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Coherent multimoded dielectric wakefield accelerators.

Description: There has recently been a study of the potential uses of multimode dielectric structures for wakefield acceleration [1]. This technique is based on adjusting the wakefield modes of the structure to constructively interfere at certain delays with respect to the drive bunch, thus providing an accelerating gradient enhancement over single mode devices. In this report we examine and attempt to clarify the issues raised by this work in the light of the present state of the art in wakefield acceleration.
Date: July 16, 1998
Creator: Power, J.
Partner: UNT Libraries Government Documents Department

Measurements of the Argonne Wakefield Accelerator's low charge, 4 MeV RF photocathode witness beam.

Description: The Argonne Wakefield Accelerator's (AWA) witness RF photocathode gun produced its first electron beam in April of 1996. We have characterized the charge, energy, emittance and bunch length of the witness beam over the last several months. The emittance Was measured by both a quad scan that fitted for space charge using an in house developed Mathematica routine and a pepper pot technique. The bunch length was measured by imaging Cherenkov light from a quartz plate to a Hamamatsu streak camera with 2 psec resolution. A beam energy of 3.9 Mev was measured with a 6 inch round pole spectrometer while a beam charge was measured with both an ICT and a Faraday Cup. Although the gun will normally be run at 100 pC it has produced charges from 10 pC to 4 nc. All results of the measurements to date are presented here.
Date: April 1, 1998
Creator: Power, J.
Partner: UNT Libraries Government Documents Department


Description: A new current structure for the fast traveling-wave 2.5-MeV beam chopper in the front end of the Spallation Neutron Source (SNS) has been suggested in [1]. The structure is based on the meander-folded notched stripline with dielectric supports and separators. Its design has been optimized using electromagnetic 3-D modeling with the MAFIA code package to provide rise and fall times in the range of 1 to 2 ns. A full-length (50 cm) prototype has been manufactured, and its preliminary measurements showed a good agreement with the calculations. Detailed measurements results and their comparison with simulations are presented. The latest front-end design requires a shorter, 35-cm chopper with a higher pulse voltage. Its meander-line current structure, based on the same principles, has also been optimized with MAFIA.
Date: October 1, 2000
Creator: KURRENOY, S. & POWER, J.
Partner: UNT Libraries Government Documents Department

Impact of transverse irregularities at the photocathode on the production of high-charge electron bunches

Description: The properties of electron beams produced in a photoinjector are strongly dependent on the initial conditions, i.e. the photocathode drive laser shape and its uniformity. We explore the impact of well-defined transverse laser perturbations on the evolution of the electron beam both in configuration and velocity spaces and especially investigate how certain types of perturbations evolve as the beam propagates in the Argonne Wakefield Accelerator facility. Numerical simulations performed with IMPACT-T are presented. Finally preliminary experimental results aimed at validating our simulations are discussed.
Date: June 1, 2007
Creator: Rihaoui, M.; Bohn, C. L.; Piot, P. & Power, J. G.
Partner: UNT Libraries Government Documents Department

Dielectric Wakefield Accelerator to drive the future FEL Light Source.

Description: X-ray free-electron lasers (FELs) are expensive instruments and a large part of the cost of the entire facility is driven by the accelerator. Using a high-energy gain dielectric wake-field accelerator (DWA) instead of the conventional accelerator may provide a significant cost saving and reduction of the facility size. In this article, we investigate using a collinear dielectric wakefield accelerator to provide a high repetition rate, high current, high energy beam to drive a future FEL x-ray light source. As an initial case study, a {approx}100 MV/m loaded gradient, 850 GHz quartz dielectric based 2-stage, wakefield accelerator is proposed to generate a main electron beam of 8 GeV, 50 pC/bunch, {approx}1.2 kA of peak current, 10 x 10 kHz (10 beamlines) in just 100 meters with the fill factor and beam loading considered. This scheme provides 10 parallel main beams with one 100 kHz drive beam. A drive-to-main beam efficiency {approx}38.5% can be achieved with an advanced transformer ratio enhancement technique. rf power dissipation in the structure is only 5 W/cm{sup 2} in the high repetition rate, high gradient operation mode, which is in the range of advanced water cooling capability. Details of study presented in the article include the overall layout, the transform ratio enhancement scheme used to increase the drive to main beam efficiency, main wakefield linac design, cooling of the structure, etc.
Date: April 20, 2011
Creator: Jing, C.; Power, J. & Zholents, A.
Partner: UNT Libraries Government Documents Department

Limiting Effects in the Transverse-to-Longitudinal Emittance Exchange Technique for Low Energy Relativistic Electron Beams

Description: 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 {approx}12 MeV electron beam. In this paper we explore the limiting effects of this phase space manipulation method including high order and collective effects. A realistic start-to-end simulation of the planned proof-of-principle experiment including sensitivity studies is also presented.
Date: May 1, 2009
Creator: Rihaoui, M. M.; Piot, P.; Power, J. G. & Gai, W.
Partner: UNT Libraries Government Documents Department

Measurement and Simulation of Space Charge Effects in a Multi-Beam Electron Bunch from an RF Photoinjector

Description: 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 multibeam configuration to tailor the transverse distribution of an electron beam.
Date: May 1, 2009
Creator: Rihaoui, M. M.; Piot, P.; Power, J. G.; Yusof, Z. & Gai, W.
Partner: UNT Libraries Government Documents Department

Verification of the AWA Photoinjector Beam Parameters Required for a Transverse-to-Longitudinal Emittance Exchange Experiment

Description: A transverse-to-longitudinal emittance exchange experiment is in preparation at the Argonne Wakefield Accelerator (AWA). The experiment aims at exchanging a low ({var_epsilon}{sub z} < 5 {micro}m) longitudinal emittance with a large ({var_epsilon}{sub x} > 15 {micro}m) transverse horizontal emittance for a bunch charge of {approx}100 pC. Achieving such initial emittance partitioning, though demonstrated via numerical simulations, is a challenging task and needs to be experimentally verified. In this paper, we report preliminary emittance measurements of the beam in the transverse and longitudinal planes performed at {approx}12 MeV. The measurements are compared with numerical simulations.
Date: May 1, 2009
Creator: Rihaoui, M. M.; Piot, P.; Power, J. G.; Mihalcea, D. & Gai, W.
Partner: UNT Libraries Government Documents Department

Development of a fast traveling-wave beam chopper for the SNS project

Description: High current and stringent restrictions on beam losses, below 1 nA/m, in the designed linac for the Spallation Neutron Source (SNS) require clean and fast--with the rise time from 2% to 98% less than 2.5 ns--beam chopping in its front end, at the beam energy 2.5 MeV. The development of new traveling-wave deflecting current structures based on meander lines is discussed. Three-dimensional time-domain computer simulations with MAFIA are used to study transient effects in the chopper and to optimize current structure design. Two options for the fast pulsed voltage generator--based on FETs and vacuum tubes--are considered, and their advantages and shortcomings for the SNS chopper are discussed.
Date: December 31, 1998
Creator: Kurennoy, S.S. & Power, J.F.
Partner: UNT Libraries Government Documents Department

Characterization of beam position monitors for measurement of second moment

Description: A dual-axis beam position monitor (BPM) consists of four electrodes placed at 90{degree} intervals around the probe aperture. The response signals of these lobes can be expressed as a sum of moments. The first order moment gives the centroid of the beam. The second order moment contains information about the rms size of the beam. It has been shown previously that the second order moment can be used to determine beam emittance. To make this measurement, the authors must characterize the BPM appropriately. The approach to this problem is to use a pulsed wire test fixture. By using the principle of superposition, they can build up a diffuse beam by taking the signals from different wire positions and summing them. This is done two ways: first by physically moving a wire about the aperture and building individual distributions, and, second, by taking a two dimensional grid of wire positions versus signal and using a computer to interpolate between the grid points to get arbitrary wire positions and, therefore, distributions. The authors present the current results of this effort.
Date: May 1, 1995
Creator: Russell, S.J.; Gilpatrick, J.D.; Power, J.F. & Shurter, R.B.
Partner: UNT Libraries Government Documents Department

Transformer ratio enhancement using a ramped bunch train in a collinear wakefield accelerator.

Description: We present a practical method for achieving a transformer ratio (R) greater than 2 with any collinear wakefield accelerator--i.e. with either plasma or structure based wakefield accelerators. It is known that the transformer ratio cannot generally be greater than 2 for a symmetric drive bunch in a collinear wakefield accelerator. However, using a ramped bunch train (RBT) where a train of n electron drive bunches, with increasing (ramping) charge, one can achieve R = 2n after the bunch train. We believe this method is feasible from an engineering standpoint. We describe a proof of principle experiment using an disk-loaded waveguide, of frequency 13.65 GHz, driven by a RBT of 4 electron bunches. We expect to achieve R > 6 using 4 electron bunches. Details of the simulation and experimental design are presented.
Date: July 19, 2001
Creator: Power, J. G.; Gai, W.; Sun, X. & Kanareykin, A.
Partner: UNT Libraries Government Documents Department

Development of a fast traveling-wave beam chopper for the National Spallation Neutron Source

Description: High current and severe restrictions on beam losses, below 1 nA/m, in the designed linac for the National Spallation Neutron Source (NSNS) require clean and fast--with the rise time from 2% to 98% less than 2.5 ns to accommodate a 402.5-MHz beam structure--beam chopping in its front end, at the beam energy 2.5 MeV. The R and D program includes both modification of the existing LANSCE coax-plate chopper to reduce parasitic coupling between adjacent plates, and development of new traveling-wave deflecting structures, in particular, based on a meander line. Using analytical methods and three-dimensional time-domain computer simulations the authors study transient effects in such structures to choose an optimal chopper design.
Date: October 1, 1997
Creator: Kurennoy, S.S.; Jason, A.J.; Krawczyk, F.L. & Power, J.
Partner: UNT Libraries Government Documents Department

Design and construction of a high charge and high current 1-1/2 cell L-Band RF photocathode gun.

Description: The Argonne Wakefield Accelerator has been successfully commissioned and used for conducting wakefield experiments in dielectric loaded structures and plasmas. Although the initial wakefield experiments were successful, higher drive beam quality would substantially improve the wakefield accelerating gradients. In this paper we present a new 1-1/2 cell L-band photocathode RF gun design. This gun will produce 10-100 nC beam with 2-5 ps rms pulse length and normalized emittance less than 100 mm mrad. The final gun design and numerical simulations of the beam dynamics are presented.
Date: March 26, 1999
Creator: Conde, M. E.; Gai, W.; Konecny, R.; Power, J. G. & Schoessow, P.
Partner: UNT Libraries Government Documents Department

Wakefield excitation in multimode structures by a train of electron bunches.

Description: We discuss wakefield excitation and propagation in dielectric structures, particularly concentrating on the case of multiple drive beam excitation in multimode structures. We emphasize calculations of the energy loss of the drive bunch train, the amplitude of the wakefield, and the relation between power flow and stored energy in the dielectric wakefield device. We show that for a collinear multimode structure the amplitude of the wakefield generated by a bunch train is less than or equal to the wakefield generated by a single bunch of the same total charge. Furthermore, the transformer ratio, R, is shown to be always less than 2, even in the multiple drive beam case. Plans for an experiment to measure wakes in a multimode structure at AWA are presented.
Date: March 26, 1999
Creator: Conde, M. E.; Gai, W.; Konecny, R.; Power, J. G. & Schoessow, P.
Partner: UNT Libraries Government Documents Department

Witness gun for the Argonne Wakefield Accelerator

Description: The witness gun for the Argonne Wakefield Accelerator (AWA) is a six-cell, copper, iris loaded, rf photocathode operating at 1.3 GHz in a {pi}/2 standing wave mode. An intense drive beam (up to 100 nC {at} 20 psec FWHM) used in the AWA project to excite (i.e. drive) wakefields in at least two separate test devices: a dielectric loaded cylindrical waveguide and a plasma cell. In both cases a low charge, low emittance witness beam (0.1 nC charge, 1 {pi}mm-mrad 90% physical emittance) is required to probe (i.e. witness) the wakefields left behind by the drive beam. This paper will primarily discuss the recent progress in the construction of the witness gun, while also briefly summarizing the central design issues of the gun. A brief status report on the dielectric witness gun option is also included. The authors conclude with a short statement on the near term future plans.
Date: June 1, 1995
Creator: Power, J.; Simpson, J.; Chojnacki, E. & Konecny, R.
Partner: UNT Libraries Government Documents Department

Stability requirements of rf linac-driven free-electron lasers

Description: Fluctuations in the output power and wavelength have been observed in two high-power, rf-driven, free-electron lasers (FELs): the 10-/mu/m FEL that has operated at the Los Alamos National Laboratory and the visible FEL at the Boeing Physical Sciences Research Center. The fluctuations have been traced primarily to instabilities in the electron beam. Specifically, these are variations in the electron energy, the charge per micropulse, and the time interval between micropulses. The effects of these instabilities on the performance of one of the FELs is demonstrated. Efforts made to minimize these instabilities are discussed and the subsequent improvements in the operation of each of the FELs are presented. 7 refs., 7 figs.
Date: January 1, 1989
Creator: Stein, W.E.; Johnson, W.J.D.; Power, J.F. & Russel, T.J.
Partner: UNT Libraries Government Documents Department

Proposed RF Breakdown Studies at the AWA

Description: A study of breakdown mechanism has been initiated at the Argonne Wakefield Accelerator (AWA). Breakdown may include several factors such as local field enhancement, explosive electron emission, Ohmic heating, tensile stress produced by electric field, and others. The AWA is building a dedicated facility to test various models for breakdown mechanisms and to determine the roles of different factors in the breakdown. We plan to trigger breakdown events with a high-powered laser at various wavelengths (IR to UV) to determine the role of explosive electron emission in the breakdown process. Another experimental idea follows from the recent work on a Schottky-enabled photoemission in an RF photoinjector [1] that allows us to determine in situ the field enhancement factor on a cathode surface. Monitoring the field enhancement factor before and after the breakdown can shed some light on a number of observations such as the crater formation process.
Date: March 21, 2007
Creator: Antipov, S.; Conde, M.; Gai, W.; Power, J. G.; Spentzouris, L.; Yusof, Z. et al.
Partner: UNT Libraries Government Documents Department

Magnet design concepts for the 100 MeV isotope production facility

Description: The North Port Target Facility proposal includes a 100 MeV beam line to be is built at the LANSCE accelerator. In developing cost and schedule estimates for the proposal the greatest uncertainties are associated with the kicker magnet that is needed to divert portions of the beam into the new beam line. This magnet must fit into a rigidly defined space within the transition region of the existing accelerator and must operate in synchrony with the current accelerator operations systems, In addition, it must not degrade the beam qualify when beam is directed to other areas of the complex. Because of these constraints the magnet must be specifically designed and built for this intended application. We have produced conceptual designs of a kicker-magnet and power supply that will meet all of the design requirements. The power supply design is based on a working design for the RIKI kicker magnet that switches 800 MeV beam into the PSR. This report presents the kicker-magnet and power-supply designs and cost and schedule estimates for incorporation into the EP facility proposal. The feasibility of various design alternatives are briefly discussed.
Date: October 7, 1996
Creator: Wadlinger, E.A.; Merrill, F.E. & Power, J.F.
Partner: UNT Libraries Government Documents Department