Management:

We extended the summer run for four days this week. Highlights for the week include: the conclusion of a very successful 5-week summer run for the FEL  To recap the most important results: (1) 1.7 kW of average power was demonstrated at 3.1 microns with the linac operating at full power (48 MeV, 4.8mA) in the energy recovery mode. A kilowatt was also demonstrated at 6 microns. (2) The first user tests were initiated with experiments on metal surface processing (Armco/Va Power),polymer surface processing (CWM/DuPont/Northrop Grumman/NASA-Langley), laser ablation (CWM, NSU, Siemens), harmonic laser radiation (NSU), and terahertz radiation generation (RPI). (3)  Accelerator physics data were recorded relating to the average current limits due to the beam break-up (BBU)instability and emittance growth effects due to coherent synchrotron radiation.

Preliminary analyses of much of the above run data will be presented at the next International FEL Conference which is scheduled for 22-27 at the DESY Laboratory in Hamburg, Germany.  Th summer run was completed early this morning (August 13th).  The next FEL run is planned tentatively for the period Oct. 25-Nov. 24th.  A one week run is planned for Sept. 13 for injector optimization studies. Weekly reports will be suspended until the next run period. We thank all of our support staff at Jefferson Lab and our colleagues in the user community for a very productive run period.

Today is the last day for Court Bohn to be a member of the FEL Team.  Court joined the FEL project early in 1995 and served as the Deputy Program Manager in charge of the design, delivery and commissioning of the FEL driver accelerator. The successful commissioning of the IR Demo is a testament to Court's contributions. We thank Court for his numerous
technical and personal contributions to the FEL project and wish him well as he makes a transition back to hadron machines at Fermilab next month.
 

FEL User Tests

The week began with a change of FEL optics back to the 3 micron set followed by re-establishment of high power lasing at 3 microns.  Numerous parametric tests of laser ablation were performed on ceramics and epoxies (for NPS) for metals and carbon (for CWM).  Ann Reilly of CWM succeeded in laser ablating stainless and carbon targets.  Several samples of laser-ablated films were removed for surface analysis.  To the eye the films appeared specular and free of particulates.  A second run was made on laser micromachining of kapton-backed aluminum films which is the material of interest for the NASA "solar sail" project.  Some gaps in the previous data set of ablation vs intensity were filled in.  A remarkably beautiful photograph was captured of the FEL harmonic radiation dispersed by a prism.  The image slows visible light from red
to violet coresponding to coherent emission from the FEL from the 5th to 9th harmonics.
 

FEL Commissioning

This week measurements continued in an attempt to discern the effects of coherent synchrotron radiation on electron-beam quality as the beam passes through the first recirculation arc.  Both transverse emittance and energy-spread measurements were made; CSR would influence both of these.  We were never able to establish sufficiently low transverse emittance at the arc entrance to provide a low enough signal-to-noise to see definitively an emittance growth at the arc exit.  In the case of energy spread, it was generally difficult to convince ourselves that we had removed all transverse effects of transport from the viewer images, something that needs to be done to be sure only energy spread is being observed on the viewers.  However, for a few select cases corresponding to short bunch length through the arc, we believe we have good data, and
within the resolution of these measurements (roughly ±10%), they show no increase in energy spread. Moreover, simulations with the corresponding beam parameters indicate growths in energy spread of about 5%, meaning that the growth would not be discernable given the measurement error.  Thus, we can conclude at a minimum that it is possible set up a beam that will transport around the Bates arc without significant growth in energy spread (which translates to no significant growth in emittance), and therefore for the FEL Upgrade it seems reasonable to plan on putting the FEL systems in the back leg -- a bunch charge of 135 pC  (versus the 60 pC used for these data) would roughly double the effect provided all else were equal, but then it still would not be significant.  Once the lattice design of the Upgrade has matured, then one should be able to
use the IR Demo to set up a beam at the entrance to the first Bates bend that mimics the Upgrade beam, and then check whether that beam propagates around the arc without undue degradation.  The simulation code is also available as another check.

We also spent some time this week attempting to measure the electron beam distribution as a function of the FEL power ( detuning curve). We will have to make some changes in the hardware that drive our scrapersbefore we come back to these measurements in the next run.