Management

Highlights for the week include:  (1)  generation of kilowatt levels of IR light at 5.85 microns and delivery of approximately 700 watts at this wavelengths to Lab 1 for polymer processing experiments.  This wavelength overlaps strong absorbances in polymers due to the carbonyl (-CO) resonance.

This week's operations completes four weeks of successful operation of the FEL for (1) the high power (kilowatt) power demonstration; (2) accelerator physics and lasing studies to support the FEL upgrade; and (3) FEL user tests involving six different user groups.  We thank these users for their patience and willingness to collect data while we are also commissioning the user labs. Operations will continue for a few days next week to collect some additional data prior to the International FEL Conference.  A tentative schedule for operations for the remainder of the year is given in the FEL Commissioning Section below.

The project's chief lattice designer, Dave Douglas, has produced a conceptual design for the accelerator driver for the FEL Upgrade that preserves the "Bate's" type recirculation arcs that are used in the present IR Demo.  A Technote describing the design (TN-99-019) is being circulated within the Lab for comments.
 

FEL User Tests

Last Friday after the press deadline for last week's report, we completed an intial test run for the metals group (c/o G.Neiheisel of Armco).  Up to 200 watts of light at 3.1 microns was delivered to Lab 3 for irradiation of 5 different types of steel samples.  The first experiments involve investigation of the IR FEL beam for oxide removal and surface amorphization.  The samples have been returned to Armco for analysis and planning of subsequent FEL exposures.

Early this week, the FEL optical cavity mirrors were changed from reflectivity at 3 microns to reflectivity at 6 microns in preparation for M. Kelley's experimental tests of polymer processsing in Lab 1.

The most attractive opportunities for modifying polymer surfaces with IR light are found at the longer wavelengths, e.g., 5.85 microns for polyester and polyimide, 6.11 microns for nylon.  These wavelengths are accessible with sufficient power for processing only from JLabs’s IR FEL.  This week saw the first such experiments ever, with the FEL delivering in excess of 600 watts to User Lab 1.  We sought to demonstrate a series of effects previously seen with UV excimer lasers:  polyester fabric surface roughening, polyester (PET)film surface amorphization, Kapton* polyimide film carbonization and nylon film surface roughening.  So far the materials have been examined only by
optical microscopy.  The Kapton* response was striking, consisting of profuse fiber growth from the irradiated region, unlike the adherent black carbon layer formed by excimer irradiation.  The PET film showed, under certain conditions, clear streaking characteristic of amorphization; dye studies which will confirm the amorphization are planned.  Conditions satisfactory for surface roughening of polyester fabric or of nylon film were not successfully attained in this initial experimental run.
 

FEL Commissioning Activities

During the day on Monday of this week, additional data concerning beam breakup was taken, and the principals are in the process of analyzing all of the data off-line.

Work also continued through the week toward progressing on the CSR studies.  The main focus of this activity was to try to reduce the transverse emittance after the wiggler to increase the "signal-to-noise" for observing CSR-induced emittance growth.  Progress to that end was slow, but the outcome of this morning's Owl Shift looked promising.

Early this week we set up 34.3 MeV electron beam for 6 µm lasing, and without undue difficulty achieved up to 3.1 mA average current while lasing cw at up to 872 W.  Interestingly, and perhaps somewhat coincidentally, the output power scaled from the electron-beam power that was present when we achieved our laser-power record is (34.3/48.7)(3.1/4.4) 1.72 kW = 0.853 kW, practically identical to the actual 0.872 kW delivered.

Yesterday we successfully recesiated the gun and brought it back to voltage.  This is the first time this cathode was recesiated; it delivered 568 C total charge on its first cesiation.  Later yesterday, we delivered 700 W cw power of 5.85 µm light upstairs to the Optical Control Room, which implies (based on the 30% loss previously measured at 5 µm in the optical transport line) generation of about a full kW out of the FEL.  The associated average current was 3.9 mA, and scaling the previous numbers by this current yields (3.9/3.1) 0.853 kW = 1.07 kW.

Plans are to continue running for at least part of next week, both to support additional lasing studies and to bring the CSR studies to reasonable closure.  FEL operations would then cease for the remainder of the month to allow proper time for  analysis of data and preparation of papers for the International FEL Conference in Hamburg on August 23-27.

Tentatively we are planning a short 2-day run for electron irradiation studies for Duke University on Aug.28-29, a one week run for injector testing starting on Sept. 13, and will resume lasing operations for a month period on or about Oct. 25 depending on available resources.

At this point, the state of the machine is such that it can deliver high-average-current beam in the energy range ~34 MeV through ~49 MeV in the presence of high-average-power lasing at ~6 µm through ~3 µm, respectively.  The process of making a gross change in laser wavelength is now straightforward: change the optical-cavity mirrors (takes about 4 hours), then restore the corresponding machine settings and turn on the machine (takes about 1 hour).  Put in programmatic terms: all facets of machine performance advertised in the Lab's IR Demo contract with the Navy have been achieved.  Next Friday we will celebrate with an "End of IR Demo Commissioning Party."