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Particle-In-Cell/Monte Carlo Simulation of Ion Back Bombardment in Photoinjectors

Description: In this paper, we report on studies of ion back bombardment in high average current dc and rf photoinjectors using a particle-in-cell/Monte Carlo method. Using H{sub 2} ion as an example, we observed that the ion density and energy deposition on the photocathode in rf guns are order of magnitude lower than that in a dc gun. A higher rf frequency helps mitigate the ion back bombardment of the cathode in rf guns.
Date: March 2, 2009
Creator: Qiang, Ji; Corlett, John & Staples, John
Partner: UNT Libraries Government Documents Department

Engineering Design and Fabrication of an Ampere-Class Superconducting Photocathode Electron Gun

Description: Over the past three years, Advanced Energy Systems and Brookhaven National Laboratory (BNL) have been collaborating on the design of an Ampere- class superconducting photocathode electron gun. BNL performed the physics design of the overall system and RF cavity under prior programs. Advanced Energy Systems (AES) is currently responsible for the engineering design and fabrication of the electron gun under contract to BNL. We will report on the engineering design and fabrication status of the superconducting photocathode electron gun. The overall configuration of the cryomodule will be reviewed. The layout of the hermitic string, space frame, shielding package, and cold mass will be discussed. The engineering design of the gun cavity and removable cathode will be presented in detail and areas of technical risk will be highlighted. Finally, the fabrication sequence and fabrication status of the gun cavity will be discussed.
Date: November 17, 2008
Creator: Ben-Zvi,I.
Partner: UNT Libraries Government Documents Department

Experimental investigation of the longitudinal beam dynamics in a photoinjector using a two-macroparticle bunch

Description: We have developed a two-macroparticle bunch to explore the longitudinal beam dynamics through various components of the Fermilab/NICADD photoinjector. Such a two-macroparticle bunch is generated by splitting the ultraviolet pulse from the photocathode drive laser. The presented method allows the exploration of radiofrequency-induced compression in the 1.625 cell rf-gun and the booster cavity. It also allows a direct measurement of the momentum compaction of the magnetic bunch compressor. The measurements are compared with analytical and numerical models.
Date: March 1, 2006
Creator: Piot, P.; Tikhoplav, R.; Mihalcea, D. & Barov, N.
Partner: UNT Libraries Government Documents Department

Experience at Fermilab with high quantum efficiency photo-cathodes for rf electron guns

Description: As part of the A0 Photo-injector collaboration at Fermi-lab [1, 2] and the TeSLA collaboration [3], a high bright-ness, low emittance electron source has been developed. In the process, a system was constructed for coating molybde-num cathodes with a layer of c├Žsium telluride (Cs2 Te), a photo-emissive material of high quantum efficiency (QE). The use of Cs2 Te was first investigated at CERN [4] and LANL [5]. The development of the systems for the TeSLA Test Facility Linac and the Fermilab Photo-injector was done in Milano [6]. The system at Fermilab incorporates manipulator arms to transfer a cathode from the preparation chamber into a 1.3 GHz photo-electron RF gun while it re-mains in an ultra-high vacuum (UHV) environment, in or-der to avoid the deleterious effects of residual gases on the QE. A first prototype electron gun has been operated with a photo-cathode for several months [1]. This paper describes preliminary results obtained with the first 2 photo-cathodes and the first gun. Some of the desired parameters for the TeSLA Test Fa-cility beam are given in Table 1. The desired characteristics for the photo-cathodes include (i) high QE, (ii) high current density (>500 A/cm{sup 2} ), (iii) long lifetime, and (iv) low field emission. The choice of Cs2 Te is a compromise between long lifetime, rugged metal cathodes with low QE (typi-cally between 10{sup -6} and 10{sup -4} and semiconductor cathodes with high QE (>10%), which generally have a short life-time because of their sensitivity to contamination.
Date: October 1998
Creator: Fry, A.; Hahn, E.; Hartung, W.; Kuchnir, M.; Michelato, P. & Sertore, D.
Partner: UNT Libraries Government Documents Department

Edge reclamation in position-sensitive photomultiplier tubes

Description: We have investigated the performance of a position-sensitive, gamma-ray detector based on a CsI(Na) scintillator coupled to a Hamamatsu R3292 Position-Sensitive Photomultiplier Tube (PSPMT). The R3292 has an active area 10.0 cm in diameter (quoted). Utilization of the full active area of the photocathode is a goal that has been previously unrealized due to edge effects. Initial measurements with a 0.75 cm thick CsI(Na) crystal indicate that the performance (position resolution linearity) starts to degrade as one reaches a radius of only 3.5 cm, reducing the active area by 60%. Measuring the anode wires we have found that this fall off is not solely due to crystal edge effects, but rather is inherent to the tube crystal system. In this paper we describe the results of our measurements and how good performance can be maintained across a full 10cm of the tube face through the use of a few additional electronics channels.
Date: June 16, 1999
Creator: Nakae, L & Ziock, K
Partner: UNT Libraries Government Documents Department

Surface and bulk contribution to Cu(111) quantum efficiency

Description: The quantum efficiency (QE) of Cu(111) is measured for different impinging light angles with photon energies just above the work function. We observe that the vectorial photoelectric effect, an enhancement of the QE due to illumination with light with an electric vector perpendicular to the sample surface, is stronger in the more surface sensitive regime. This can be explained by a contribution to photoemission due to the variation in the electromagnetic potential at the surface. The contributions of bulk and surface electrons can then be determined.
Date: November 4, 2008
Creator: Pedersoli, Emanuele; Greaves, Corin Michael Ricardo; Wan, Weishi; Coleman-Smith, Christopher; Padmore, Howard A.; Pagliara, Stefania et al.
Partner: UNT Libraries Government Documents Department

QE Tests with Nb-Pb SRF Photoinjector and Arc Deposited Cathodes

Description: In this contribution, we report Quantum Efficiency (QE) test results with a hybrid lead/niobium superconducting RF (SRF) photoinjector at 2K and new Pb arc deposited cathodes at 300K. The ultimate goal of our effort is to build a Nb injector with the superconducting cathode made of lead, which, as reported in the past, demonstrated superior QE compared to other metallic superconducting elements. At first, we present the test results obtained with a 1.6-cell high purity Nb cavity with the emitting lead spot in the center of the back plate. The QE test results at room temperature and the SEM surface analysis of eight Pb cathodes, deposited recently under various conditions, are discussed in the second part of this contribution.
Date: May 1, 2010
Creator: J.K. Sekutowicz, P. Kneisel, R. Nietubyc, T. Rao, J. Smedley
Partner: UNT Libraries Government Documents Department

Charge and fluence lifetime measurements of a dc high voltage GaAs photogun at high average current

Description: GaAs-based dc high voltage photoguns used at accelerators with extensive user programs must exhibit long photocathode operating lifetime. Achieving this goal represents a significant challenge for proposed high average current facilities that must operate at tens of milliamperes or more. This paper describes techniques to maintain good vacuum while delivering beam, and techniques that minimize the ill effects of ion bombardment, the dominant mechanism that reduces photocathode yield of a GaAs-based dc high voltage photogun. Experimental results presented here demonstrate enhanced lifetime at high beam currents by: (a) operating with the drive laser beam positioned away from the electrostatic center of the photocathode, (b) limiting the photocathode active area to eliminate photoemission from regions of the photocathode that do not support efficient beam delivery, (c) using a large drive laser beam to distribute ion damage over a larger area, and (d) by applying a relatively low bias voltage to the anode to repel ions created within the downstream beam line. A combination of these techniques provided the best total charge extracted lifetimes in excess of 1000 C at dc beam currents up to 9.5 mA, using green light illumination of bulk GaAs inside a 100 kV photogun.
Date: April 1, 2011
Creator: J. Grames, R. Suleiman, P.A. Adderley, J. Clark, J. Hansknecht, D. Machie, M. Poelker, M.L. Stutzman
Partner: UNT Libraries Government Documents Department

Survey of SRF guns

Description: Developing Superconducting RF (SRF) electron guns is an active field with several laboratories working on different gun designs. While the first guns were based on elliptic cavity geometries, Quarter Wave Resonator (QWR) option is gaining popularity. QWRs are especially well suited for producing beams with high charge per bunch. In this talk we will describe recent progress in developing both types of SRF guns. SRF guns made excellent progress in the last two years. Several guns generated beams and one, at HZDR, injected beam into an accelerator. By accomplishing this, HZDR/ELBE gun demonstrated feasibility of the SRF gun concept with a normal-conducting Cs{sub 2}Te cathode. The cathode demonstrated very good performance with the lifetime of {approx}1 year. However, for high average current/high bunch charge operation CsK{sub 2}Sb is preferred as it needs green lasers, unlike UV laser for the Cs{sub 2}Te, which makes it easier to build laser/optics systems. Other high QE photocathodes are being developed for SRF guns, most notably diamond-amplified photocathode. Several QWR guns are under development with one producing beam already. They are very promising for high bunch charge operation. The field is very active and we should expect more good results soon.
Date: July 25, 2011
Creator: Belomestnykh, S.
Partner: UNT Libraries Government Documents Department

ION TRAPPING AND CATHODE BOMBARDMENT BY TRAPPED IONS IN DC PHOTOGUNS.

Description: DC photoguns are used to produce high-quality, high-intensity electron beams for accelerator driven applications. Ion bombardment is believed to be the major cause of degradation of the photocathode efficiency. Additionally to ions produced in the accelerating cathode-anode gap, the electron beam can ionize the residual gas in the transport line. These ions are trapped transversely within the beam and can drift back to the accelerating gap and contribute to the bombardment rate of the cathode. This paper proposes a method to reduce the flow of ions produced in the beam transport line and drifting back to the cathode-anode gap by introducing a positive potential barrier that repels the trapped ions. The reduced ion bombardment rate and increased life time of photocathodes will reduce the downtime required to service photoinjectors and associated costs.
Date: June 25, 2007
Creator: POZDEYEV,E.
Partner: UNT Libraries Government Documents Department

Design of a VHF-band RF Photoinjector with Megahertz BeamRepetition Rate

Description: New generation accelerator-based X-ray light sources require high quality beams with high average brightness. Normal conducting L- and S-band photoinjectors are limited in repetition rate and D-C (photo)injectors are limited in field strength at the cathode. We propose a low frequency normal-conducting cavity, operating at 50 to 100MHz CW, to provide beam bunches of up to the cavity frequency. The photoinjector uses a re-entrant cavity structure, requiring less than 100 kW CW, with a peak wall power density less than 10 W/cm{sup 2}. The cavity will support a vacuum down to 10 picoTorr, with a load-lock mechanism for easy replacement of photocathodes. The photocathode can be embedded in a magnetic field to provide correlations useful for emittance exchange. Beam dynamics simulations indicate that normalized emittances smaller than 1 mm-mrad are possible with gap voltage of 750 kV, with fields up to 20 MV/m at the photocathode, for 1 nanocoulomb charge per bunch after acceleration and emittance compensation. Long-bunch operation (10's of picosecond) is made possible by the low cavity frequency, permitting low bunch current at the 750 kV gap voltage.
Date: June 1, 2007
Creator: Staples, J.W.; Baptiste, K.M.; Corlett, J.N.; Kwiatkowski, S.; Lidia, S.M.; Qiang, J. et al.
Partner: UNT Libraries Government Documents Department

CONCEPTUAL DESIGN OF A BRIGHT ELECTRON INJECTOR BASED ON A LASER-DRIVEN PHOTOCATHODE RF ELECTRON GUN

Description: Conceptual design of a bright electron injector for the 1 GeV high gradient test experiment. envisaged by the LLNL-SLAC-LBL collaboration on the Relativistic Klystron is presented. The design utilizes a high-brightness laser-driven RF photocathode electron gun, similar to the pioneering LANL early studies in concept (different parametrically however), together with achromatic magnetic bunching and transport systems and diagnostics. The design is performed with attention to possible use in an FEL as well. A simple but realistic analytic model including longitudinal and transverse space-charge and RF effects and extensive computer simulation form the basis of the parametric choice for the source. These parameters are used as guides for the design of the pico-second laser system and magnetic bunching section.
Date: September 1, 1988
Creator: Chattopadhyay, S.; Chen, Y.J.; Hopkins, D.; Kim, K.J.; Kung, A.; Miller, R. et al.
Partner: UNT Libraries Government Documents Department

A Masked Photocathode in Photoinjector

Description: In this paper, we propose a masked photocathode inside the photoinjector for generating high brightness election beam. Instead of mounting the photocathode onto an electrode, an electrode with small hole is used as a mask to shield the photocathode from the accelerating vacuum chamber. Using such a masked photocathode will make the replacement of photocathode material easy by rotating the photocathode behind the electrode into the hole. Furthermore, this helps reduce the dark current or secondary electron emission from the photocathode material. The masked photocathode also provides transverse cut-off to a Gaussian laser beam that reduces electron beam emittance growth from nonlinear space-charge effects.
Date: December 14, 2010
Creator: Qiang, Ji
Partner: UNT Libraries Government Documents Department

Applications of Laser and Synchrotron Based ARPES to Photocathode Research

Description: Laser angle resolved photoelectron spectroscopy (ARPES) provides unique information about angle and energy distribution of photoelectrons. Laser ARPES gives unique insight into how NEA materials work. ARPES combined with some ancillary measurements gives a very complete picture of system electronic physics. For H:C[100] there is now a clear program for engineering as well as development analogous systems. ARPES well suited for identifying 'ideal' photocathodes with intrinsically low emittance and high QE.
Date: October 12, 2010
Creator: J., Rameau; J., Smedley; Muller, E.; Kidd, T.; Johnson, P.; Allen, P. et al.
Partner: UNT Libraries Government Documents Department

Observation of transverse space charge effects in a multi-beamlet electron bunch produced in a photo-emission electron source

Description: A 'multiple beamlet' experiment aimed at investigating the transverse space charge effect was recently conducted at the Argonne Wakefield Accelerator. The experiment generated a symmetric pattern of 5 beamlets on the photocathode of the RF gun with the drive laser. We explored the evolution of the thereby produced 5 MeV, space-charge dominated electron beamlets in the 2m drift following the RF photocathode gun for various external focusing. Two important effects were observed and benchmarked using the particle-in-cell beam dynamics code IMPACT-T. In this paper, we present our experimental observation and their benchmarking with Impact-T.
Date: September 1, 2008
Creator: Rihaoui, M.; /Northern Illinois U. /NICADD, DeKalb; Gai, W.; /Argonne; Piot, P.; /Northern Illinois U. /NICADD, DeKalb /FERMILAB et al.
Partner: UNT Libraries Government Documents Department

In-Situ Cleaning of Metal Photo-Cathodes in rf Guns

Description: Metal cathodes installed in rf guns typically exhibit much lower quantum efficiency than the theoretical limit. Experimenters often use some sort of in situ technique to ''clean'' the cathode to improve the QE. The most common technique is laser cleaning where the laser is focused to a small spot and scanned across the cathode surface. However, since the laser is operated near the damage threshold, it can also damage the cathode and increase the dark current. The QE also degrades over days and must be cleaned regularly. We are searching for a more robust cleaning technique that cleans the entire cathode surface simultaneously. In this paper we describe initial results using multiple techniques such as several keV ion beams, glow discharge cleaning and back bombarding electrons. Results are quantified in terms of the change in QE and dark current.
Date: January 3, 2007
Creator: Schmerge, J.F.; Castro, J.M.; Clendenin, J.E.; Colby, E.R.; Dowel, D.H.; Gierman, S.M. et al.
Partner: UNT Libraries Government Documents Department

Particle-In-Cell/Monte Carlo Simulation of Ion Back BomBardment in a High Average Current RF Photo-Gun

Description: In this paper, we report on study of ion back bombardment in a high average current radio-frequency (RF) photo-gun using a particle-in-cell/Monte Carlo simulation method. Using this method, we systematically studied effects of gas pressure, RF frequency, RF initial phase, electric field profile, magnetic field, laser repetition rate, different ion species on ion particle line density distribution, kinetic energy spectrum, and ion power line density distribution back bombardment onto the photocathode. Those simulation results suggested that effects of ion back bombardment could increase linearly with the background gas pressure and laser repetition rate. The RF frequency has significantly affected the ion motion inside the gun so that the ion power deposition on the photocathode in an RF gun can be several orders of magnitude lower than that in a DC gun. The ion back bombardment can be minimized by appropriately choosing the electric field profile and the initial phase.
Date: October 17, 2009
Creator: Qiang, J.
Partner: UNT Libraries Government Documents Department

A Spin Manipulator for Electron Accelerators

Description: We have designed and constructed a novel optical system capable of manipulating the orientation of the polarization direction. vector P, of a 100 keV beam of polarized electrons relative to the momentum vector, k, in an arbitrary manner. This spin manipulator is fully compatible with the UHV requirements of the photocathode sources that are typically used for accelerator-based experiments involving polarized electrons. We describe the design and operation of the system and its components, and document its performance.
Date: June 1, 1992
Creator: Dunham, Bruce; Sinclair, Charles; Engwall, David; Heddle, David & Cardman, Lawrence
Partner: UNT Libraries Government Documents Department

Simulations And Experiments of Electron Beams Pre-Modulated at the Photocathode

Description: The University of Maryland and the Source Development Laboratory at Brookhaven National Laboratory have been collaborating on a project that explores the use of electron beam pre-modulation at the cathode to control the longitudinal structure of the electron beam. This technique could be applied to creating deliberate modulations which can lead to the generation of terahertz radiation, or creating a smooth profile in order to suppress radiation. This paper focuses on simulations that explore some of the pre-modulated cases achieved experimentally.
Date: February 15, 2006
Creator: Neumann, J.G.; Fiorito, R.B.; O'Shea, P.G.; U., /Maryland; Carr, G.L.; Shaftan, T.V. et al.
Partner: UNT Libraries Government Documents Department

MAPMT H7546B anode current response study for ILC SiD muon system prototype

Description: The proposed Silicon Detector (SiD) concept for the ILC has barrel and end cap muon systems. An SiD scintillator based muon system prototype has 256 strips and was constructed from extruded strips, WLS fibers, clear fibers, and multianode photo multiplier tubes (MAPMT) Hamamatsu H7546B. Six MAPMTs were used. As a first step to understand strip output, the response of every anode to a given brightness of light and applied voltage must be measured. For the test, a custom made light source was used. Each MAPMT output was measured independently. The anode currents were measured at constant (green) input light brightness and the same photocathode to anode voltage (800V). The anode currents have a wide spread; for all tubes the maximum value is 5.23 times larger than the minimum value. The MAPMT cross talk was measured for one of the central inputs. The maximum cross talk value is about 4.9%. The average cross talk for the nearest four neighboring channels is 3.9%, for the farthest four is 1%. To assure the reproducibility and repeatability of the measurements, the double reference method was used.
Date: October 1, 2007
Creator: Dyshkant, A.; Blazey, G.; Francis, K.; Hedin, D.; Zutshi, V.; U., /Northern Illinois et al.
Partner: UNT Libraries Government Documents Department