Management

Highlights of the week include successfully initiating the beam breakup experiment, re-establishing lasing into User Lab 1, and progressing with the coherent-synchrotron-radiation study.

FEL Installation/Maintenance Activities

After the high-power laser beam melted the plastic mounting pins of an optical-transport mirror late last Friday and thereby erratically
sprayed itself through the optical transport line, we tracked down and replaced damaged transport optics.  Software for controlling translation stages in User Labs 1 and 3 was installed and tested.

FEL Commissioning Activities

This week L. Merminga and I. Campisi were able to get their beam-breakup (BBU) experiment running and have been collecting data most of the week.  The motivation for the study is that the threshold current for the BBU instability in the FEL Upgrade is roughly estimated to be ~20mA, only a factor ~2 away from its 10 mA specification.  The phenomenon therefore deserves attention, and the code TDBBU used for the BBU calculation has never been benchmarked.

Given that the threshold current for the BBU instability in the IR Demo is about 27 mA, much less than the 5 mA hardware limit, no BBU can nominally be deciphered.  Therefore one needs to "force" its creation.  The scheme of the experiment is as follows:  A special stripline beam-position monitor is installed in the injection line.  It is driven at the rf frequency matching that of the most troublesome (from the
perspective of BBU) deflecting dipole mode in the cryomodule cavities.  Doing so creates a seed beam for injection to the cryomodule; the seed beam has the same properties as if cavities prior to the cryomodule were triggering BBU.  The amplitude of the deflecting mode in a given cryomodule cavity is being measured as a function of average current.  The plan is to look at all the cryomodule cavities in this way, and also look at different electron-beam energies (so far they have been using 36.7 MeV beam).  The data will then be used to infer by extrapolation the threshold current for the instability and compare the result with the TDBBU code prediction.  Plans are to continue the measurements on Day Shift through the end of next week.

As mentioned above, the optical transport system self-destructed, apparently due to high- cw-power lasing on Friday evening.  Plastic
mounting pins on the second mirror of the collimator at the exit aperture of the FEL melted.  As a result, and in view of anomalous and
persistent poor optical beam quality subsequently observed in the Optical Control Room (OCR), we ultimately had to inspect the entire
optics suite, ranging from the FEL itself through upstairs in the OCR.  We had to replace several damaged mirrors, as well as the brewster
pickoff window in the OCR.  And we replaced the plastic mounting pins with metal ones.  After completion of this refurbishment, we quickly
began delivering quality optical beam into User Lab 1 and commenced user tests there.

Early in the week, beam measurements comprising the coherent-synchrotron-radiation study were plagued by a persistent longitudinal tail that vitiated good measurements of energy spread based on viewer images in high-dispersion regions of the beamline.  It was traced to missteering through the injector cryounit; a slight adjustment of the horizontal corrector magnet prior to the cryounit removed the tail.  Systematic measurements of energy spread versus position around the machine and versus the cryomodule gang phase (which loosely
correlates with bunch length) were taken.  A study was done to determine the contribution of transverse-longitudinal coupling to the viewer
images (and therefore to the inferred energy spread); preliminary indications are that it is a small effect.  CSR studies are slated to continue on Owl Shift through the end of next week.

An initial look at lasing on the 5th harmonic at 1.03 microns was attempted however gain was too low to initate lasing. A second attempt
will be made next week utilizing higher reflectivity mirrors.

Further experiments on the ablation of materials is to be performed Friday evening now that the transport system has been repaired and the
optical beam characterized.  This work will take place at 3.1 microns.