CA 421: Commissioning Preparations

This account has been closed to further obligations since October.

CA 521: IR FEL COMMISSIONING

The primary accomplishment of the month was running low-average-current beam (at 38 MeV and a few µA cw) to the straight-ahead dump and processing liquid-ammonia targets for the nuclear-physics program. We processed three targets that subsequently will be inserted into a strong magnetic field and thereby become polarized for use in Gen experiments with the CEBAF accelerator. We ran stable beam for an integrated time of about 20 hours over a 40-hour period during 21-23 Mar 98 resulting in a very successful program for our nuclear-physics users. (The users needed the other 20 hours to make adjustments to their target chamber and dewar.) The machine was successfully turned on and off several times during the course of the program and behaved itself very well. At month's end we were configuring the Machine Protection System to support mA- level electron beam in support of going after first light.

All SRF cavities were measured for level of microphonics and operation of their higher-order- mode (HOM) loads. Subsequently, more extensive modeling of beam-breakup (BBU) ensued given the latest measurements of HOMs in the cryomodule. The simulations were for two passes with energy recovery. The threshold current is calculated to be about 25-30 mA. Accordingly, one might be able to tune the HOMs in such a way as to "make" BBU for observation and experiments. During nominal operation with 5 mA beam, BBU should not be a problem. It is conceivable that the machine could be run with three passes to amplify the BBU by another factor of order two as part of BBU experiments. Ultimately, the interest is ascertaining what is a sufficient safety margin with regard to BBU when one designs higher-current machines.

Simulations of the effect of coherent synchrotron radiation (CSR) in the IR Demo's magnetic bending systems progressed to the point of application to the optical chicanes and first recirculation bend. Predictions are that CSR from nominal 60 pC bunches would generate emittance growths of 7%, 21% and 44% after the first optical chicane, second optical chicane, and east arc, respectively. We are continuing to develop experiments in which the effects of CSR can be enhanced or suppressed.

FEL INSTALLATION

Installation highlights in March included: