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Title: Reducing the Heat Load on the LCLS 120 Hz RF Gun with RF Pulse Shaping

Abstract

The LCLS injector must operate at 120 Hz repetition frequency but to date the maximum operating frequency of an S-band rf gun has been 50 Hz. The high fields desired for the LCLS gun operation limit the repetition frequency due to thermal expansion causing rf detuning and field redistribution. One method of addressing the thermal loading problem is too reduce the power lost on the cavity walls by properly shaping the rf pulse incident on the gun. The idea is to reach the steady state field value in the gun faster than the time constant of the gun would allow when using a flat incident rf pulse. By increasing the incident power by about a factor of three and then decreasing the incident power when the field reaches the desired value in the gun, the field build up time can be decreased by more than a factor of three. Using this technique the heat load is also decreased by more than a factor of three. In addition the rf coupling coefficient can be increased from the typical critically coupled designs to an overcoupled design which also helps reduce the field build up time. Increasing the coupling coefficient from 1 tomore » 2 reduces the heat load by another 25% and still limits the reflected power and coupling hole size to manageable levels.« less

Authors:
Publication Date:
Research Org.:
Stanford Linear Accelerator Center (SLAC), Menlo Park, CA
Sponsoring Org.:
USDOE Office of Science (SC)
OSTI Identifier:
839687
Report Number(s):
SLAC-TN-05-027
TRN: US0503322
DOE Contract Number:
AC02-76SF00515
Resource Type:
Technical Report
Country of Publication:
United States
Language:
English
Subject:
43 PARTICLE ACCELERATORS; DESIGN; THERMAL EXPANSION; HEATING LOAD; RF SYSTEMS; ELECTRON GUNS; LIGHT SOURCES; BEAM INJECTION; PULSE SHAPERS; TEMPERATURE CONTROL

Citation Formats

Schmerge, J. Reducing the Heat Load on the LCLS 120 Hz RF Gun with RF Pulse Shaping. United States: N. p., 2005. Web. doi:10.2172/839687.
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Schmerge, J. Reducing the Heat Load on the LCLS 120 Hz RF Gun with RF Pulse Shaping. United States. doi:10.2172/839687.
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Schmerge, J. Mon . "Reducing the Heat Load on the LCLS 120 Hz RF Gun with RF Pulse Shaping". United States. doi:10.2172/839687. https://www.osti.gov/servlets/purl/839687.
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@article{osti_839687,
title = {Reducing the Heat Load on the LCLS 120 Hz RF Gun with RF Pulse Shaping},
author = {Schmerge, J.},
abstractNote = {The LCLS injector must operate at 120 Hz repetition frequency but to date the maximum operating frequency of an S-band rf gun has been 50 Hz. The high fields desired for the LCLS gun operation limit the repetition frequency due to thermal expansion causing rf detuning and field redistribution. One method of addressing the thermal loading problem is too reduce the power lost on the cavity walls by properly shaping the rf pulse incident on the gun. The idea is to reach the steady state field value in the gun faster than the time constant of the gun would allow when using a flat incident rf pulse. By increasing the incident power by about a factor of three and then decreasing the incident power when the field reaches the desired value in the gun, the field build up time can be decreased by more than a factor of three. Using this technique the heat load is also decreased by more than a factor of three. In addition the rf coupling coefficient can be increased from the typical critically coupled designs to an overcoupled design which also helps reduce the field build up time. Increasing the coupling coefficient from 1 to 2 reduces the heat load by another 25% and still limits the reflected power and coupling hole size to manageable levels.},
doi = {10.2172/839687},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Mon Jan 31 00:00:00 EST 2005},
month = {Mon Jan 31 00:00:00 EST 2005}
}
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DOI:  10.2172/839687
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  • A load lock for the LCLS 120 Hz RF gun is required for the rapid interchange of cathodes onto the gun as well as to avoid exposing the cathode to contamination during installation. Implementing a load lock allows the gun to be operational in less then 1 hour after a cathode change since no bake out is required and minimal rf conditioning will be needed. A conceptual drawing of the load lock proposed by SLAC is shown in the figure. The load lock approach adopted by SLAC requires two load-lock transporters: one mounted on the gun and another in themore » cathode processing lab where cathodes are prepared and stored for use on the gun. A single cathode is mounted in each transporter, eliminating the need for transferring cathodes under vacuum from a storage carousel, or ''6-pack''. This design has the advantage of greatly reducing the mechanical complications of transferring cathodes and connecting them to cooling lines while under high vacuum. Cooling of the cathode is necessary, given the gun cooling requirements as detailed in the LCLS Technical Note ''Design Considerations for the LCLS RF Gun''. The cathode size is approximately the same as that of the standard BNL/SLAC/UCLA gun which has an RF seal at the outer diameter of the cathode cavity to avoid RF breakdown.« less

  • ; ; ; ...
    A preliminary design studies were carried out at Brookhaven National Laboratory for a photocathode RF gun injection system for LCLS 120 Hz operation. The starting point for the design is 50 Hz BNL Gun IV developed by a BNL/KEK/SHI collaboration. The basic parameters of the 120 Hz gun is discussed in this report. The complete photocathode RF gun injection system is described for a 120 Hz operation. The injector system includes photocathode RF gun, emittance compensation solenoid magnet, laser system and laser beam delivery system, and electron beam diagnostics. The basic design parameters, mechanical modification and the performance will bemore » presented in this report.« less

  • The review was held at SLAC on September 11 and 12, 2001. Presentations concerning the thermal analysis, mechanical design, integration with the laser and accelerator, general beam dynamics considerations, a load lock mechanism, and symmetric power feed options comprised the review. Slides from these presentations are available elsewhere. The review committee was charged with evaluating the 120 Hz gun design including proposed load lock and power feed options and recommending improvements. Broader evaluation of the injector as a whole (including focusing and diagnostic systems that do no impact the envelope of the gun itself) is expected to be covered inmore » a future review and will not be commented on here. In general, the long operational experience with four generations of s-band RF guns at numerous labs worldwide has led to considerable experience in design, fabrication, and operation aspects, and provides an excellent base on which to design the higher duty factor LCLS injector. While open questions remain on obtaining the design performance from these injectors, the microwave design of the gun has reached a state of relative maturity.« less

  • The LCLS rf gun design requires several modifications from existing S-band rf guns. The modifications that have the largest impact on the design of the gun are a dual rf feed and additional cooling capacity for 120 Hz operation. A list of electrical and mechanical specifications have been developed to produce the desired rf field. The simulation code ANSYS was used to study the steady state thermal properties of a 120 Hz gun. It was determined that four cooling channels at appropriate locations could be used to maintain the gun at a nearly constant frequency. The gun body stresses weremore » all at or below the yield strength of Cu except for at the rf apertures. The stress at the apertures is over twice the yield strength and additional work is necessary to reduce the stress and still produce the desired rf coupling coefficient. Only steady state temperatures were calculated and no pulsed heating effects were considered in this study.« less

  • This note highlights the possible re-configuration options for the existing WR284 S-band waveguide in the klystron gallery, to enable those existing 5045 klystrons to power the RF Gun and the L0 accelerating structures for LCLS. A reconfiguration decision based on the pros and cons for each option is subsequently identified.