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RHIC Accelerator R&D DivisionHigh-quantum-efficiency photocathode groupThe group is headed by John Skaritka. We are developing a variety of high QE photocathodes, including a CW 50 mA polarized electron gun for eRHIC.
EIC LDRD Projects EIC Polarized Electron Gun, PI: Ilan Ben-Zvi The objective of this program is to test the feasibility of a multicathode DC gun that can deliver ~ 50 mA of polarized electrons. The questions to be addressed are: 1) Achievement of good vacuum in this complex, tight geometry. 2) Insertion and activation of the individual gallium arsenide cathode. 3) Questions of isolation of the near-by cathodes, to prevent cross-interference. 4) Question of jitter caused by the combination of multiple beams. The critical elements are the multicathode-anode assembly that can operate at XHV vacuum even during the electron beam generation and the bender-combiner pair that would bring the beamlets together without compromising their polarization and quality. Multicathode gun: The mechanical design of the multi-cathode gun and the cathode preparation system has been completed. The multi-cathode system would contain 20 cathode-anode elements capable of maintaining 10-12 Torr vacuum and able to support 200 kV bias on the cathode. Vacuum windows are incorporated for the injection of the laser beam for photoemission. This vacuum chamber is connected to the cathode preparation chamber where fresh cathodes are prepared. Spent cathodes in the gun can be replaced with fresh ones in situ without breaking the vacuum in either of the systems, using a load-lock assembly. The 3-D designs of the multi-cathode gun and cathode preparation chamber are shown in Figures 1 and 2 respectively. Procurement of components is underway. A list of items already ordered and the status are shown in the attached spread sheet. One student has already successfully activated a number of bulk GaAs cathodes in an existing XHV chamber. Another student is currently evaluating different methods to generate ultra pure oxygen for activating the sample. The electron beam transport including the bending and combining magnets have been simulated in 2-D. The bend angles are being optimized and the current optical lay out is shown in Figure 3. With a simplified, straight line transport, emittance of a 4 nC beam of 3.2 mm length has been calculated to be ~ 25 mm-mrad for the full beam and ~ 15 mm- mrad for 73% of the beam at ~ 10 MeV. The corresponding longitudinal emittance is ~ 112 mm keV and energy spread is 0.83% . The combiner that can provide the high field uniformity required is being designed. The beam parameters and the characteristics of the bending and combining elements are being optimized further using 3-D codes. Development of a laser system for driving the photocathode of the polarized electron source for the EIC, Triveni Rao and Brian Sheehy Two crucial tests for the feasibility of the Gatling gun are the successful combining of the beams from multiple cathodes and the acceptable life time of the cathodes delivering ~ 2 mA individually and ~ 50 mA combined. In order to deliver the electron beam for the EIC, the ultimate laser system would be operating at 780 nm with an average power of 2 w/mA and pulse duration of 0.5-1 ns. In the laser LDRD project, the objective is to develop/procure a system that can drive 2 of the Gatling Gun cathodes such that the gun concept and the impact of the operation of one cathode on the other can be determined.
In the R&D phase of the Gatling gun development, we require a laser capable
of delivering the full 2 mA current required of an individual cathode, and
the ability to demonstrate simultaneous operation of two cathodes. However,
in the full implementation, 20 cathodes must be driven synchronously, so the
laser solution must be scalable, modular, robust, and easily synchronized.
For this reason we have chosen to work with a fiber-based master
oscillator power amplifier configuration.
The system is shown schematically in
the figure below. The
oscillator is a CW distributed feedback (DFB) laser operating at 1560 nm,
which is electro-optically modulated (EOM) to produce 1.2-1.7 nsec long
pulses. The synchronization of
the optical pulse train is then accomplished through the simpler task of
synchronizing the RF pulse train driving the EOM (the jitter in the
subsequent optical stages is negligible).
The 704 kHz optical pulse train is then amplified to 10W (14 uJ/pulse)
in the amplifier chain.
The linearly-polarized 1560 nm wavelength light is then
frequency-doubled in periodically-poled Lithium Niobate to produce 4 Watts
at a wavelength of 780 nm. The pulse width may be adjusted via the pulser
driving the EOM; in the current design, it will vary between 1.2 and 1.7
nsec, but this will likely be shortened in later stages.
Figure 1. Schematic of Gatling gun laser operation principle. A 1560 nm laser (CW DFB) is modulated synchronously with a subharmonic of the accelerating RF. After amplification in an Erbium-doped fiber amplifier chain (EDFA), the light is frequency doubled in periodically poled Lithium Niobate. The output will be 4 Watts at 704 kHz (5.4 uJ/pulse), with an adjustable pulse width of 1.2-1.7 nsec.
The laser is being constructed in collaboration with Optilab LLC of Phoenix
AZ, and Covesion Ltd, of Hampshire, UK. The
unit is undergoing final testing now and should be delivered in December
2011. Photographs of the 1560
nm laser units are shown in Figure 2, below, and the frequency doubling
module is shown in Figure 3.
The low-power 1560 nm pulse train has been separated into a separate module
to allow for later development.
Note that the laser itself is a very compact rack-mounted device.
The frequency doubling is currently done with a free space coupling;
while the module has an acceptably small footprint, we hope in the future to
develop a fiber coupling, which will further enhance the compactness of the
design and its scalability for the operation of the full 20-cathode gun.
Figure 2. As-built 1560 nm laser components from Optilab.
Figure 3.
The frequency doubling module
Last Modified: April 25, 2012 |
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