Management

FEL Project weekly reports have been on hiatus since the summer run ended on August 13.  The next full operational period is scheduled for Oct. 24-Nov. 24.  In this report we offer a brief summary of several tests that were performed with the FEL over the last month.

Highlights for the month

(1)  The IR Demo FEL successfully demonstrated last Friday (Sept. 24)the production of sub-picosecond pulses of soft X-rays (at approximately 5 keV) by Compton scattering of the IR laser pulse off of the electron pulse.  There exist very few ps or sub-ps sources of X-rays in the world.  Short, intense X-ray pulses can be very useful for material science and chemical dynamics studies.  A more detailed description of the experiment is appended below.  The management of the FEL project congratulates all members of the FEL team for their efforts in successfully detecting the X-ray emission.  Particular acknowledgement goes to the team members who planned and executed the experiments:  Jim Boyce, Geoff Krafft, Kevin Jordan, Dave Douglas, Steve Benson, and George Neil.

Other highlights for the month include:

(2)  The FEL linac was used at the end of last month for electron irradiation tests of biological "stents" that are used in heart
surgery.  These tests were performed for the Duke University Medical School.
(3)  Successful operation of the injector and linac in lasing operation at peak charges of 110 pC.  We have raised the ante from our previous standard range of 60 pC to a range closer to the original specification for the IR/UV Demo (135 pC).

Considerable documentation efforts were proceeding this past month.  Documents that are necessary for the FEL to be certified as an operational "User facility" were prepared for approval by the Director's Council.  Draft documents pertaining to the expected FEL Upgrade project are in preparation.

Activity Details

Upgrade Design

Machine running during the month was on a reduced basis so that the group could focus on designing the IR Upgrade.  Design meetings on the upgrade covered the FEL wiggler specifications and options and the baseline approach for electron beam transport.  Scheduled for the next design meeting is the SRF cavity specifications and program options.

Medical Research

A short test run was performed of the tungsten X-ray target to be used for activating stents (arterial wire mesh inserts) for a Duke medical research activity.  Although the test was generally satisfactory it was determined that the target overheated and additional cooling will be used for the actual medical run in the next month.

X-ray production

At approximately 12:06 EDT, on Friday, September 24, 1999, Thomson scatter 5 keV X-rays were observed.  The first observations confirmed the overall production rate, the low bremstralung background, and the small energy spread (of order several percent) of the X-rays scattered through a single-arm spectrometer.  It remains to confirm the short ``pulse-length'' (< 400 fsec rms) of the X-rays.  The X-rays are produced in an intracavity collision in the center of the FEL wiggler, at location where the electron orbit is canted vertically up by about 18 mrad.  This allows extraction of the X-rays by a single LiF crystal 4 cm above the beam-line 1.5 m downstream of the collision point.  The X-rays were detected with a standard Si detector without the need for complicated background subtractions.  The ease with which these observations were made is demonstrated by the fact that the crystal and detector were first installed Monday, September 20 and beam operations
started Wednesday morning.  About 2.5 shifts were needed to first find the expected signal, a tribute to the care that was taken in the pre-alignment process.  Also, the event rates are such that single counts correspond to individual X-rays; there is no detector pileup uncertainty in the emerging data.  Because the FEL is CW, we obtain respectable rates at charge-per-bunch much lower than alternative Compton/Thomson scatter sources.

The following table summarizes conditions during most of the running and data collection:

Parameter Value Unit
Beam Energy 36.7 MeV
Beam Size @ Wiggler 200 microns
Charge 40 pC
Macropulse duration 250 microsec
Micropulse duration 350 fsec
Bunch Repetition Rate 37.5 MHz
Macropulse Repetition Rate 60 Hz
Laser Frequency 5.2 microns
Laser Macropulse Power 0.5 kW
Laser Circulating Macropulse Power 10 kW
Laser Average Power 5 W
X-ray Energy 5 keV
Source Macropulse Luminosity 2.5e34 cm^-2 per sec
Source Average Luminosity 4e32 cm^-2 per sec

We checked briefly that there were no significant limitations to CW data collection.  Using third harmonic lasing we should be able to produce 30 keV X-rays at our highest operating energy.

Pulsed mode operation

A test of external triggering of the FEL led to a demonstration of the ability to operate the FEL in macropulses at repetition rates up to 2.5 kHz with pulse widths down to 20 microseconds.  A number of materials processing users will be interested in this capability.

Injector Studies:

Injector Studies started after Labor Day.  A trial of some injector configuration changes was long anticipated.  The second solenoid was brought up 5 mm to its intended position (It had been lowered during early commissioning to compensate for a large, unexplained vertical kick in the Quarter cryomodule).  One pair of correctors was shifted from before to after this solenoid to avoid coupling that correction with the solenoid.  Another set of correctors was installed in the center of the
four quadrupole telescope.  This set provided a missing degree of freedom, a way of centering beam through the last two quadrupoles.

After dealing with a number of glitches, recirculating beam was established.  The unexplained angular kicks (now in both axis) from the Quarter cryomodule remained but were easier to compensate for with the new configuration.  A bettor match was established that reduced beam envelope height at the entrance to the Cryomodule and reduced beam envelope width at the exit.  Because strong lasing was established and 4.5 mA was cleanly recirculated with this new configuration, it was
dubbed the new standard and we didn't back out to the original configuration.  Up to 110 pC of charge was successfully recirculated with this configuration.

We plan to continue next week with fundamental studies such as cavity calibration and phasing verification.