Closeout Report of FEL Gun Committee

Thursday, 5 Aug 99
From: C. Bohn, on behalf of the FEL Gun Committee

This report constitutes the formal closeout of the activities of the FEL Gun Committee. The principal committee members were: G. Biallas, C. Bohn, L. Phillips, T. Siggins, C. Sinclair, and yourself, although a number of others contributed to the overall process. The Committee now feels it appropriate to transition to a design effort that ultimately would fall into a subtask of the Injector WBS of the forthcoming FEL Upgrade Program.

The report comprises two parts. First is an task-by-task reply to the original charge you issued on 13 Nov 98. Second is a list of topics that should be pursued, or in some cases at least be considered, during the design phase.

In reference to the original charge, the tasks and their status are delineated in the following:

Task 1 -- Existing Gun:

(1a) Analyze the performance of the gun since its first operation in the ITS to the present.
Status: Records have been pulled together into a single volume for which T. Siggins is the caretaker. In addition, an electronic logbook (FUNLOG) was created and is now in routine use. The essential outcome of this activity was the identification of a clear, reproducible pattern in the field-emission behavior of the gun versus the high-voltage-processing procedure employed. It resulted in a processing recipe (the "520 kV, quiet at 510 kV" procedure) that worked well enough to enable sufficiently rapid progress in commissioning the FEL. Now that a new cathode ball is in use, the old trends are no longer strictly relevant, but new data is being judiciously compiled and recorded in FUNLOG.

(1b) Establish a set of standard written procedures for gun maintenance and operation that ensures best results with the existing hardware. Status: Comprehensive procedures for resurrecting and recycling the gun are written in draft. T. Siggins has the draft and needs to polish it (which he will be doing when taking breaks from polishing stainless-steel plates). One fundamental change to the overall procedure is to retract the cathode into the stalk before HV processing to aid in preserving its QE. Doing so was not viable with the old cathode ball because it raises the peak surface electric field to the point that a sufficiently high processing voltage could not be reached. Such was not the case with the new cathode ball.

(1c) Recommend routine test measurements that would improve reliability using existing and augmented instrumentation. Status: A cathode scanner was built to map the QE across the wafer surface, and it is in routine use. It was found from experience that the strip-chart recorder (hardware, not software!) is essential to monitor gun-current fluctuations during high-voltage processing. That is why the strip-chart recorder is now in the FEL Control Room instead of the vault, and why processing is now routinely conducted from the FEL CR.

One test that lent confidence to the "gun resurrection" procedure was measurement of the cathode QE after the vacuum bake but prior to HV processing. The test established that good cathodes were indeed being installed in the gun, and poor QE was a result of HV processing. We currently have insufficient data with the new cathode ball to make a statement about QE versus processing.

The Committee recommends, in addition, that the QE be monitored routinely with a cathode scan and QE measurement after every, e.g., 100 C of delivered charge, and the data be logged in FUNLOG. Doing so would establish a base of systematic data with which to compare one cathode against another, and ultimately permit one to distinguish between beneficial and deleterious trends that could accommodate trial changes in the standard processing procedures.

(1d) Investigate and recommend possible incremental improvements to the existing gun hardware.
Status: The Committee's assessment of gun performance led to replacement of the cathode ball. In addition, the Committee identified the essential need to keep cesium off of the electrodes. Four general recommendations surfaced: installing an apertured cesiator that will direct cesium to the wafer and not elsewhere (ideally), devising the means to cesiate the cathode within the ball so the outer surfaces of the ball stay unexposed to cesium, installing a shield to protect the cathode during HV processing, and developing a load lock. Of these four, the apertured cesiator is now in construction and will be tried first. Notional shield designs are now available, but none are being implemented. The Committee recommends shield design, construction, and testing be completed as part of the FEL Upgrade Program. A load lock is generally viewed to be an expensive (few 100 k$) and labor-intensive (8-12 months to build and install) proposition.

The Committee recommends that prior to committing to a load lock, a test chamber be constructed off-line to support experiments of processing techniques to improve and/or sustain cathode QE. For example, the chamber could be used to develop and demonstrate the cathode shield and activation of the cathode within the ball. There is not enough data available to justify a load-lock program. As more data becomes available with the present gun, it may or may not prove wise to invest in the test chamber.

The Committee also recommended R&D to identify electrode materials that would be more field-emission resistant. Consequently, ion-implantation studies are forthcoming in collaboration with W&M. The procedures underlying these studies can also be applied to alternative metals, particularly titanium, and to coatings such as diamond-like coatings. The Committee recommends this R&D effort continue long-term in parallel with the incremental upgrades discussed above.

The idea of tapering the lip of the cathode ball was investigated but found to increase the surface gradient; consequently, the idea was abandoned.

(1e) Incorporate new procedures and hardware developed by the polarized source group that could benefit FEL gun operation.
Status: Hardware involved with deuterium-cleaning the wafers for the FEL gun was checked in consultation with members of the Polarized Source Group. It was modified to provide improved vacuum near the cathode surface during cleaning. As mentioned above, the QEs of newly-installed cathodes have been good: a few percent, which is better than the historical values of a few tenths of a percent, but worse than the values of around ten percent achieved by the Polarized Source Group in CEBAF's sources. Because the QE checks were done only after the hardware was modified, there is no data to indicate whether the modification improved the pre-HV-processing QEs.

Task 2 -- Upgraded Gun:

(2a) Identify improvements to the existing gun design.
Status: Gun performance has not been an overriding concern at 60 pC bunch charge, and since being installed in the FEL Facility, it has produced bunch charges up to ~90 pC. Prior to replacing the cathode ball, 135 pC bunches (needed for the Upgrade) could not be generated, although during initial testing in the Injector Test Stand, the gun delivered bunch charges in excess of 200 pC. Initial performance of the "new" gun is favorable, yet it has not been tested at 135 pC. Moreover, gun availability has historically been much too low. At this writing there is insufficient data with the "new" gun to make a statement about its availability. Tasks that are now ongoing, as described in (1d) above, should improve availability in any case. The Committee recommends staying the course with the program of incremental improvements, while judiciously documenting the quantitative results of their implementation.

The Committee also recommends, for the record, that early in FY00 a time period be set aside for comprehensive injector tests. The tests should establish (1) the ability of the gun to deliver cw current at 135 pC, (2) its lifetime during that mode of operation, and (3) the delivery of good beam quality from the injector at that bunch charge. The Committee also advises that a 150 MHz capability (e.g., drive laser) be procured as part of the Upgrade Program to allow additional flexibility in commissioning the Upgrade.

(2b) Consider an alternative gun, such as a hybrid rf photogun that couples a load lock with the front end of the best technology gun being developed by the Polarized Source Group.
Status: The Committee feels there is insufficient motivation to pursue an alternative gun for the Upgrade, especially in view of the paucity of resources. The dc photocathode gun should suffice, especially given the performance of the "new" gun that shows it is capable of generating well over the 10 mA cw required for the Upgrade.

In view of the findings delineated above, the Committee recommends transitioning to a design effort for the Upgrade gun. Recommended ingredients of the design effort are: