Next Meeting

Date: 9 Jul 99 (Friday next week)

Time: "0845"-0945

Place: FEL Facility Break Room

Agenda for Next Meeting

• Status of Machine (Bohn, 5 min)
• Program/Two-Week Schedule (Bohn, 10 min)
• New Hardware (Jordan, 10 min)
• New Software (Grippo, 10 min)
• Update on Feasibility of BBU Experiment (Merminga, 20 min)

This Week's Attendees

J. Bennett, S. Benson, G. Biallas, C. Bohn, J. Boyce, I. Campisi, D. Douglas, K. Jordan, R. Li, L. Merminga, G. Neil, P. Piot, T. Siggins, B. Yunn

Closed Action Items

None.

New Action Items

None.

Old Action Items

• Biallas: Establish a staffing plan for the week-long injector studies. Status: No progress. (due 12 Jul 99)
• Biallas: Install the proper magnet excitation curves into software. Status: Working with Douglas. (due 6 Jul 99)
• Douglas/Biallas: Create new BURT Save/Restore files to reflect the changed magnet excitation curves. Status: No progress. (Due 9 Jul 99)
• Merminga: Calculate modulation amplitude of beam-current orbit needed for BBU experiment. Status: No progress. (due 9 Jul 99)
• Douglas: Check difference-orbit data against "48.2 MeV beam". Status: No progress. (due 30 Jun 99)

Items of Discussion

T. Siggins briefly reported the status of the gun. It has been processed without cesium to a quiet 440 kV, and the processing is therefore deemed to be complete. The new cathode electrode behaves much differently from the old one. Specifically, field emission stayed quiet up to 370 kV, whereas the onset of field emission with the old ball was much lower, ~200 kV. Apparently the old ball has a persistent field-emission site that degraded with time. The HVPS still needs to be checked thoroughly. As it stands now, the current output seems to fluctuate inexplicably, and the HV stack has yet to be carefully inspected for damage. K. Jordan has the work planned and coordinated, and it will start today.

G. Neil proposed that we plan to begin the next run sometime next week. C. Bohn said that if progress is rapid, then we'll do it. However, hot checks remain (e.g., gun with cesium, the MPS, rf systems, the HVPS), most of which must be done next week. The optimistic plan for hot checkouts is as follows:

• HVPS and HV system (Jordan is lead): this week.
• Gun at 350 kV with cesium (Siggins is lead): Tuesday next week.
• RF systems (Merminga is lead): Wednesday next week.
• MPS (Jordan and Coleman): Wednesday and Thursday next week.
• 60 pC beam (Siggins and Biallas): Friday next week (can then run the rest of Day Shift and Swing Shift).

Today's meeting focused on discussions of coherent synchrotron radiation (CSR). R. Li gave a very nice presentation of her simulation results to date, including how they contrast with P. Piot's measurements of emittance growth around the first recirculation arc. Li's approach is as follows:

1. Start the simulation at the exit of the wiggler;
2. Construct the initial beam phase space from measurements (at this stage, the measurements constitute rms transverse and longitudinal beam properties, from which she constructs gaussian density profiles);
3. Use actual magnet settings in the simulations;
4. Ignore CSR effects in the pre-arc chicanes because they are known to be small;
5. Use Twiss parameters measured after the second chicane and back-propagate them to the start point;
6. Mimic changes in cryomodule gang phase by tilting the phase space at the cryomodule exit and back-propagating the Twiss parameters to that point, for which the results correlate well with PARMELA-based expectations.

One result of the theoretical and numerical studies is that the CSR-induced longitudinal wakefield dominates the beam dynamics. The transverse wakefield is negligibly small and can be ignored, thereby aiding the run time. Basically what happens is that the longitudinal wakefield induces and energy spread during the arc which spoils the achromaticity and gradually induces an emittance growth as the beam traverses the arc.

Li's simulations nominally predict small emittance growths ( 7% or less) with bunch charges in the range 20-50 pC, which is a puzzle given Piot's measurements of up to 80% growth. To try to resolve the puzzle, Li looked at several aspects of the problem. First, she noted that M_26 is antisymmetric through the arc, but M_16 is not. Consequently, the effect of CSR-induced energy spread on x' must be small, but the effect on x can be substantial. If the phase space at the entrance to the arc is rotated to make the horizontal beam size smaller, then the influence of CSR on the emittance (through x) will be more pronounced. The CSR-induced energy spread is identical, but it's influence is enhanced. Li can "see" 20% emittance growth at 60 pC in her simulations by rotating the phase space in the manner described. She also noted that the measured Twiss parameters change (modestly) as the cryomodule gang phase changes, and so follow- on runs should include this dependence. In addition, fine structure (meaning gross deviations from gaussian density) may influence the emittance growth; it is thought based on analytic calculations that the effect will be small, but it still needs to be checked via simulation.

Implications for the upcoming experimental program are as follows:

1. Need to check to what extent the beam must be centered through the arc when changing the cryomodule gang phase to ensure non-CSR wakefields are unimportant (Piot said no BLMs were lighting during his measurements, but the check is still warranted);
2. Need to try to measure the arc-induced energy spread because it is a key signature of CSR (method will be to compare images on viewers ITV3F01 and ITV5F01, both of which are located at the same high-dispersion point in the first and second arcs, respectively -- subtracting the digitized images and looking at the residual would probably aid the sensitivity of the measurement);
3. Vary the matching to the arc (tilt the input phase space) to enhance the influence of the CSR-induced energy spread on the horizontal beam size;
4. Measure the vertical component of emittance at the entry and exit to the arc (significant change would be the signature of non-CSR wakefield).

Li will continue her simulations per the focus delineated above. Bohn, Li, Douglas, and Piot likewise will continue to evolve and iterate the overall CSR program as new information becomes available.

New Items

None.