could use a short and energetic electron beam scattered through a set of carefully selected targets to simulate certain behaviors of the positron beam, such as beam emittance and energy spread. The basic idea is to make the phase space distribution of the scattered electron beam to reflect certain aspects of the positron beam distributions. Subsequently, the positron source elements such as capture optics, linacs and even damping ring could be effectively commissioned before ILC colliding electron beam is ready. The simulation results using EGS4 for beam scattering and PARMELA for beam dynamics are presented.

• C. Tennant, D. Douglas, K. Jordan, L. Merminga, E.P. Pozdeyev, H. Wang
  Jefferson Lab, Newport News, Virginia
• I.V. Bazarov
  Cornell University, Department of Physics, Ithaca, New York
• G. Hoffstaetter
  Cornell University, Laboratory for Elementary-Particle Physics, Ithaca, New York
• S. Simrock
  DESY, Hamburg
• T.I. Smith
  Stanford University, Stanford, Califormia

In recirculating accelerators, and in particular energy recovery linacs (ERLs), the maximum current has been limited by multipass, multibunch beam breakup (BBU), which occurs when the electron beam interacts with the higher-order modes (HOMs) of an accelerating cavity on the accelerating pass and again on the energy recovered pass. This effect is of particular concern in the design of modern high average current energy recovery accelerators utilizing superconducting technology. Experimental observations of the instability at the Jefferson Laboratory 10 kW Free-Electron Laser (FEL) are presented. Measurements of the threshold current for the instability are presented and compared to the predictions of several BBU simulation codes. To further characterize the instability, beam based measurements were made to determine the orientation of the dangerous HOMs. With BBU posing a threat to high current beam operation in the FEL, several suppression schemes were developed. These include direct damping of the dangerous HOMs and appropriately modifying the electron beam optics. Preliminary results of their effectiveness in raising the threshold current for stability are presented.

• A.T. Wu, R.C. Ike, H.L. Phillips, A-M. Valente, H. Wang, G. Wu
  Jefferson Lab, Newport News, Virginia

Deposition of thin niobium (Nb) films on copper (Cu) cavities, using an electron cyclotron resonance (ECR) plasma appears to be an attractive alternative technique for fabricating superconducting radio frequency cavities to be used in particle accelerators. The performance of these Nb/Cu cavities is expected to depend on the surface characteristics of the Nb films. In this paper, we report on an investigation of the influence of deposition energy on surface morphology, microstructure, and chemical composition of Nb films deposited on small Cu disks employing a metallographic optical microscope, a 3-D profilometer, a scanning electron microscope, and a dynamic secondary ion mass spectrometer. The results will be compared with those obtained on Nb surfaces treated by buffered chemical polishing, electropolishing, and buffered electropolishing. Possible implications from this study for Nb deposition on real Cu cavities will be discussed.

• H. Wang, H.L. Phillips, R.A. Rimmer, A-M. Valente, A.T. Wu, G. Wu
  Jefferson Lab, Newport News, Virginia

A system for the deposition, using an ECR plasma source, of a thin film of niobium inside a copper cavity for superconducting accelerator applications has been designed and is being constructed. The system uses a 500-MHz copper cavity as the substrate and the vacuum chamber. The ECR plasma will be created to produce direct niobium ion deposition. The central cylindrical grid is biased to realize the energy controlled deposition. This report describes the design of several subcomponents including the vacuum chamber, RF supply, biasing grid and magnet coils. Operational parameters are compared between a working small-sample deposition system and this system. Initial plasma simulation also suggested that plasma ignition in this cavity system is feasible.

• H. Wang, R.A. Rimmer, G. Wu
  Jefferson Lab, Newport News, Virginia

A normal design process for a superconducting cavity shape is to maximize the R/Q (shunt impedance/intrinsic quality factor) and geometry factor G for a given RF field limit of Bpeak/Eacc or Epeak/Eacc. For the application of an Ampere-class, high current energy recovery linac or storage ring, heavy HOM damping is required. This paper reports on a survey of single cell shapes developed for multi-cell cavities for different projects. Using a set of normalized parameters, we compare the designs for different frequencies and ß structures for the fundamental mode. Using dispersion curve (frequency verse phase advance) calculated by MAFIA for a single cell, we explore further how to optimize the cavity shape to avoid a light cone line crossing at the dangerous resonance frequencies determined by the beam bunch structure or the dangerous (trapped or high R/Q) modes with a low group velocity. We expect such a formulation to inform our development of a 5-cell, optimized cavity shape, with good real estate accelerating gradient and strong HOM damping waveguide structure for the JLab 1MW ERL-FEL project.

• R.A. Rimmer, E. Daly, J. Henry, W.R. Hicks, J.P. Preble, M. Stirbet, H. Wang, K. Wilson, G. Wu
  Jefferson Lab, Newport News, Virginia

We describe the concepts and developments underway at JLab as part of the program to develop a new CW cryomodule capable of transporting ampere-level beam currents in a compact FEL. Requirements include real-estate gradient of at least 10 MV/m and very strong HOM damping to push BBU thresholds up by two or more orders of magnitude compared to existing designs. Cavity shape, HOM damping, power couplers, tuners etc. are being designed and optimized for this application. Cavity considerations include a large iris for beam halo, low-RF losses, HOM frequencies and Q’s, low peak surface fields, field flatness and microphonics. Module considerations include high packing factor, low static heat leak, image current heating of beam-line components, cost and maintainability. This module is being developed for the next generation ERL based high power FELs but may be useful for other applications such as electron cooling, electron-ion colliders, industrial processing etc.

• M. A. Drury, E. Daly, G.K. Davis, J.R. Delayen, C. Grenoble, W.R. Hicks, K. King, T. Plawski, T. Powers, J.P. Preble, H. Wang, M. Wiseman
  Jefferson Lab, Newport News, Virginia

Thomas Jefferson National Accelerating Facility (Jefferson Lab) has completed production of 24 Superconducting Radio Frequency (SRF) cryomodules for the Spallation Neutron Source (SNS) superconducting linac. This includes one medium-beta (0.61) prototype, eleven medium-beta and twelve high-beta (0.81) production cryomodules. Ten medium-beta cryomodules as well as two high beta cryomodules have undergone complete operational performance testing in the Cryomodule Test Facility at Jefferson Lab. The set of tests includes measurements of maximum gradient, unloaded Q (Q0), microphonics, and response to Lorentz forces. The Qext’s of the various couplers are measured and the behavior of the higher order mode couplers is examined. The mechanical and piezo tuners are also characterized. The results of these performance tests will be discussed in this paper.