Next Meeting

Date: 30 Jun 99 (Wednesday next week)

Time: "0845"-0945

Place: FEL Facility Break Room

Agenda for Next Meeting

• Status of Gun (Siggins, 5 min)
• Status of CSR Simulations (Li, 20 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, R. Evans, R. Li, L. Merminga, G. Neil, P. Piot, T. Siggins

Closed Action Items

• Biallas: Incorporate latest injector measurements into scope of injector test plans. Status: Done; summary of test plans was presented at today's meeting.

New Action Items

• Biallas: Establish a staffing plan for the week-long injector studies. (due 12 Jul 99)

Old Action Items

• Biallas: Install the proper magnet excitation curves into software. Status: Planned, not forgotten. (due 30 Jun 99)
• Douglas/Biallas: Create new BURT Save/Restore files to reflect the changed magnet excitation curves. Status: Planned, not forgotten. (Due 9 Jul 99)
• Merminga: Calculate modulation amplitude of beam-current orbit needed for BBU experiment. Status: No progress. (due 30 Jun 99)
• Douglas: Check difference-orbit data against "48.2 MeV beam". Status: No progress. (due 30 Jun 99)

Items of Discussion

C. Bohn opened by remarking that according to the latest information the new sapphire mirrors are on track for early- July installation, and so it appears probable that the upcoming run will involve "exclusively" 48 MeV beam energy. Plans are to operate the gun at its nominal 350 kV.

G. Biallas summarized the status and plans of injector studies slated for the last week of the upcoming run. Briefly, the goals are to improve the beam quality and orbit out of the injector and into the cryomodule, and to establish a "simple", reproducible method for setting up the injector. In order of priority, the essential tasks are as follows:

• Fix the orbit, both through the cryounit and through the injection line.
• Calibrate the srf gradients with beam and cross-check against previous rf calibration.
• Test the effects of the gun solenoids on focusing and beam quality.
• Fix the transient phasing, to include: establishing specifications for stability of the buncher, identifying hardware and controls necessary to meet the specification, and should transient phasing prove unfeasible, identify a viable alternative.
• Establish the influence of cryounit cavity #4 settings on bunch length.
• Determine an efficient procedure for using injector diagnostics to set up the injector unambiguously.

Based on the outcome of the empirical studies, Biallas will coordinate the development of the best possible procedure for setting up the injector. It is to be useable by CEBAF operators during machine turn-on in support of a follow-on user program. One suggestion (by P. Piot) was to include a study of beam quality versus gun voltage (spanning ~250 kV through ~375 kV). The suggestion is a good one in principle, but due to the limited time available during the upcoming run, the task will likely need to wait for future machine-development runs.

Piot summarized the available "emittance growth" measurements at the first recirculation arc, as well as the status of the requisite diagnostics and the plans for CSR measurements during the upcoming run. Key points include:

• Have measurements of emittance change versus cryomodule gang phase at 20 pC and 41 pC and at 38 MeV beam energy. The data are not yet fully understood, and they need to be reanalyzed because they are based on incorrect quadrupole excitation curves.
• The gang phase needed to produce the shortest bunch length at the arc entrance is about 3 deg less than that needed to produce the shortest bunch at the wiggler; this is an important piece of information for interpreting the emittance data.
• Only the horizontal component of the emittance has been studied to date; strong vertical mismatch in the back leg prevented good vertical measurements.
• Bimodal distributions are sometimes seen in the transverse phase space; double peaks are sometimes seen in the longitudinal density profile.
• No data could be taken during the last run because the ITV2G00 foil was badly wrinkled. (It is being replaced during the shutdown.)

Piot has taken a cursory look at the relative importance of non-CSR-induced emittance growth. Based on PARMELA and TLIE simulations he finds that both chromatic effects and head-tail (rf-induced) effects contribute little (~10% or less) to the emittance growth. He is looking into the possible contributions of transverse and longitudinal wakefields induced by beamline hardware (specifically, the unshielded viewers and the vacuum-chamber transitions).

Changes in diagnostic hardware underway during the shutdown include installing red filters on the "quad-viewer" cameras to suppress ghost pulses from the viewer images and replacing the viewer foil in ITV2G00. In addition, Piot has modified the software to record the quadrupole current so it can be input into revised field maps (if any come up).

Specific measurements during the upcoming run include:

• Measuring rms vertical and horizontal emittance before and after the first recirculation arc versus cryomodule gang phase. At least for the settings that generate the most emittance growth, do tomography in the horizontal plane both before and after the arc (providing as much phase-space detail as possible before the arc as initial conditions for a simulation, and providing as much phase-space detail after the arc for comparison with simulation results).
• Vary the dispersion in the arc to force as much coupling as possible between energy spread and the transverse phase space to see the extent to which energy spread influences emittance growth (as opposed to other influences, thereby verifying the mechanism, an important check given that CSR-induced emittance growth results from CSR-induced energy spread).

Further discussion of CSR test plans will likely ensue at the next meeting at which R. Li is planning to discuss output from her simulation code.

New Items

L. Merminga commented that she would like to do a set of microphonics measurements in the cryomodule cavities at full cw current to aid in anticipating their levels in the FEL Upgrade. The measurements can be done parasitically during high-power-lasing runs.