Next Meeting
Agenda for Next Meeting
Item Person Responsible Time ---- ------------------ -------- Status of Open Action Items All 5 min Status of Gun Biallas 15 min Commissioning Schedule Bohn 10 min Plans for BBU Experiments (tentative) Merminga 15 min New Issues All 5 min
This Week's Attendees
J. Bennett, S. Benson, C. Bohn, J. Boyce, D. Douglas, F. Dylla, J. Fugitt, E. Gillman, A. Grippo, K. Jordan, G. Krafft, L. Merminga, P. Michel, D. Oepts, P. Piot, C. Rode, M. Shinn, T. Siggins, R. Walker, B. Yunn
Closed Action Items
New Action Items
None.
Old Action Items
Items of Discussion
T. Siggins gave a summary of the status of the photocathode gun. Last week's helium processing turned out to have no discernable influence on either the cathode's photoresponse or the field emission. Consequently this week was spent doing high-voltage processing, ultimately up to 480 kV, prior to opening the gun. During the processing, a new site of field emission was generated, and as it processed (up to 480 kV), a strong nitrogen component prevailed in the emission. Based on this observation, a conjecture is that the site may have been associated with the coating on the stalk.
In view of the ultimately successful outcome of high-voltage processing, we are in the process of replacing the cathode wafer, after which we will do a vacuum bake, heat clean and cesiate the new cathode, and then bring the gun on-line. Once the bad cathode is removed, it will be subjected to Auger analysis in the hope of gaining information about the cause of the gun problems. At this writing (1115, 9 Oct 98) the estimated date for gun turn-on is 20 Oct 98. If successful (meaning delivery of 60 pC pulses at 350 kV), commissioning activities will resume per the plans already in place.
Regarding lasing, it is not known how much cw power the installed CaF mirrors can sustain. S. Benson believes the power will peak at ~400 W due to thermal distortion, but the actual limit awaits empirical determination by way of successful commissioning of the full machine. Presuming the limit is well below a kilowatt, and recognizing the need to lase at wavelengths other than 5 µm, it is of interest to start considering in detail other lasing configurations.
In this light, Benson presented a set of requirements for lasing at 3 µm with sapphire mirrors. The strategy he took was to follow the philosophy of the "High Power Setup Procedure", i.e., first lase straight ahead, then lase with recirculation. He further provided for lasing using 60 pC bunch charge with the electron-beam quality that has already been achieved. The fundamental outcome of his analysis is that a total beam energy of 47.5±0.5 MeV will be needed at the wiggler. A 4 mA average current (60 pC at 75 MHz? -- we didn't discuss this or its implication for the MPS) should be sufficient to provide 1 kW light near 3 µm. These numbers are to be compared to the original design parameters of 42 MeV with 135 pC bunch charge at 37.4 MHz (5 mA average current).
Subsequent discussion centered on the Procedure (all concurred with the strategy of using the High Power Setup Procedure as the starting point) and implications of the required energy. D. Douglas expressed concern about whether the acceptance of the injection/extraction chicanes is sufficient to accommodate the energy in view of unknowns about the transverse beam halo. We should try it, and if problems show up, then perhaps increasing the energy of the injected beam will help.
M. Shinn provided an update on the status of the sapphire mirrors. Analysis at China Lake has shown that the vendor has yet to succeed in applying the high-reflectivity coating. The coatings have delaminated, and they contain scratches and pits. Consequently they cannot sustain the high-average-power light associated with kilowatt lasing. Last week Shinn met with the vendor to discuss the problem and determine remediary measures. Our intent will be to work with the vendor to expedite delivery of acceptable optics.
Another potential avenue for kilowatt lasing that was mentioned but not discussed in detail is to do it at 6 µm with ZnSe optics. We are due imminently to receive a set of these optics, after which we will mount them and subject them to acceptance tests. The option for using them, of course, depends on the outcome of their tests.
In view of all of these considerations, Bohn declared that we will stay the course of commissioning at 5 µm with the installed CaF optics and extract as much power from them as possible. Afterward, at an opportune time, we will conduct a machine-development program to explore the feasibility of providing 47.5 MeV beam at high cw currents. In parallel, work on the sapphire and ZnSe optics will continue, and we will have some basis for deciding the timing of their installation.
G. Krafft reported that the two additional M_56 cavities and peripheral hardware are nearing the point at which they can be installed. In the meantime, Krafft is assisting Bohn in specifying the requirements of the associated electronics for C. Hovater.
New Issues
Krafft stated that Duke continues to plan for irradiation of stents in the straight-ahead machine to be done in conjunction with the next preparation of GEn targets. The motivation for the irradiated stents is for use in conjunction with angioplasty. By inserting stents during angioplasty, there is the prospect of substantially increasing the time interval between successive treatments. Duke is fabricating the hardware necessary for installing stents in our 2G beamline. The "new issue" is that presently it is not possible to provide Duke even a rough date for the irradiation; perhaps it could be early next year.
Procedures in Work
Unfinished Subsidiary Tasks for Phase Space Metaprocedure
Task Principal
---- ---------
I. Stabilize Drifts in Drive-Laser RF Phase Walker/Fugitt
II. Calibrate RF Gradients
A. Buncher Yunn
B. Cryounit Cavities (fix control screen) Merminga/Krafft/Yunn
C. Cryomodule Cavities Merminga/Krafft/Yunn
III. Reconcile Methods for Setting RF Phase Merminga/Krafft/Yunn
A. Buncher Cavities
2. Minimum bunch length/time of flight
B. Cryounit Cavities
1. Transient phasing
2. Cresting
3. Time of flight
IX. Complete Diagnostics Procedures and Users' Guide Krafft/Piot
A. M_55 #1, #2
B. Happek #1, #2
C. BPMs
1. button calibration factors
2. rotated MEDM spike charts
3. difference orbit data acquisition script
D. Multislit #1, #2
E. Quad/Viewer (at Wiggler, after Wiggler)
F. Multimonitor (at Wiggler)
G. SLM/CSR
H. Zero Phasing
I. Momentum Spread (Injector, Linac Pre-Wiggler, Linac Post-Wiggler)
J. Energy
XIV. Laser Turn-On Procedure Benson