MEMORANDUM


To: J. Albertine, D. Helms, W. Skinner

cc: Division (M7), FEL Coordination Group

From: F. Dylla

Subject: IRFEL Weekly Report, June 16-20, 1997

Date: June 20, 1997

Management

Highlights for the week: we are on track for completing the list of final activities for the injector test stand that would enable shut down of the ITS next Monday (June 23). The activity list included completion of longitudinal emittance and bench length measurements and an extended run to monitor the dark current in the gun. On Monday of this week, the 1/4 cryounit was removed from the ITS and placed in the injector area of the FEL Building. Removal and transfer of the gun and other ITS hardware slated for transfer to FEL Building is scheduled to begin on June 23 (see ITS report for details).

This week Jefferson Laboratory hosted a visit from DOE Office of Energy Research senior management for their review of the laboratory's institutional plan. The visit included a status report on the IR Demo project and a tour of the FEL Facility.

Final preparations were made for next week's Laser Processing Consortium Workshop (June 25-26). Approximately 50 attendees from outside the laboratory are expected. The focus of the meeting will be the generation of proposals for initial fit-up and use of the FEL user labs.

At the request of Navy program office, revised proposals were forwarded on June 16 for use of the $3.7M of FY97 DOD appropriations planned for allocation to Jefferson Lab for July 1997-Sept. 1998 FEL activities.

Injector Test Stand

Regarding the photocathode gun:

We worked on taking longitudinal measurements at high bunch charges and followed that activity with a 48-hour run to monitor dark current versus time and thereby gain some additional information regarding the field-emission behavior of the gun.

The status of the gun at this writing (0930, 20 Jun 97) is as follows: We have measurements of bunch length and longitudinal emittance at bunch charges of 10, 60, and 135 pC. One qualitative feature observed at high bunch charges is a hollow longitudinal density profile, one that shows clumping at the head and tail of the bunch. This observation correlates with simulations (PARMELA), at least qualitatively, and therefore is attributed to space charge. However, there remains a question concerning the sensitivity of the density profile with respect to the solenoid setting. The drive laser introduced a complication in that it was unstable during the high-bunch-charge measurements, and this generated uncertainties in some of the data. For example, at 135 pC, there was considerable jitter in the energy spread. Moreover, there was a discrepancy between measurements of quantum efficiency and output current. This lead to an uncertainty in bunch-charge measurements at 60 pC; they could in fact have been 40 pC. However, after revamping the drive laser as described below, and during a subsequent set of measurements at 10 pC, the drive laser operated very stably. In view of these circumstances, and to complete a solid set of experiments, we are embarking on two more 10-hour runs through the weekend, during which we will repeat and augment the 60 and 135 pC measurements. We made a new cathode yesterday after completing the field-emission run, and are gearing up to begin the additional runs today. The plan remains to begin dismantling the gun on Monday, 23 Jun 97, to prepare for moving it to the FEL Facility, but this precise date remains tentative pending the success of this weekend's activities.

The pump volute on the drive laser was replaced last Friday with one made of PVC. This will eliminate problems with lowered (and often unstable) power that occurs when using the stock electroless nickel-coated volutes. The lamps were also cleaned and the deionizer cartridge replaced. The laser now performs at the level it did several months ago. At 1700 today (20 Jun 97) it will have operated this week for 100 hours in support of gun experiments. The power output was quite stable over this period (+/- 0.1 W at 19.6 W infrared output). The intensity "ringing" of long macropulses continues to be investigated. The vendor is no longer convinced it is due to piezoelectric resonances in the KD*P crystals, but instead may be in the electronics. We will continue to investigate this as soon as the laser is not being used for gun experiments. We are also preparing to move the drive laser as soon as the utilities are available in the new clean room located in the FEL Facility.

Regarding the cryounit:

The cryounit was moved to the FEL Facility this week. Rough alignment was completed and final alignment started.

Regarding development of 50 kW ceramic warm windows:

We tested a Wesgo ceramic to 50 kW in the resonant ring test assembly and found very little heating. One of our staff will travel to SLAC next week to make RF measurements with the goal of optimizing the window design for low losses. Eight Wesgo ceramics were sent out to be metallized and are expected back within a week.

Accelerator Systems

Regarding the beam-transport system:

In anticipation of the assembly of the optical chicane dipoles, Everson Electric completed two trials of the gluing of shim materials to sample cores. The first, using 30 psi, failed in that the glue thickness ranged from .0005 inch to .0025 inch, with one reading at .0045 inch. However, the second trial, at 120 psi, was successful, with a combined, uniform thickness of two layers of epoxy at 16 points of less than .0005 inches.

Cores for the reverse-bend dipoles are in final machining, and the mechanical quality of their cut-to-size mu metal passed qualification. However, because of the changed dimension of the thickness of the mu metal and brass associated with the non-interchangeable parts assembly plan, the before-final-grinding back-leg height is already at the nominal final dimension. The existing plan was to grind an additional .010 in. to achieve the final, accurate dimension which leaves the back legs short by this amount. At week's end we are working the issue. A simple solution is to reduce the 180-degree bend's gap height by .010 in. to maintain the original match between families. Reduction in the size of the gap affects the chambers and coils. The chamber slot reduction has been coordinated and cleared with the designer, and the coils have to have thinner bedding strips for both styles of magnet.

Core parts for the injection/extraction dipoles lack only the final delivery of mu metal. The manufacturing process has been qualified, and those parts need only go through their laser cut. A final detail is being worked out on the position of the bus leads on the coils before the complete set can be delivered.

Additional parts and modifications to our Magnet Test Stand that will allow comparative measurement of the dipole magnets are nearing sign-off.

Sign-off of the Panofsky trim quad drawing set is being delayed by checking of the assembly tooling. The tooling is being included with the order because it is necessary for an accurate assembly by the vendor. Sextupole cores and coils remain on track for a delivery on 8 Jul 97.

Regarding corrector magnets, the mu-metal phasing dipole is being drawn up. (Note: We still have to qualify these mu-metal correctors with regard to cross talk and fringe field.) Three-dimensional magnet modeling of correctors is continuing, and we seem to be converging on a design for a constant-perimeter, two-shell air-core-coil set that will achieve the 1% uniformity specification. This design could be made by a simple process involving fabrication of a flat coil followed by bending it around a tube. In parallel, Advanced Magnet Lab in Florida submitted a preproposal wherein they could make similar style coils with their unique conductor lay-up process that would achieve the same quality. However, because of development and prototyping, they are way out in schedule, and cost is considerably more than we project for our process.

Regarding vacuum chambers, completion of the injection X-chamber was delayed as we assessed the impact of the contamination that our Machine Shop introduced by using cutting oil when machining the ends. This was contrary to the informal, written procedure worked up with the Shop to produce the chamber. We are working on a procedure that will be signed off and which has a higher probability of being followed. At the same time, inspection of the weld of the extraction Y-chamber (as well as of all future chambers) with a boroscope should increase the welder's skill at producing good welds. The parts for the optical-chicane chambers are being prepared for weld. We are working to ensure the necessary manpower is available to complete all of the vacuum-chamber welds on time.

Drawings for the insertable dumps are being signed off, and the design of the beam scraper is nearing completion.

Regarding cryomodule fabrication:

The status of the cryomodule is as follows: First cryounit -- complete. Second cryounit -- will be moved to cryomodule staging area next week. Third cryounit -- inserted into the helium vessel; welding will be completed early next week. Fourth cryounit -- spacer flanges were fabricated and assembled.

Regarding electron-beam instrumentation and controls (I&C):

Six sets of beam instrumentation controls drawings were signed off. All beam viewer and motor control interface chassis are installed.

The PSS systems are proceeding well, and the ODH system will be operational by 6/27/97. This is required prior to cooling down the quarter cryomodule. A review of the MPS is scheduled for next Thursday.

A preliminary design for an analog monitoring system is complete. A cost comparison is being made between an HP 4:1 mux (24 required ~ $22K) and home built one.

All VME crates are installed, the cryogenic controls are operational. This IOC is connected to the Central Helium Liquifier directly by a fiberoptic link. The controls database is being loaded and instrumentation checkout is proceeding for the July 1 cooldown of the quarter cryomodule.

FEL Systems

Wiggler

Further measurements on the quad girders resulted in a few other design errors being discovered. The girders must be reworked before further alignment work can proceed. We are also working on making up borescoping fixtures to check for potential systematic errors in the quadrupole alignment which might lead to an offset of up to 200 microns in the alignment of the quadrupoles and the wiggler.

Optics

This has been a busy week in terms of receipt of optics and components. We received the sapphire mirrors (coated for 3 micron operation) from Research Electro-Optics. Their metrology shows the ROC to be within our spec (+/- 0.2%) for the output couplers, and somewhat out of spec (+0.4%) for the high reflectors. We will perform our own checks, and await results from China Lake before determining if they need to be sent back for refiguring. We also received a partial order (8 of 18) of the optical transport mirrors from SPAWAR. We should receive the rest of the order next week. We received the mirror can stands on Monday, but rejected some of them because of poor welds. They were returned, fixed, and we received them back on 19 Jun 97. Once time permits the installation crew to set the anchors, we will install the stands and the mirror cans. We received the Brewster window pickoff (one plus a spare) and are double-checking a few of the critical dimensions before having them cleaned. Optical cavity components still await final cleaning to reduce particulates. We are delayed by staff unavailability due to work on the main accelerator. We determined that a 50/50% beamsplitter pellicle can be imaged (if sufficiently attenuated) by our vidicon cameras. We needed to make this determination before ordering the pellicles that are used for optical cavity alignment. We measured the sensitivity of an Electrophysics infrared camera at 633 nm (a possible diagnostic for 3rd harmonic detection.) We moved the Mach-Zehnder interferometer to the FEL Facility for testing. It is being used on one optical table until the other table is aligned. The indium braze used to heat sink a calcium fluoride window (simulating one of our cavity mirrors) to a copper holder was cycled to 50 C above ambient nine times without exhibiting signs of separation. We will now mount a calcium fluoride mirror (coated for 633 nm) in one of the holders for mirror heating tests.

Commissioning/Operations

We assimilated the available data from the experiments with the photocathode gun in the ITS, which led to the plans discussed above in the "Injector Test Stand" section.

There was considerable interplay during the week between our lattice designer and magnet engineers to assess the relative importance of various manufacturing complications involving the mounting of mu-metal sheets in the reverse-bend dipole magnets and the nature of the corrector magnets in general. The status of these magnets is summarized above in the "Accelerator Systems" section.

Investigation of the observed beam scraping at the aperture in the light box of the photocathode gun with 135 pC bunch charges gave way to some puzzling results. The simulation (PARMELA) says that, with a 4 mm laser spot at the cathode, the 4-sigma size of the beam at that aperture is 1.8 cm, which is a factor of 4/1.8 = 2.22 times smaller than the aperture size there. Furthermore, with a 6 mm laser spot at the cathode, the 4-sigma size of the beam at that aperture is 1.5 cm, which is a factor of 4/1.5 = 2.67 times smaller than the aperture size there. So, the code has thus far offered no explanation about why beam was scraped at that point. One possibility is that the electrons scraped away were not from the usable beam itself, but were, for example, from ghost pulses. We are in process of investigating whether this was the case.

Facility

Finish work continued on the Facility. Outside grading is just underway to get the grounds restored and the parking areas installed. The vinyl tile was completed in the break room and upstairs hall. Ceramic tile work was completed in the bathrooms. The ceramic fixtures are being installed. The cab and doors are being constructed on the elevator. The HVAC turns on today (20 Jun 97. On the Jefferson Lab side we brought 18 workbenches into the optics labs to support buildup. The first 50 kW RF unit including power supply and klystron was craned into the Facility and placed. A number of power supplies and support electronics were also brought in. Electronic hookup continued in the instrumentation and control racks. The clean room safety system was completed and is operational. The cryogenic controls were hooked up to the transfer lines and the control computer brought on line and connected to the net. The crane rail for moving the cryounit was qualified early in the week. The cryounit was extracted from the ITS and installed in the injector pit. Initial survey and positioning was performed. Survey and alignment was done on the optical cavity tables. They are close to final position but will have to await replacement of a broken positioning bolt on Monday before they are locked down. A laser interferometer was set on the optical table and measured maximum vibrational excursions in the tens-of-nanometer range at 15 to 25 Hz despite lots of mechanical pumps, forklifts, etc., going in the area. A longer-term drift will be quantified next.

The clean room is substantially complete and ready for checkout by Clean Air Technology. The Laser Safety system is also complete and an electronics rack for the laser electronics has been moved to the clean room. Installation of the optical bench is awaiting technician time. The quarter cryounit is installed. Preparations continue for a cooldown during the July shutdown. Mirror cans for the drive laser transport are in check. The elbow for the high voltage power supply was delayed and may not be available until Aug. 8. Work on installing the power supply has been delayed due to other conflicting work in the injector area.