Greetings all,

This week was dedicated to a four day STELLA run. The results are outstanding and at this point all the components of the STELLA experiment are commissioned. The acceleration quality of the Inverse Cerenkov laser accelerator is the best ever and proved adequate for demonstrating high-quality laser acceleration by the staged laser accelerators. We are looking forward to the beginning of staging experiment next running season.

During the maintenance period on Friday the process to install the Compton Experiment chamber started and VISA beam line work was carried out.

Ilan Ben-Zvi.

 

Experiments

 

STELLA Experiment (Reported by Karl Kusche)

The STELLA group, (resident team + Wayne Kimura & Christian Dilley who flew in from Seattle), successfully concluded a 4-day marathon this week with the long-awaited observation of "flat top" modulation from the Inverse Cerenkov Acceleration (ICA) portion of the experiment. This allows us to report that the second major phase of the STELLA Experiment, recommissioning the ICA system, has been completed!

Summary of goals & qualitative results:

1) E-beam tuning - Vitaly ultimately achieved a measured electron beam size of ~50x70 micron sigma in the gas cell interaction region, compared with

~80x80 micron sigma predicted via his MathCad/VAX feedback tuning routine.

 

Not only does this tune now meet the stringent requirements for staging (see STELLA scientific plan), it allowed us to validate the usefulness of our new high resolution OTR signal. We observed a shortcoming of the YAG crystal BPM (same beam showed as ~120 micron sigma, indicating possible saturation and/or other problems at modest currents). A maximum of 150 pC was delivered to the end of the beamline during these runs. The quality of the electron beam is one of the major contributors to the acceleration quality (see item 3).

2) Alignment BPM - necessary for the overlap of the laser and e-beam to better than 100 micron accuracy are high resolution targets in the ICA interaction region. Several promising schemes were tested, each with the ability to visualize the positions of both beams. A miniature VOx (Vanadium oxides) -on-mica chip inserted into the e-beam at 45 degrees allows for visualization of well-defined OTR signal. The same chip is expected to pass the annular CO2 beam and show the waist profile after axicon-reflection. Maintained in a narrow temperature range, the VOx produces an observable local change in reflectivity upon CO2 laser impingement - this feature is in need of further testing. Another approach used a variety of fine-wire (1-3 mil) crosses to produce shadows in the images of both beams as seen from IR and CCD cameras. Final alignment was verified using a 3-mil metal wire cross. Also of interest was the observation of visible light emission for an optical fiber cross, which may obviate the need for the YAG crystal as a light source when observing the electron beam.

3) Energy modulation - in previous years, a pseudo-Gaussian shaped spectrum was the only evidence observed of the ICA effect. Recent simulations indicated that we should expect to see a slightly concave "flat top" under real conditions. Indeed, we did observe such a strong modulation (0.7 % @ 45 MeV) with the profile that appears to qualitatively agree well with the models.

The electron energy spectra with and without the ICA acceleration effect are show in:

August_27_STELLA

 

Operating conditions: Spectrometer set to +8 degrees (maximum resolution), CO2 laser output ~300mJ @ 200 psec (est.), delivered ~100mJ MAX (optical damage to cell input window), reduced by 3.4X attenuation for typical observations. Axicon mirror alignment and timing dependencies were determined (laser pulse length follows) and optimized.

The tremendous success achieved this week was possible as a result of the improvements made to the ATF’s linac and laser systems, as well as the continued support of the entire ATF staff. Coupled with the higher accuracy BPM in the gas cell, laser and e-beam quality and stability allowed for optimized beam profile and alignment in the interaction region.

The months of September and October will be used for design and fabrication of the new permanent magnet wiggler and ICA cell input window extension at STI, as well as other system improvements. Testing of these components will follow in November, prior to the first attempts to demonstrate staging.

Thanks again to all who contributed to this latest success!

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VISA Experiment (Reported by Roger Carr and Xijie Wang)

The status of the undulator segments is that they are at SLAC, the tooling balls have been purchased and have had holes put into them. The posts for the tooling balls are being made as I write, and the tooling ball assembly will be done next week. As soon as they are installed in the undulator segments, they will be fiducialized on the CMM. We are still shooting for delivery in the week following Labor Day.

New adjusting screws have been received. Silicon bronze stock has been received to make the new nuts for the adjusting screws, but we are short of machinist labor, and this will take longer than the tooling balls. I would hope to send the new nuts about the same time as we send the undulators, but this depends on shop scheduling.

On Tuesday, August 24, the NSLS Beam Line Committee reviewed the VISA experiment. Xijie Wang gave an overview of VISA experiment, and status of the major components. We also discussed radiation safety, laser safety, vacuum and space issue involve in VISA experiment. Ilan Ben-Zvi and John Skaritka were present at the review. We want to thank the NSLS design group for providing a strong support for this review. The committee made many constructive comments and suggestions. We will implement those suggestions. The Committee walk through will take place in two stages: First stage for the VISA test beam line, on Friday, Sept.3, 1999. Second stage, the assembled experiment TBD.

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Facility

 

Computer and Control System (Reported by Bob Malone)

 

Software changes, needed to accommodate beam line 3 magnet reassignments, are underway. Changes will be implemented during the first half of next week with final testing during the last half.

NEW CONTROL SYSTEM:

Motif libraries have been installed on the temporary development system. Vsystem is now operational on this machine. Small test displays were created and used to verify interoperability between new Linux-based system and present VMS-based host. Results show that operator displays can incorporate control elements from both the new system and the old without conflict. This implies that migration from the old system to the new may be possible in a gradual fashion. Once Ethernet crate controllers are purchased in October, similar tests will be performed to check for shared control between the old and new crate controllers. After this hardware exercise is completed, we can develop a more firm sequence of steps to effect the migration.

Existing VMS system uses Vsystem 2.6. It will have to be upgraded to V2.7 to fully exploit some conversion tools which Vista has developed for VMS-->Linux migration. Work continues on developing some in-house conversion/validation tools to help move the more than 700 operator displays to the new system. Present temporary development system is inadequate to handle the storage needs during this transition.

Technical Operations (Reported by Bill Cahill)

This week proved quite interesting! The remaining low level rf temperature controller was installed in the gun system and the results were overwhelming. Phase jitter once observed as high as 50% is now stable at a peak of 3%. This massive increase in stability we feel added to the success on the STELLA Experiment. Also during the week, the defective transistors in the power supply system were replaced and the vacuum system power supply was also replaced. The trim magnet assembly on the VISA matching section is bolted into position and the waveguide pumping port is finished to be installed on Monday. The STELLA spectrometer is being removed from beamline #1 and the first of four vacuum pumps for the VISA Experiment has been installed on the ceiling awaiting survey configuration.

ATF OPERATIONS SCHEDULE (Prepared by Xijie Wang)

August: Schedule August 99

 

September: Schedule September 99