MEMORANDUM

**To:** Distribution

**From:** F. Dylla

**Subject:** FEL Upgrade Project Weekly Brief --May 24-28, 2004

**Date:** May 28, 2004

**Highlights:**

We had a very productive week for the start-up of the FEL Upgrade after
the completion of the three week shut down for the third cryomodule
installation and photogun cathode replacement. To ensure start-up at the
earliest time this week, we devoted last Saturday to checking out the
new or modified magnet systems and Sunday to high voltage processing of
the gun. For the week we have accomplished the following:

-   Completed the gun set-up with a new GaAs cathode that showed an
    initial quantum efficiency of 6.2% (the highest we have recorded).

-   Set up the injector (the vacuum improvements we incorporated appear
    to be worthwhile-we see no vacuum activity with 3.6mA operation).

-   Accelerated e-beam through all 3 cryomodules.

-   Matched and recovered 88 MeV e-beam.

-   Measured 1st BBU threshold (\~ 3 mA @ 88 MeV) which is remarkably
    close to the prediction (within \~10%) of the TDBBU code based on
    the HOM measurements of the 3^rd^ module cavities. Kudos to the CASA
    BBU team!

-   With only a brief attempt we were able to raise the BBU threshold to
    \~3.6 mA by phase advance.

-   Lased at 2.2 kW with the above 88 MeV configuration at \~3 mA.

-   Measured losses in the new 6 micron HR optic (5x less than previous
    HR).

-   Set-up initial 145 MeV beam with the 3^rd^ cryomodule fully powered,
    successfully re-circulated and recovered beam at 1 mA cw, (matching
    activities are planned next week to optimize and push current).

At press time we have successfully completed Steps 1-10 of the 10 kW
Commissioning Plan (shown below) that we sent to the Navy and FEL
Steering Committee on May 14^th^. Of particular note is this important
result:

\- We now know that the 3^rd^ FEL module carries sufficient current that
it will remain in the accelerator. The models indicate the BBU threshold
should scale up conservatively with the square root of the energy ratio,
so that the present threshold observed at \~3.6mA at 88 MeV should scale
by \~1.3x 3.6mA = 4.7 mA (taking credit for none of the planned
amelioration techniques). We remind our readers that Dr. Benson's models
are predicting 10kW at either 6 or 3 microns with the linac operating at
145 MeV and 5 mA.

![](media/image1.wmf){width="6.782638888888889in"
height="4.177777777777778in"}

**Commissioning Summary (Steve Benson):**

This was a short but productive week of beam operations  Problems with
cryounit trips and drive laser software freezes threw some tacks in the
road but we were still able to make significant progress. Monday and
Tuesday were dedicated to dealing with punch list items from the shut
down and in bringing up the gun.  Since this is a new wafer we proceeded
slowly and carefully in high voltage processing. The high voltage
processing with the cathode withdrawn was completed on Monday afternoon
and we set up for a cathode heat clean.  On Tuesday we made a good
cathode (highest quantum efficiency seen to date in this gun) and high
voltage processed with the cathode in place.  This went slowly due to a
noisy current signal that worried us.  We finished the processing,
shorted out the conditioning resistor, and brought the gun back up with
the running resistor.  The current noise was gone.  We then did the
drive laser alignment.  The aperture for this wafer is smaller than the
last so we are exploring the use of smaller spot to reduce scatter and
give us more maneuvering room.\
Wednesday we started beam operations.  After tuning up the injector, we
phased up the linac and set the energy to 88 MeV.  We then threaded the
beam through the beam transport and to the energy recovery dump.  When
we got clean transmission we increased the macropulse current up to 5 mA
while looking at the loss monitors.  We had evidence of halo loss at a
few points around the machine but nothing in the vicinity of the new
module.  This showed that we could transport high current beam through
the new module without loss and did not have to pull the module out. 
There was no conclusion on the BBU except that the threshold was
probably higher than 1 mA.\
On Thursday we recovered the configuration from Wednesday and started to
carefully match the beam around the machine.  We also turned on two
families of sextupoles in the first arc.  This eliminated the halo loss
at two points that we thought were due to transverse beam blowup due to
non-linear dispersion.  We managed to minimize both the linear and
non-linear dispersion in the first arc.  We had some difficulties in
matching because the frame grabber (the WesCam) was not working
properly.  We did manage to get a decent match through the machine
however and the losses were low.   We then got the laser lasing at 5.75
MHz and roughly setup the energy recovery with lasing.  Finally we tried
to run up the CW current and found a hard limit of 0.7 mA regardless of
duty cycle.  No evidence was seen for BBU during the trips.  We realized
that the current limit in the gun high voltage power supply was still
set to its high voltage processing setting.  When it was raised to 10 mA
we immediately managed to get 135 pC at 18.7 MHz

(2.5 mA).  We switched to 75 MHz and found a BBU current limit of 3 mA. 
This time there was a clear signature on the HOM diodes for the cavity
with the lowest threshold.  We then switched to 37.5 MHz so that we
could lase and see the effect of lasing on the threshold.  Lasing raised
the threshold only slightly from 3 to 3.2 mA.  Finally we changed the
betatron phase advance to see if we could raise the threshold that way. 
We raised the threshold to 3.6 using this technique.  Problems with the
injector cryounit tripping off and the drive laser attenuator freezing
up slowed us down in this effort and we finally decided to shut down so
that some work could be done to fix these problems.\
The plan for Friday was to try 145 MeV operation to see if there were
any big problems.  If we could operate easily there we would work on BBU
suppression at that energy.  So far we have been able to operate
relatively easily at the higher energy and we are optimizing the system
so that we can ramp up the current.\
During high current running we noticed that beam loss was still
significant after the cryounit but was just about non-existent before
the cryounit.  The reduction in the diameter of the active area seems to
have been quite effective at reducing halo loss before the cryounit.
Halo loss is concentrated at two locations in the machine\--the
injection chicane and the extraction chicane.  Neither showed much
vacuum pressure changes when we ran 3 mA CW beam.

**Management:**

Everyone's attention was devoted this week to making the start-up from
the shut down go as smoothly as possible.

On Monday, May 24^th^, George Neil and Fred Dylla gave a presentation on
the Navy funded FEL program at JLab to DoD's Office of Force
Transformation in Arlington,Va.

On Wednesday, our new ONR Program Officer, Quentin Saulter made a site
visit and witnessed the machine start-up.

**WBS 3 (Beam Physics) Lia Merminga:**

Beam breakup instability was observed in the FEL Upgrade operating at 88
MeV at \~3 mA of average current. Signals from the Schottky diodes
placed on all 16 HOM ports (2 per cavity) of the CM3 were observed on
oscilloscopes, as the average current ramped up. At approximately

3 mA of beam current, both diode signals from cavity 4 grew
exponentially from 0 power level, while signal levels from all other
cavities remained unchanged. At the same time the beam tripped. The
growth rate was measured to be \~ 5 msec (see attached). Growth rate,
threshold current and mode with the lowest threshold are all in
agreement with simulation results.

Further, lasing was initiated and the phase advance of the recirculator
was changed by a fraction of betatron wavelength (no systematic search
was conducted) resulting in threshold current increase to 3.6 mA.

![](media/image2.wmf){width="5.995833333333334in"
height="4.107638888888889in"}

**WBS 4 (Injector):**

After three weeks of gun and beamline work, we successfully completed
the gun HV conditioning at 424 kV (20% higher than the nominal operating
voltage) on Monday afternoon. Shortly after that, we started a heat
clean cycle on the newly installed anodized GaAs wafer. On Tuesday
morning, we made the first cathode and achieved 6% QE. The QE obtained
on first cathode made one year ago after installing a new wafer was
0.6%. We experienced some current noise in the transmission line during
HV processing the new cathode with the conditioning resistor. There was
no evidence of field emission in the vacuum side. The noise went away
once we switched to the running resistor. We will check the conditioning
resistor once we have a chance.

On Wednesday morning the new cathode was delivering beam, although halo
was observed despite the anodized wafer. Because of the smaller active
area, the positioning of the drive laser spot on the cathode is very
limited. The laser beam is very close to the edge of the mirror and to
the aperture of the window beam pipe, generating significant amount of
scattered light. Halo loss used to be a limiting factor while running
high CW current because of vacuum rise in the beamline between the gun
and the quarter cryo unit. Running 3 mA CW beam used to raise the vacuum
from 2 to 3 times in that region. There is almost no vacuum rise now
when running the same CW current, indicating the success of the added
pumping in reducing halo loss.

**WBS 6 (RF):**

Report next week. (writer devoted to hardware at press time).

**WBS 8 (Instrumentation):**

Ditto.

**WBS 9 (Beam Transport):**

Ditto.

**WBS 10 (Wiggler):**

Ditto.

**WBS 11 (Optics):**

**Plan A is to obtain low loss 6 um optics**

While lasing at reasonably high power, \~ 2.2 kW at 3 mA, we used the
opportunity to check the loss in the new HR.  Our first measurement,
made at a predicted lasing wavelength of 5.75 um, is 450ppm +/- 175
ppm.  In speaking with the vendor, \~ 100 ppm is due to transmission
through the coating and substrate into the backplane cooler.  This is \~
at least 5X lower than the mirror we removed. 

 

**Our three Plan B\'s had the following progress:**\
***Cryomirror:***\
Producing a uniform indium annulus continues to stump our braze expert. 
In part, this is due to the tolerance of the fixtures that provide
a base for the solidifying indium.  New fixtures are being made.

***2.8 micron mirrors: ***\
3\" dia OC mirrors are in fabrication.  We did metrology on one that was
fabricated but uncoated.  It surpasses the rms roughness spec of 1 nm by
3X.  Metrology for the mirrors in hand continues.  Mirrors checked so
far meet specifications.\
***Scraper outcoupler:***\
No progress since the last report. 

 

***Other activities:***\
A new Nd:YLF laser rod was mounted and installed, resulting in
significant improvement in performance.  We now produce 5 W of SHG with
\~ 1 deg of phase noise.  SHG powers over

7 W are easily in reach.  Kudos to Shukui and David for their weekend
efforts.  We have been bedeviled with the software for the attenuator
suspending, and causing headaches for the commissioning team. I&C is
working on it.  A lot of work is going on in User Lab 2 to organize
hardware more efficiently.  The loaner streak camera\'s minimum
resolution was checked, and found to be long, 18 ps.  The vendor had
given us a unit with a total sweep aperture of \~ 1ns.  (Guess you get
what you pay for.)  We\'ll probably install it in the vault to wring out
the matching optics and remote control.  New fixtures to hold the PbS
and pyroelectric arrays arrived.  The PbS array will be installed on the
spectrometer when we have some time.  The pyroelectric array is working
on the benchtop.  New PIC-based readout electronics are being designed. 
It will be used in the further IR.  We held planning and coordination
meetings on the optical transport procurement and installation, with the
goal to install vault hardware during the early August down period.

**Terahertz:**

Additional shielding was placed adjacent to the long vertical beam tube
in the vault.  The alignment laser at the backward tangent point was
aligned to generate a beam exiting the M1 chamber, which now has a 20mm
clear aperture diamond window so that we can carry out tests prior to
the installation of M1, and at the same time provide machine beam
diagnostics.  The Nicolet Impact-400 FTIR THz Michelson interferometer
was re-aligned on this port.  Meanwhile, the mirror mount components
were identified and parts sent for cleaning prior to assembly.