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We have built a high-DC-voltage photoemission gun and a diagnostic
beamline permitting us to measure rms transverse emittance
(ε˜<sub>x</sub>) and rms momentum spread (δ) of
short-duration electron pulses produced by illuminating the cathode with
light from a mode-locked, frequency-doubled Nd:YLF laser. The electron
gun is a GaAs photocathode...
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Citations
... At Jefferson Lab, an inverted-geometry ceramic insulator approach has been adopted, where the term " inverted " describes an insulator that extends into the vacuum chamber serving as the cathode electrode support structure [4]. This represents an alternative to designs that employ large cylindrical insulators with long metal electrode support tubes passing through the insulator bore [5][6][7]. The inverted-insulator design helps to reduce field emission because there is considerably less metal biased at high voltage. ...
Jefferson Lab is constructing a 350 kV direct current high voltage photoemission gun employing a compact inverted-geometry insulator. This photogun will produce polarized electron beams at an injector test facility intended for low energy nuclear physics experiments, and to assist the development of new technology for the Continuous Electron Beam Accelerator Facility. A photogun operating at 350kV bias voltage reduces the complexity of the injector design, by eliminating the need for a graded-beta radio frequency "capture" section employed to boost lower voltage beams to relativistic speed. However, reliable photogun operation at 350 kV necessitates solving serious high voltage problems related to breakdown and field emission. This study focuses on developing effective methods to avoid breakdown at the interface between the insulator and the commercial high voltage cable that connects the photogun to the high voltage power supply. Three types of inverted insulators were tested, in combination with two electrode configurations. Our results indicate that tailoring the conductivity of the insulator material, and/or adding a cathode triple-junction screening electrode, effectively serves to increase the hold-off voltage from 300kV to more than 375kV. Electrostatic field maps suggest these configurations serve to produce a more uniform potential gradient across the insulator.
... The size of the beam emittance will be determined at three different locations using three distinct methods. In a set of gun characterization experiments [5], a variant of the " two slit " method is used. After the beam is accelerated to 10 MeV, the large space charge in the beam makes envelope measurements questionable. ...
In this paper the current plans for the diagnostic complement for
Jefferson Lab's IRFEL are presented. Diagnostic devices include optical
transition radiation beam viewers, both stripline and button beam
position monitors, multislit beam emittance measuring devices, coherent
synchrotron and transition radiation bunch length monitoring devices,
and synchrotron light cameras for measuring the beam profile at high
average power. Most devices have update rates of order 1 sec or shorter,
and all are controlled through an EPICS control system
... To fully characterize the high-DC-voltage photocathode gun [1] for the FEL injector at the Jefferson Lab, transverse emittance measurements covering a wide range of bunch charges are planned. Such measurements are necessary both to verify that the gun meets the specifications as well as to quantify agreement between the measurements and particle simulations using PARMELA [2]. ...
... Such measurements are necessary both to verify that the gun meets the specifications as well as to quantify agreement between the measurements and particle simulations using PARMELA [2]. In this paper we will describe the emittance measurement system and its performance, while the data is described in another paper [1]. ...
... Here, we sample the beam using a movable copper disk with a ∼50 µm rectangular slot cut in it. The sampled portion of the beam traverses a drift distance L until it reaches a scanning wire device (see figures 1 and 2; see [1] for more details). The wire scanner (or harp) is the standard design used at the Jefferson Lab consisting of a 50 µm tungsten wire supported on a light-weight fork which is attached to a stepper motor through a bellows. ...
As a part of the emittance measurements planned for the FEL
injector at the Thomas Jefferson National Accelerator Facility
(Jefferson Lab.), we have developed an emittance measurement system that
covers the wide dynamic range of bunch charges necessary to fully
characterize the high-DC-voltage photocathode gun. The measurements are
carried out with a variant of the classical two-slit method using a slit
to sample the beam in conjunction with a wire scanner to measure the
transmitted beam profile. The use of commercial, ultralow noise
picoammeters makes it possible to cover the wide range of desired bunch
charges, with the actual measurements made over the range of 0.25 pC to
125 pC. The entire system, including its integration into the EPICS
control system, is discussed
... NEA photocathodes have two broad areas of application as electron sources for accelerators. They are at the heart of essentially all polarized electron sources in use at the present, and they provide a high quantum efficiency photocathode for the generation of high brightness beams which may be modulated at very high frequencies [1]. These cathodes are formed on atomically clean surfaces of direct bandgap semiconductors by the addition of cesium and an oxidant, typically oxygen or nitrogen trifluoride. ...
Negative electron affinity (NEA) semiconductor photocathodes are
widely used for the production of polarized electron beams, and are also
useful for the production of high brightness electron beams which can be
modulated at very high frequencies. Preparation of an atomically clean
semiconductor surface is an essential step in the fabrication of a NEA
photocathode. This cleaning step is difficult for certain
semiconductors, such as the very thin materials which produce the
highest beam polarization, and those which have tightly bound oxides and
carbides. Using a small RF dissociation atomic hydrogen source, we have
reproducibly cleaned GaAs wafers which have been only degreased prior to
installation in vacuum. We have consistently prepared very high quantum
efficiency photocathodes following atomic hydrogen cleaning. Details of
our apparatus and most recent results are presented
... The injector is the critical technology for operation of systems such as this; it must produce high average currents at high brightness. Although ultimately a srf photocathode gun such as under development in Dresden [17] is believed to be the most desirable, this system utilizes a DC photocathode operating at 320 kV to produce a 37.4 MHz pulse train of 60 pC [18]. The 20 ps beam is bunched by a copper fundamental cavity to around 3 ps and accelerated to 9.5 MeV in a srf cavity pair operting at 1497 MHZ. ...
High power Free Electron Lasers (FELs) have been an elusive promise since the development of FEL oscillators over two decades ago. Limitations in duty factor, ability to carry high brightness beam, and the high cost of RF wall losses has stymied progress in room temperature accelerator systems. The application of SRF technology has now permitted two orders of magnitude increase in FEL average power and at the same time shown that high quality FEL beam can be produced by the first demonstration of lasing at the 5th harmonic of the fundamental. A concurrent key technical development which leverages the high efficiency of SRF linacs was the demonstration of beam energy recovery while lasing. This leads to high overall efficiency, especially in high average power systems. This paper will discuss the issues relating to SRF use in high average power FELs and present the results of the IR Demo FEL at Jefferson Lab demonstrating the sizeable advantages that SRF offers for kilowatt average power output.
The JLab IR Upgrade Injector has delivered up to 9.1 mA of CW electron beam current at 9 MeV. The injector is driven by a 350 kV DC Photocathode Gun. Injector behavior and beam-based measurements are in good agreement with PARMELA simulations. The injected beam envelopes were established by measuring beam spot sizes and comparing them with those predicted by a transpart matrix based model. The emittances were measured by fitting an initial trial beam matrix to the measured data. The injected bunch length was established by measuring the energy spread downstream of the Linac while operating at either side of crest.
The performance of the 320kV DC photocathode gun has met the design specifications for the 1kW IR Demo FEL at Jefferson Lab. This gun has shown the ability to deliver high average current beam with outstanding lifetimes. The GaAs photocathode has delivered 135pC per bunch, at a bunch repetition rate of 37.425MHz, corresponding to 5mA average CW current. In a recent cathode lifetime measurement, 20h of CW beam was delivered with an average current of 3.1mA and 211C of total charge from a 0.283cm2 illuminated spot. The cathode showed a 1/e lifetime of 58h and a1/e extracted charge lifetime of 618C. We have achieved quantum efficiencies of 5% from a GaAs wafer that has been in service for 13 months delivering in excess 2400C with only three activation cycles.
Degradation of the photocathode materials employed in photoinjectors represents a challenge for sustained operation of nuclear physics accelerators and high power Free Electron Lasers (FEL). Several photocathode degradation processes are suspected, including defect formation by ion back bombardment, photochemistry of surface adsorbed species and irradiation-induced surface defect formation. To better understand the mechanisms of photocathode degradation, we have conducted surface and bulk analysis studies of two GaAs photocathodes removed from the FEL photoinjector after delivering electron beam for a few years. The analysis techniques include Helium Ion Microscopy (HIM), Rutherford Backscattering Spectrometry (RBS), Atomic Force Microscopy (AFM) and Secondary Ion Mass Spectrometry (SIMS). In addition, strained super-lattice GaAs photocathode samples, removed from the CEBAF photoinjector were analyzed using Transmission Electron Microscopy (TEM) and SIMS. This analysis of photocathode degradation during nominal photoinjector operating conditions represents first steps towards developing robust new photocathode designs necessary for generating sub-micron emittance electron beams required for both fourth generation light sources and intense polarized CW electron beams for nuclear and high energy physics facilities.
Jefferson Lab is building a free-electron laser (FEL) to produce
continuous-wave (cw), kW-level light at 3-6 μm wavelength. A
superconducting linac will drive the laser, generating a 5 mA average
current, 42 MeV energy electron beam. A transport lattice will
recirculate the beam back to the linac for deceleration and conversion
of about 75% of its power into rf power. Bunch charge will range up to
135 pC, and bunch lengths will range down to 1 ps in parts of the
transport lattice. Accordingly, the space charge in the injector and
coherent synchrotron radiation in magnetic bends come into play. The
machine will thus enable studying these phenomena as a precursor to
designing compact accelerators of high-brightness beams. The FEL is
scheduled to be installed in its own facility by 1 October 1997. Given
the short schedule, the machine design is conservative, based on
modifications of the CEBAF cryomodule and MIT-Bates transport lattice.
This paper surveys the machine design
