MEMORANDUM
To: J. Albertine, D. Helms, W. Skinner
cc: Division (M7), FEL Coordination Group
From: F. Dylla
Subject: IRFEL Weekly Report, June 23-27, 1997
Date: June 27, 1997
Management
A successful Laser Processing Workshop was held on June 25-26.
Approximately 50 attendees representing most of the LPC institutions
participated in the workshop. Accomplishments at the workshop
included initiation of a series of proposals to be developed over
the summer for initial R&D projects with the FEL and planning
by three of the working groups (polymers, metals and microfabrication)
for fit-up of the User Laboratory.
The week's highlights included considerable installation activity
including moving the gun into the FEL building. Details are discussed
below.
Injector Test Stand
The gun was moved to the FEL Facility this week, and the cryounit
is there, as well. Accordingly, this will be the last weekly
report on the Injector Test Stand.
Regarding the photocathode gun:
Prior to moving the gun, we completed the planned measurements
of bunch length and longitudinal emittance at high bunch charges,
obtaining a complete set of "good" data for 135 pC bunch
charge. Although we still need to analyze the totality of the
data carefully, preliminary indications are that they agree very
closely with the predictions of simulations (PARMELA).
We also assessed the results of last week's run to measure the
influence of dark current on photocathode lifetime. The cathode
quantum efficiency was measured for eight six hour periods, divided
into four separate conditions, each repeated for two six hour
periods. These conditions were: 1. HV off, 2. HV on, solenoid
unpowered, 3. HV on, solenoid powered to nominal correct strength,
4. HV on, solenoid powered to about double strength, to try to
capture electrons emitted from the edges of the cathode, nearby
areas of the ball, etc. The net result is that there is no obvious
dependence of the cathode lifetime on HV on or off, nor on the
solenoid strength. For the conditions of this measurement, whatever
field-emission currents were present did not materially affect
the photocathode lifetime, no matter where the solenoid was used
to put the beam, nor did the fact that the high voltage was on
have any discernible effect. A rough estimate of the cathode
e-folding lifetime under these conditions is 220 hours. This
isn't great, but it does imply that the photocathode should stay
alive long enough to conduct operations over some reasonable period
of time. Typical initial quantum efficiencies (QEs) are in the
neighborhood of 7% or better, and if there were 2.5 W of green
light incident on the cathode, (corresponding to having a nominal
5 W of green at 74.85 MHz from the laser and operating at a bunch
repetition rate of 37.425 MHz) we could maintain 5 mA of beam
current with cathode QEs down to 0.5%. If the cathode decay were
truly exponential, started from 7% initially, and had a 220 hour
1/e lifetime, this would imply operation for almost 600 hours.
This means that during our experiments something other than the
bad effects of field emission or the simple presence of high voltage
on the cathode was shortening the photocathode lifetime. Whatever
is doing the damage is associated with electrons being emitted,
and the decay is likely determined by where these electrons go.
It is conceivable that things will be better in the FEL injector,
i.e., when the beam goes through the cryounit as opposed to going
directly into a nearby dump, as long as there is no significant
scraping along the way.
The drive laser was operated this week to make some light box
transmission measurements, and some high dynamic range autocorrelation
measurements. We will move the drive laser to the FEL building
next Monday (6/30), with the rest of the optics following the
next day. We plan on having the drive laser operational by the
end of the week. In preparation for the move, the optical table
was set in the Clean Room, and we cleaned the room. Plumbing
for the laser's chiller will be completed when the laser is moved.
Accelerator Systems
Regarding the beam-transport system:
Everson Electric glued mu metal on one of optical chicane dipole
successfully and are continuing with the assembly, anticipating
completion Monday.
Reverse bend cores continue in final machining at Process Equipment Co. while core slabs for the 180 degree bends are in heat treat and its coils may start winding next week. At Master Machine, they are performing the last operation (grinding to height) on the last parts (back legs) required for the injection/extraction dipoles. All remaining injection/extraction dipole coils were shipped Wednesday.
We signed-off of the Panofsky trim quad drawing set. Sextupole
cores and coils remain on track for a delivery on July 8.
In the corrector area, all three mu metal correctors (4 each required)
are being designed. Steve Lassiter is using TOSCA to analyze
a constant perimeter design, including end effects. So far he
has achieved a magnet good to 2.7 % using crude angle changes
in the coil shells and is confident that he can improve on this
to the required 1 % level. This sets the stage for designing
the basic coil of the 15 nested and 11 single pole air core correctors
that we need.
More formalized welding and cleaning specifications were signed
off at the start of the week to aid in making the fabrication
and welding of chambers a more defined process. Welding on the
extraction dump Y Chamber body was completed while parts for the
optical chicane chambers were being readied for welding. The
60-inch test piece chamber was deliberately contaminated with
machining oil and was cleaned with our steam cleaner. It will
be capped and pumped down to establish a base pressure for that
level of treatment. This will help us assess if we can use the
Injection X Chamber.
Design continued on the Scraper while the insertable dumps are
out for bid.
Regarding cryomodule fabrication:
Cryounit assembly production is on accelerated schedule providing
small amount of slack to support on time delivery of the cryomodule.
Cryomodule assembly is on schedule with no issues. The cryomodule
should be delivered on schedule.
Regarding electron-beam instrumentation and controls (I&C):
The network for CEBAF connections are 80% installed. 50% of telephone
cables are in place. Computer server racks have begun to arrive,
with the balance arriving next week. The network hub has been
set up and will be installed and commissioned late next week.
The ITS equipment will begin to move in on Monday.
The ODH system has been certified in preparation for next week's
cooldown. The cryogenic controls are fully operational. Trim
magnet trunk cables were installed. The control room tables are
complete.
FEL Systems
Wiggler
The vacuum chamber supports were delivered this week. The e-beam
welder is continuing to refine the welding technique on the vacuum
chamber parts. The long tubes being made outside should be complete
next week. The quadrupole girders which mount on the wiggler
are being modified and should be reassembled next week.
Optics
The Brewster window pickoffs (one plus a spare) passed inspection
and are being submitted for cleaning. All optical cavity components,
save the mirror holders and bellows, have been cleaned. This
will begin on 7/7, and then we will reassemble the cavities in
the FEL building. The mirror can stands were received; the installation
team is beginning to mark positions to set the anchors for them.
They are also marking the position for the optical transport
line through the User Labs. The Mach-Zehnder interferometer measurements
in the FEL building indicate a relative change in separation between
the two optical tables of about +/- 60nm. The fundamental frequency
of the oscillation is ~28-29 Hz. While the level of oscillation
shouldn't impact us, we'd like to identify the source of the vibration,
so tests will be conducted next Monday evening, where we shut
down LCW and the HVAC to the building for a brief time. We twice
attempted to braze calcium fluoride mirrors into their copper
holders, both times they cracked during cooled down. We believe
we are cooling too quickly and will try again next week, once
we receive another mirror.
During the LPC meeting, we met with Dr. Sadegh Siahatgar and updated
him on optics results from China Lake and gave him a tour of the
optics lab and the FEL Building.
Commissioning/Operations
Some effort was spent to ascertain whether the tight alignment
tolerances on the quadrupoles immediately preceding the wiggler
could be met by way of the conventional Jefferson Lab alignment
procedures. Specifically, residual twist, yaw, and pitch in these
magnets are potentially of concern. We are in process of modelling
the effects of these possible alignment "errors" on
the beam leading to the wiggler, and this will lead to a more
refined specification on alignment tolerances.
Follow-up studies with PARMELA (mentioned in last week's report)
were done to investigate whether ghost pulses during the 135 pC
runs of the photocathode gun were scraping in the light box.
The results indicate tighter confinement of the ghost pulses than
of the bunches, indicating no significant scraping. We therefore
are investigating where on the ball of the cathode-support assembly
would field-emission sites likely lead to scraping in the light
box. We know scraping was occurring in the light box during the
high-bunch-charge experiments because the resultant radiation
saturated Geiger-Muller tubes located there. So, it is clearly
of interest to ascertain the source of the scraped electrons.
Facility
Finish work and punch lists are the order of the day for the rest
of the contractor effort. Partitions went up in the men's bathroom.
The toilets now flush. Vinyl tile is in the lower lobby. The
kitchenette cabinets were installed but incorrectly and will require
rework. Site work is heavily underway to put in the road to the
loading dock, the west loading ramp, and curbs and parking lots
in front. By weeks end the areas had been graded and the curbs
poured. West emergency exit stairs were also being cast. They
were about 50% done at weeks end. The elevator doors are installed
and the controls are being hooked up. The plan is for certification
next week. Controls people were balancing the HVAC units through
the week. It was a good test case for high temperature excursions.
No problem keeping things cool. Chilled water was plumbed to
the clean room and set up for the photo-injector laser. Safety
system equipment was installed by Jefferson Lab folks in preparation
for the turning on of liquid helium in two weeks. ODH sensors
were successfully tested and are on line. Helium gas began circulating
in the cryo transfer lines. RF crews are heavily into hooking
up the injector cryounit and wiring up the cryomodule low level
controls. The gun was moved into the injector pit. Survey crews
are checking grids and we continued measuring vibrations and length
excursions on the optical cavity tables. So far it looks very
good. Controls are starting to go in to the control room. Fiber
and cable was pulled for the control network and phones throughout
the upstairs.
The gun is moved to the FEL facility and is in the injector pit.
The drawings for the new optical transport mirror cans are being
checked. The clean room is awaiting certification from Clean
Air Technology. The laser optical bench and the plumbing for
the laser chilled water were installed this week.