Free Electron Laser Commissioning/Diagnostic Meeting
Tuesday, March 19, 1996
Recorder: G. Krafft
Next Meeting
- Date: Tuesday, March 26, 1996
- Time: 2:30 - 4:00 **** Please note (and complain about, if need be) time
- Place: MCC Room 101
Attendees
K. Jordan, J. Denard (2nd Half), C. Bohn, G. Neil, D. Douglas, R. Legg,
D. Kehne, and G. Krafft
Items of Discussion
The main body of the meeting consisted in systematically going through the
accelerator in order to determine if the diagnostic complement was sufficient
in the current drawing. As a start, the commissioning was divided into the
steps listed below. We also tried to identify uncertain items that should
be investigated off-line in the action items below. The first cut at the
steps were:
- I-I Beam from gun
- I-II Buncher/solenoid centering
- I-III Buncher setup
- I-IV Unit centering
- I-V Unit setup (gross)
- II-VI 10 MeV Thread
- I-VII Unit setup (fine)
- I-VIII Beam on dump
- I-IX Matching
At this point one would hope to have a well-characterized beam (known
alpha, beta, epsilon, bunch length, and energy spread) entering the
main recirculation path. The commissioning steps beyond the injector
(M for main) are
- M-I ``Linac'' thread
- M-II RF Phasing
- M-III RF Amplitudes
- M-IV Chicane 1
- M-V Wiggler
- M-VI Chicane 2
- M-VII Pi bend 1
- M-VIII Back leg thread
- M-IX Pi bend 2
- M-X Recirculation
- M-XI Beam to 10 MeV dump
Associated with these steps the following comments were made:
- I-I The pulse structures for initial setup, ``viewer limited'' mode,
matching mode, etc., should be agreed upon and established.
- I-II Buncher centering is needed to minimize emittance growth, if
the solenoid is not centered at the same time, we should move
the solenoid to the buncher center. Whether there will be a problem
will be determined early in Engwall's measurements. No Action
Having a phase modulation capability, as in the nuclear physics
injector will be useful. Having a modulator on the laser reference
would make this step, and RF phase verifications easy. Deffered Action
- I-III The buncher will be phased first by transient phasing, and its
amplitude set by RF power measurements. In order for this to procede,
it is needed that the RF power measurement (say forward - reflected
power) be calibrated, and the external Q of the cavity must be known
before hand. Neil volunteered to make sure such information is available.
In this region, scraping is liable to be an issue. Jordan assured that
Geiger tubes will be available if needed. Again, this should get clarified
during Engwall's experiment. No Action
- I-V Again, transient phasing would provide the first estimates of the
phase settings. No Action
- I-VI Are stray fields an issue? Legg will investigate.
- I-VII Precise final phasing will be done by energy maximization.
There was a question about how precisely the unit gradients must
be set. Bohn agreed to talk to Liu about getting that resolved. Like-
wise, how precisely must the off-crest phase be set?
A separate but related question is, how precisely must the overall
energy out of the injector be set. Douglas to investigate.
- I-VIII It was agreed that the test stand matching was not a problem. However
in the FEL proper, it was pretty clear that there was no good way
to verify the match. If the spot was bad downstream of the full
module, there would be no way the show that the injector was/was not
the culprit. After much discussion it was agreed that a quadrupole
should be added to the first 10 MeV dump line, so that the dispersion
at the viewer could be suppressed and a measurement could be done.
Such a solution will be somewhat cumbersome compared to the system
in the nuclear physics injector, but no good proposal on the main
beamline seems possible given the space. Douglas has an action to
check the said optics and Bohn has an action to get the drawing up-
dated.
Two additional issues, not really covered above, were where and how will we
measure the injector current, and how are we going to verify that the beam
stays on the dump. Legg will investigate dump instrumentation and report
back next week.
The final issue associated with the injector, namely, slit measurements
of emittance in the injector, that seem necessary to quantify our emittance
credibly (at least to the outside world), needs work, especially in regards
to specifying the properties and design of the slits. At some point we
need to get someone working on this seriously. Action defferred.
M-II RF phasing can be accomplished in the first instance by transient
phasing. A more precise measurement follow from using the BPM at
the first chicane bend when the beam gets there. Finally, best precision
is achieved at high dispersion points in the lattice: the SLM
port in the recirculation case, or in the beam dump in ``first light
experiments''. Because of concerns expressed about possible damage to
the wiggler as these processes are going on, and partly to ensure
that emittance measurements are possible both before and after the
first pi bend, it was proposed that the layout include a wiggler bypass
and the 42 MeV dump be moved downstream of the first magnet of the
first pi bend. The layout should include some optics to allow the
dispersion to be controlled. Diagnostics in the bypass were discussed
and a tentative plan was proposed. Douglas has the action to finalize
the bypass optics and Bohn will make sure the drawing gets updated
properly.
M-IV The chicane will be verified to be ok primarily through the fact that
beam traverses the chicane properly and through the fact that the
bunch length is short after the chicane (a Happek device will be
installed there). On the first issue Douglas expressed great con-
cern that the chicane magnets be done right, especially that they
by cross calibrated to the high degree needed. We would like to
avoid problems like those that have plagued the doglegs. On the
second issue, a possible disconnect in the present design was un-
covered. With the high M56 in the chicane it is necessary to run
the module off crest to a level that was thought possibly to have RF
implications. G. Neil will find out for next time.
M-V It was thought that the wiggler looked OK. The match will be verified
by the OTR diagnostics and the steering can be done with BPMs and
OTRs. An issue arose as to whether the layout of the wiggler and
chicane could be more optimally done. Reducing M56 in the chicane
might be possible, elliminating some portion of the RF problem
mentioned above. Douglas, Neil, and Liu were drafted to resolve this
issue before the layout is frozen.
M-VI Legg suggested that it would be a convenient to have an OTR diagnostic
where the BPM was in the second chicane. The reason is that one could
check the energy spread generated by the wiggler, and have a sensitive
check that everything is just fine during operations. It was agreed
to incorporate this change.
M-VIII According to Douglas, the back leg optics had completely changed.
No comments were made other than to assure that we thought an
emittance measurement can be made. Douglas has an Action to specify
diagnostics locations on the back leg.
M-X Given the short recirculation time, given that recirculation is done
more or less 180 degrees off crest, and given that there isn't much
room, Krafft has an action to try to resolve how the path length
measurement might be done. Because of the large wiggler generated energy
spread, it is not likely that energy measurements on the second
10 MeV dump will be very usefull.
M-XI Not much thought has been given to getting the recovered beam on the
dump. This is food for a future action item.
After completing this discussion, it was pointed out that we also have to
consider longerterm operational issues. For example, if the primary way to
control the radiation wavelength is through beam energy, and requests for
energy changes are to be frequent, we should have enough capability to
make and verify energy changes as expeditiously as possible. Same for
pulse changes. How are we going to optimize the power out when a user
complains. It was requested that others add to the list.
Action Items (assigned)
- Create a new drawing with changes indicated (Bohn) 3/26/96
- Buncher gradient calibration (Neil) 3/26/96
- Stray fields in test stand (Legg) 3/26/96
- Unit gradient precision (Bohn, Liu) 3/26/96
- Injector energy precision (Douglas)
- 10 MeV dump optics (Douglas) 3/26/96
- Bypass optics (Douglas) 3/26/96
- M56/RF phasing and beam load issues (Neil, Douglas, Liu) 3/26/96
- Dump Instrumentation (Legg) 3/26/96
- Back leg diagnostic placement (Douglas) 3/26/96
- How should the path length be checked? (Krafft, Jordan) 3/26/96
Action Items (pending)
- Determine Beam Pulse structure for setup
- Laser phase modulator
- Slit emittance measurement
- Is an energy compression scheme needed?
Agenda for Next Meeting
Item Person Responsible Time
---- ------------------ ----
* Review agenda Krafft 5 min.
* Corrections to minutes from last meeting All 5 min.
* New drawing Bohn 20 min.
* Optics issues Douglas 20 min.
* Buncher gradient calibration Neil 10 min.
* M56/RF phasing issues Neil 10 min.
* Dump instrumentation Legg 15 min.
* Path length Krafft 10 min.
* Agenda for next meeting All 5 min.
Acknowledgement
A templet devised by J. Karn was used to produce these minutes. Comments back
(to G. Krafft) are appreciated.