To: J. Cook, D. Helms, W. Skinner
cc: Division (M7), FEL Coordination Group
From: F. Dylla
Subject: IR Demo Project Weekly Report, October 19-23, 1998
Date: October 23, 1998
Management
This week was spent with high voltage conditioning and diagnosti
activties with the photogun. Highlights for the week include:
determination of a likely cause of the poor photoresponse performance
with the last two installed photocathodes (suspected problems
with the GaAs cesiator).
The FEL project received a very favorable review during this week's
SURA/DOE Institutional Management Review for Jefferson Lab.
Two day "brainstorming" sessions were held this week
to examine the design issues for the 20 kW IR/1 kW UV upgrade
for the IR Demo.
An announcement was distributed this week for a one day workshop
on UV, x-ray and gamma ray applications of FELs. The workshop
will be held at SURA headquarters in Washington on Nov. 20 and
it is co-sponsored by SURA and the Southest Photon Consortium.
FEL Installation/Maintenance Activities
In addition to the Gun/photocathode work noted in the next section,
the following maintenance and installation activities were performed
this past week:
The sticky shutter on the Drive Laser was modified and tested
satisfactory.
The certification of the PSS was completed with the Drive Laser
tests.
The MPS was audited and found wanting. Corrections have been
made and the MPS audit will be finished next week.
Cables were pulled and terminated for the Happek bunch length
spectrometer picomotors.
A laser scanner is being installed today that will allow spatially
resolved measurements of responsivity of the Gun's photocathode.
The TUN1 locks are in the process of being rekeyed today.
FEL Commissioning Activities
Work continued toward resurrecting the photocathode gun. In the
end, it proved unsuccessful; however, we have evidence as to why,
and we developed a plan based on the evidence.
Specifically, following a procedure that had consistently proved
successful in the past, we did high-voltage processing of the
gun up to 430 kV and then let the gun soak at that voltage for
about 36 hours. We then heat-cleaned the cathode at 675 C for
two hours, followed by cesiation. The photoresponse of the cathode
after cesiation was very poor. Moreover, a subsequent attempt
to apply high voltage was met with field emission starting at
285 kV. This is essentially the same circumstance that befell
us with the last cathode. We tentatively concluded that the most
likely problem is that the cesiator has been introducing contaminants
into the gun, although there are other possible problems.
In parallel, William & Mary has been doing an auger analysis
of the previous cathode. Last night they informed us that they
were unable to find cesium in the spectra, lending further credibility
to our conjecture about the cesiator. Consequently, this morning
we developed a plan centered on replacing the charge in the cesiator.
Today (this is being written at 1000 Friday, 23 Oct 98) we will
map the photoresponse of the cathode using a new diagnostic developed
over the past few weeks. Then we will again do high-voltage processing
of the gun to 430 kV. On Monday morning we will begin to heat-clean
the cathode (at 600 C for 12 hours, known to be a successful recipe
from past experience) and also start refurbishing the cesiator.
On Tuesday afternoon we will recesiate the cathode and check
the photoresponse. Follow-on plans depend on the outcome; of
course, the hoped-for outcome is that we will ramp the voltage
up to 350 kV, turn on the gun, and resume commissioning per our
(now longstanding) plans.
An important side activity this week has been to work on completing
the audit of the Machine Protection System (MPS). A new problem
arose, namely that the machine-mode status was incorrectly reflected
in the EPICS control software. The problem was traced to a cable
that blew a fuse in one of the beam-viewer chassis. The cable
was fixed and the fuse was replaced, presumably correcting the
problem. Completion of the MPS audit is scheduled for early next
week in coordination with the gun activities described above.
Finally, we held an off-site meeting Thursday morning, to be wrapped
up this afternoon, to brainstorm plans for upgrading the FEL for
20 kW IR light and 1 kW UV light. One absolutely indisputable
need is a program to develop incrementally a gun capable of delivering
~10 mA cw beam, for example, 135 pC bunch charge at 75 MHz. The
gun, of course, needs also to be sufficiently reliable and available
to support a user facility. The collection of ideas will be used
as an aid to define the upgrade program during the months to follow.
IRFEL Installation and Maintenance
In addition to the Gun/photocathode work, the following maintenance
and installation activities were performed this past week:
1. The sticky shutter on the Drive Laser was modified and tested
satisfactory.
2. The certification of the PSS was completed with the Drive
Laser tests.
3. The MPS was audited and found wanting. Corrections have been made
and the MPS audit will be finished next week.
4. Cables were pulled and terminated for the Happek picomotors.
5. A laser scanner is being installed today to determine the
responsivity of the Gun's photocathode.
6. The TUN1 locks are in the process of being rekeyed today.
IRFEL Commissioning
Work continued toward resurrecting the photocathode gun. In the
end, itproved unsuccessful; however, we have evidence as to why,
and we developed a plan based on the evidence.
Specifically, following a procedure that had consistently proved
successful in the past, we did high-voltage processing of the
gun up to 430 kV and then let the gun soak at that voltage for
about 36 hours. We then heat-cleaned the cathode at 675 C for
two hours, followed by cesiation. The photoresponse of the cathode
after cesiation was very poor. Moreover, a subsequent attempt
to apply high voltage was met with field emission starting at
285 kV. This is essentially the same circumstance that befell
us with the last cathode. We tentatively concluded that the most
likely problem is that the cesiator has been introducing contaminants
into the gun, although there are other possible problems.
In parallel, William & Mary has been doing an auger analysis
of the previous cathode. Last night they informed us that they
were unable to find cesium in the spectra, lending further credibility
to our conjecture about the cesiator. Consequently, this morning
we developed a plan centered on replacing the charge in the cesiator.
Today (this is being written at 1000 Friday, 23 Oct 98) we will
map the photoresponse of the cathode using a new diagnostic developed
over the past few weeks. Then we will again do high-voltage processing
of the gun to 430 kV. On Monday morning we will begin to heat-clean
the cathode (at 600 C for 12 hours, known to be a successful recipe
from past experience) and also start refurbishing the cesiator.
On Tuesday afternoon we will recesiate the cathode and check
the photoresponse. Follow-on plans depend on the outcome; of
course, the hoped-for outcome is that we will ramp the voltage
up to 350 kV, turn on the gun, and resume commissioning per our
(now longstanding) plans.
An important side activity this week has been to work on completing
the audit of the Machine Protection System (MPS). A new problem
arose, namely that the machine-mode status was incorrectly reflected
in the EPICS control software. The problem was traced to a cable
that blew a fuse in one of the beam-viewer chassis. The cable
was fixed and the fuse was replaced, presumably correcting the
problem. Completion of the MPS audit is scheduled for early next
week in coordination with the gun activities described above.
Finally, we held an off-site meeting Thursday morning, to be wrapped up this afternoon, to brainstorm plans for upgrading the FEL for 20 kW IR light and 1 kW UV light. One absolutely indisputable need is a program to develop incrementally a gun capable of delivering ~10 mA cw beam, for example, 135 pC bunch charge at 75 MHz. The gun, of course, needs also to be sufficiently reliable and available to support a user facility. The collection of ideas will be used as an aid to define the upgrade program during the months to follow.