• P. Schoessow, C.-J. Jing, A. Kanareykin
  Euclid TechLabs, LLC, Solon, Ohio
• W. Gai, J.G. Power
  ANL, Argonne
• O.V. Sinitsyn
  UMD, College Park, Maryland

Funding: Work supported by the US Department of Energy.

The development of high gradient rf driven dielectric accelerating structures is in part limited by the problem of multipactor. The first high power experiments with an 11.424-GHz rf driven alumina accelerating structure exhibited single surface multipactor. Unlike the well understood multipactor problem for dielectric rf windows, where the rf electric field is tangential and the rf power flow is normal to the dielectric surface, strong normal and tangential rf electric fields are present from the TM01 accelerating mode in the DLA and the power flow is parallel to the surface at the dielectric-beam channel boundary. While a number of approaches have been developed, no one technology for MP mitigation is able to completely solve the problem. In this paper we report on numerical calculations of the evolution of the MP discharge, and give particular attention to MP dependence on the rf power ramp profile and the use of engineered surface features on the beam channel wall to interrupt the evolution of the multipactor discharge.

• M. BastaniNejad, A.A. Elmustafa
  Old Dominion University, Norfolk, Virginia
• J.M. Byrd, D. Li
  LBNL, Berkeley, California
• M.E. Conde, W. Gai
  ANL, Argonne
• R.P. Johnson, M.L. Neubauer, R. Sah
  Muons, Inc, Batavia
• A. Moretti, M. Popovic, K. Yonehara
  Fermilab, Batavia

Funding: Supported in part by USDOE STTR Grant DE-FG02-08ER86352 and FRA DOE contract number DE-AC02-07CH11359

Many present and future particle accelerators are limited by the maximum electric gradient and peak surface fields that can be realized in RF cavities. Despite considerable effort, a comprehensive theory of RF breakdown has not been achieved and mitigation techniques to improve practical maximum accelerating gradients have had only limited success. Recent studies have shown that high gradients can be achieved quickly in 805 MHz RF cavities pressurized with dense hydrogen gas without the need for long conditioning times, because the dense gas can dramatically reduce dark currents and multipacting. In this project we use this high pressure technique to suppress effects of residual vacuum and geometry found in evacuated cavities to isolate and study the role of the metallic surfaces in RF cavity breakdown as a function of magnetic field, frequency, and surface preparation. A 1.3-GHz RF test cell with replaceable electrodes (e.g. Mo, Cu, Be, W, and Nb) and pressure barrier capable of operating both at high pressure and in vacuum been designed and built, and preliminary testing has been completed. A series of detailed experiments is planned at the Argonne Wakefield Accelerator.

• W. Gai, M. Borland, K.-J. Kim, W. Liu, A. Xiao
  ANL, Argonne
• J. Sheppard
  SLAC, Menlo Park, California

Funding: This work was supported by the US Department of Energy Office of Science under Contract No. DE-AC02-06CH11357.

The effect of ILC positron source’s helical undulator to the drive electron beam is of great interest. People have been looking into the effect of wakefield, quad misalignment and also the effect of radiation. In this paper we’ll report an emittance damping effect of the ILC positron source undulator to the drive electron beam and our QUAD-BPM error simulation results. For 100m RDR undulator, the emittance of drive electron beam will be damped down by about 1% instead of growing as the damping is stronger than quantum excitation for this RDR undulator with the RDR drive electron beam. Quad-BPM misalignment simulations show that a 20um rms misalignment error in a 250m long undulator beamline can cause about 5% emittance growth in drive electron beam. Taking into consider the damping effect of undulator, the net emittance growth will be smaller.

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

Funding: Work supported by the U.S. Department of Energy under contract No. DE-AC02-06CH11357.

The Argonne Wakefield Accelerator Facility is dedicated to the study of advanced accelerator concepts based on electron beam driven wakefield acceleration and RF power generation. 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 (iris loaded, photonic band gap, etc). Accelerating gradients as high as 100 MV/m have been reached in dielectric loaded structures, and RF pulses of up to 44 MW have been generated at 7.8 GHz. In order to reach higher accelerating gradients, and also be able to generate higher RF power levels, a photocathode with higher quantum efficiency is needed. Therefore, a new RF gun with a Cesium Telluride photocathode will replace the electron gun that has been used to generate the drive bunches. In addition to this, a new L-band klystron will be added to the facility, increasing the beam energy from 15 MeV to 23 MeV, and thus increasing the total power in the drive beam to a few GW. 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, F. Gao, A. Kanareykin, A.L. Kustov, P. Schoessow
  Euclid TechLabs, LLC, Solon, Ohio
• M.E. Conde, W. Gai, R. Konecny, J.G. Power
  ANL, Argonne
• S. Kazakov
  KEK, Ibaraki

Funding: DoE SBIR 2008 Phase II, DE-FG02-07ER84821

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. Using a 20nC bunch train (bunch length of 1.5 mm) at the Argonne Wakefield Accelerator (AWA) facility, we expect to obtain a steady 26GHz output power of 148 MW. The extractor has been fabricated and bench tested along with a 26GHz Power detector. The first high power beam experiments should be performed prior to the Conference. Detailed results will be reported.

• 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

Funding: DoE SBIR Phase I 2008

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 from the geometric axis. These deflection modes can cause inter-bunch beam breakup and intra-bunch head-tail instabilities along the beam path, and thus BBU control becomes a critical issue. All new metal based accelerating structures, like the accelerating structures developed at SLAC or power extractors at CLIC, have designs in which the transverse modes are heavily damped. Similarly, minimizing the transverse wakefield modes (here the HEMmn hybrid modes in Dielectric-Loaded Accelerating (DLA) structures) is also very critical for developing dielectric based high energy accelerators. We have developed a 7.8GHz transverse mode damped DLA structure. The design and bench test results are presented in the article.

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

Funding: This work is supported by the US Department of Energy

Beam breakup (BBU) effects resulting from parasitic wakefields provide a potentially serious limitation to the performance of dielectric structure based accelerators. We report here on comprehensive numerical studies and planned experimental investigations of BBU and its mitigation in dielectric wakefield accelerators. An experimental program is planned at the Argonne Wakefield Accelerator facility that will focus on BBU measurements in a number of high gradient and high transformer ratio wakefield devices. New pickup-based beam diagnostics will provide methods for studying parasitic wakefields that are currently unavailable at the AWA. The numerical part of this research is based on a particle-Green’s function beam dynamics code (BBU-3000) that we are developing. The code allows rapid, efficient simulation of beam breakup effects in advanced linear accelerators. The goal of this work is to compare the results of detailed experimental measurements with accurate numerical results and ultimately to study the use of external FODO channels for control of the beam in the presence of strong transverse wakefields.

• L. Rinolfi, F. Antoniou, H.-H. Braun, Y. Papaphilippou, D. Schulte, A. Vivoli, F. Zimmermann
  CERN, Geneva
• E.V. Bulyak, P. Gladkikh
  NSC/KIPT, Kharkov
• R. Chehab
  IN2P3 IPNL, Villeurbanne
• J.A. Clarke
  STFC/DL/ASTeC, Daresbury, Warrington, Cheshire
• O. Dadoun, P. Lepercq, R. Roux, A. Variola, Z.F. Zomer
  LAL, Orsay
• W. Gai, W. Liu
  ANL, Argonne
• T. Kamitani, T. Omori, J. Urakawa
  KEK, Ibaraki
• M. Kuriki
  HU/AdSM, Higashi-Hiroshima
• I. Pogorelsky, V. Yakimenko
  BNL, Upton, Long Island, New York
• T. Takahashi
  Hiroshima University, Graduate School of Science, Higashi-Hiroshima

The CLIC polarized positron source is based on a positron production scheme in which polarized photons are produced by Compton process. Compton backscattering happens in a so-called "Compton ring" where an electron beam of 1.06 GeV interacts with a powerful laser beam amplified in an optical resonator. The circularly-polarized gamma rays are sent on to a target, producing pairs of longitudinally polarized electrons and positrons. An Adiabatic Matching Device maximizes the capture of the positrons. A normal-conducting 2 GHz Linac accelerates the beam up to 2.424 GeV before injection into the Pre-Damping Ring (PDR). The nominal CLIC bunch population is 4.4x10⁹ particles per bunch. Since the photon flux coming out from a "Compton ring" is not sufficient to obtain the requested charge, a stacking process is required in the PDR. Another option is to use a "Compton Energy Recovery Linac" where a quasi-continual stacking in the PDR could be achieved. A third option is to use a "Compton Linac" which would not require stacking. We describe the overall scheme as well as advantages and constraints of the three different options.

• S.H. Gold
  NRL, Washington, DC
• W. Gai, R. Konecny, W. Liu, J.G. Power
  ANL, Argonne
• C.-J. Jing, A. Kanareykin
  Euclid TechLabs, LLC, Solon, Ohio
• A.K. Kinkead
  Icarus Research, Inc., Bethesda, Maryland

Funding: Department of Energy, Office of Naval Research, and DoE SBIR Phase II grant DE-FG02-06ER84463

Dielectric-loaded accelerating (DLA) structures, in which a dielectric liner is placed inside a cylindrical metal tube, offer the potential of a simple, inexpensive alternative to copper disk-loaded structures for use in high-gradient rf linear accelerators. A joint Naval Research Laboratory/Euclid Techlabs/Argonne National Laboratory study is under way to investigate the performance of X-band DLA structures using high-power 11.43-GHz radiation from the NRL Magnicon Facility*. The initial goal of the program has been to develop structures capable of sustaining high accelerating gradients. The two significant limitations that have been discovered relate to multipactor loading of the structures and rf breakdown at joints between ceramic sections. We will report the results of several recent structure tests that have demonstrated significant progress in addressing both of these issues. The longer-range goal of the program is to study electron acceleration in DLA structures. For this purpose, we are developing an X-band DLA test accelerator. We will also report the results of initial operation of a 5-MeV injector for the new accelerator.

*C. Jing, W. Gai, J. Power, R. Konecny, S. Gold, W. Liu and A. Kinkead, IEEE Trans. Plasma Sci., vol. 33, pp.1155-1160, August 2005.

• C.S. Park, M. Hess
  IUCF, Bloomington, Indiana
• W. Gai, J.G. Power
  ANL, Argonne

Funding: Supported in part by DOE(DE-FG029ER40747) and in part by NSF(PHY-0552389)

Near the cathode in a photoinjector, the electron beam is emitted with low energy, and its dynamics are strongly affected by the beam's space-charge fields. This can cause beam loss at the cathode due to virtual cathode formation. In general, a fully electromagnetic code can correctly predict the beam space-charge fields, beam dynamics, and beam loss. However, an electrostatic type algorithm would overestimate the space-charge fields since it does not incorporate relativistic time-retardation effects which limit the size of the fields near the cathode. IRPSS (Indiana RF Photocathode Source Simulator) can calculate the electromagnetic space-charge fields using a Green’s function method to a high-precision, and can track beam dynamics in the RF photoinjector. Using IRPSS, we simulated the beam dynamics and beam loss near the cathode for the Argonne Wakefield Accelerator 1.3 GHz gun* and compared those results to electrostatic codes, such as PARMELA and ASTRA.

*P. Schoessow, PAC 2009.

• S.P. Antipov, W. Gai, J.G. Power
  ANL, Argonne
• A. Kanareykin, P. Schoessow
  Euclid TechLabs, LLC, Solon, Ohio
• G. Semouchkin, E. Semouchkina
  PSU, University Park, Pennsylvania
• A.V. Tyukhtin
  Saint-Petersburg State University, Saint-Petersburg

Funding: DOE

Metamaterials (MTMs) are periodic artificially constructed electromagnetic structures. The periodicity of the MTM is much smaller than the wavelength of the radiation being transported. With this condition satisfied, MTMs can be assigned an effective permittivity and permeability. Areas of possible application of MTMs in accelerator science are Cherenkov detectors and wakefield devices. MTMs can be designed to be anisotropic and dispersive. The combination of engineered anisotropy and dispersion can produce a Cherenkov radiation spectrum with a different dependence on particle energy than conventional materials. This can be a basis for novel non-invasive beam energy measurements. We report on progress in the development of these media for a proof-of-principle demonstration of a metamaterial-based beam diagnostic.

• J.G. Power, M.E. Conde, W. Gai, W. Liu
  ANL, Argonne
• P. Piot
  Northern Illinois University, DeKalb, Illinois

Funding: The work is supported by the U.S. Department of Energy under Contract No. DE-AC02-06CH11357 with Argonne National Laboratory.

Phase space measurements of RF photoinjectors have usually been done with multislit masks or scanning slits. These systems implicitly ignore the correlations between the X and Y planes and thus yield measurements of the projected 2D phase space distributions. In contrast, a grid-patterned pepper-pot is capable of measuring the full 4D transverse phase space distribution, f(x,x',y,y'). 4D measurements allow precise tuning of electron beams with large canonical angular momentum, important for electron cooling and flat beam transformation, as well as zeroing the magnetic field on the photocathode is zero for ultra low emittance applications (e.g. SASE FEL, ERL FEL). In this talk, we report on a parametric set of measurements to characterize the 4D transverse phase space of the 1 nC electron beam from the Argonne Wakefield Accelerator (AWA) RF photoinjector. The diagnostic is simulated with TStep, including the passage of the electron beam trough the mask and tracking of the beamlets to the imaging screen. The phase space retrieval algorithm is then bench marked against simulations and measurements.

• M.M. Rihaoui, P. Piot
  Northern Illinois University, DeKalb, Illinois
• W. Gai, J.G. Power
  ANL, Argonne

Funding: M.R. and P.P. was supported by the US DOE under Contracts No. DE-FG02-08ER41532 with NIU. W.G. and J.P are supported by the U.S. DOE under Contract No. DE-AC02-06CH11357 with ANL.

Transverse to longitudinal phase space manipulation hold great promises, e.g., as a potential technique for repartitioning the emittances of a beam. A proof-of-principle experiment to demonstrate the exchange of a low longitudinal emittance with a larger transverse emittance is in preparation at the Argonne Wakefield Accelerator using a 15 MeV electron beam. In this paper we explore the limiting effects of this phase space manipulation method associated to high order optics and collective effects. A realistic start-to-end simulation of the planned proof-of-principle experiment including jitter studies is also presented.

• C. F. Papadopoulos, R.B. Fiorito, R.A. Kishek, P.G. O'Shea, A.G. Shkvarunets
  UMD, College Park, Maryland
• M.E. Conde, W. Gai, J.G. Power
  ANL, Argonne

Funding: This work is funded by the US Dept. of Energy Offices of High Energy Physics and High Energy Density Physics, and by the US Dept. of Defense Office of Naval Research and Joint Technology Office.

Single foil OTR and two foil OTR interferometry have been successfully used to measure the size and divergence of electron beams with a wide range of energies. To measure rms emittance, two cameras are employed: one focused on the foil to obtain the spatial distribution of the beam, the other focused to infinity to obtain the angular distribution. The beam is first magnetically focused to a minimum size in directions which are orthogonal to the propagation axis, using a pair of quadrupoles. Then simultaneous measurements of the rms size (x,y) and divergence (x’,y’) of the beam are made. However, in the process of a quadrupole scan, the beam can go through a spot size minimum, a divergence minimum and a waist, i.e. the position where the cross-correlation term is zero. In general, the beam size, divergence and focusing strength for each of these conditions are different. We present new algorithms that relate the beam and magnetic parameters to the rms emittance for each of these three cases. We also compare the emittances, obtained using our algorithms and measurements made at the ANL AWA facility, with those produced by computer simulation.

• M.M. Rihaoui, D. Mihalcea, P. Piot
  Northern Illinois University, DeKalb, Illinois
• W. Gai, J.G. Power
  ANL, Argonne

A transverse-to-longitudinal emittance exchange experiment is in preparation at the Argonne Wakefield Accelerator (AWA). The experiment aims at exchanging a low (< 5 mm-mrad) longitudinal emittance with a large (>15 mm-mrad) transverse horizontal emittance for a bunch charge of 100 pC. Achieving such emittance partitioning, though demonstrated via numerical simulations, is a challenging task and need to be experimentally verified. In this paper, we report emittance measurements of the beam in the transverse and longitudinal planes performed at 12 MeV. The measurements are compared with numerical simulations using Impact-T.

• M.M. Rihaoui, P. Piot
  Northern Illinois University, DeKalb, Illinois
• W. Gai, J.G. Power, Z.M. Yusof
  ANL, Argonne

Funding: M.R. and P.P. was supported by the US DOE under Contracts No. DE-FG02-08ER41532 with NIU. W.G., J.P., and Z.Y. are supported by the U.S. DOE under Contract No. DE-AC02-06CH11357 with ANL.

We report on a new experimental study of the space charge effect in a space-charge-dominated multi-beam electron bunch. A 5 MeV electron bunch, consisting of a variable number of beamlets separated transversely, was generated in a photoinjector and propagated in a drift space. The collective interaction of these beamlets was studied for different experimental conditions. The experiment allowed the exploration of space charge effects and its comparison with three-dimensional particle-in-cell simulations. Our observations also suggest the possible use of a multi-beam configuration to tailor the transverse distribution of an electron beam.

• S.P. Antipov, W. Gai, O. Poluektov, J.G. Power
  ANL, Argonne
• A. Kanareykin, P. Schoessow
  Euclid TechLabs, LLC, Solon, Ohio
• L. Schächter
  Technion, Haifa

Funding: DOE

Particle Acceleration by Stimulated Emission of Radiation (PASER) is method of particle acceleration in which a beam gains energy from an active medium through stimulated emission. To obtain the required sitmulated emission for the PASER effect the particle beam intensity is modulated at the frequency corresponding to the energy difference between the levels in which population inversion is achieved in the active medium. We propose to use solid-state active medium based on the Zeeman effect (triplet systems) for the PASER. Modulation of the beam at the frequency of the transition to obtain stimulated emission can be produced by means of a deflecting cavity. A transverse "beamlet" pattern will be produced on the AWA photocathode gun by using a laser mask. The transverse beam distribution will be transformed into a longitudinal beam modulation as the beam passes through the deflecting cavity. In this paper we report on the development of active media and the first RF bench test.

• F. Gao, W. Gai, W. Liu
  ANL, Argonne
• F. Gao, T. Wong
  Illinois Institute of Technology, Chicago, Illinois
• C.-J. Jing
  Euclid TechLabs, LLC, Solon, Ohio

We report on the design of a dual-frequency higher-order-mode dielectric-loaded power extraction set. This power extraction set consists of a dual-frequency dielectric-loaded decelerating structure (decelerator) and two changeable output couplers. In the decelerator, the TM02 mode synchronizes with an ultra-relativistic electron beam at 20.8GHz, and the TM03 mode synchronizes with the beam at 35.1GHz. These frequencies are both harmonics of 1.3GHz, the operating frequency of the electron gun and linac at the Argonne Wakefield Accelerator. The power generated in the unwanted TM01 mode is effectively suppressed for bunch train operation with a novel mode suppression technique. To extract power from the decelerator to standard rectangular waveguides, a TM02-TE10 output coupler was designed with S21 = -0.26dB at 20.8GHz, and a TM03-TE10 output coupler with S21 = -0.66dB at 35.1GHz. 90.4MW and 8.68MW rf power are expected to be extracted from a drive beam with charge of 50nC per bunch, at 20.8GHz and 35.1GHz respectively.