Accelerator Test Facility

The ATF Users' Meeting and Program Advisory Committee met on June 1-2, 2000. Presentation on the ATF systems, recent results from user experiments and new proposal filled one and a half days. Thanks go to Kathy Tuohy who organized the meeting with her usual thoroughness. Five exciting new proposals were submitted from national and international laboratories, universities and small business. The new proposals are being studied by the Program Advisory Committee. Complete details on the meeting, including agenda, list of participants and presentation slides (some of these are still missing) can be found at :

Usermtg2000.htm

The STELLA experiment achieved a major landmark in the development of laser accelerator: Two laser accelerator units were phased to a fraction of an optical wavelength. The process involves the generation of few femtosecond long electron bunches in one laser accelerator and their acceleration by a second laser accelerator located over one meter downstream of the first unit. The optical bunching has been optimized by adjusting the laser power to the first accelerator. Control of the acceleration was done by shifting the phase to the second unit by a fraction of the laser wavelength. The energy spectra at the output of the accelerator clearly demonstrate the ability of the experimenters to shift anywhere from acceleration to deceleration. The relative phase was stable over period of up to half an hour. Read more in the newsletter.

New office space for the ATF: We got a block of new offices in building 820M. This will locate nearly all ATF staff and users at a single area in close proximity to the ATF machine. We will also enjoy a nice meeting room in the same area. The move is expected to be completed by the middle of June.

Ilan Ben-Zvi.

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STELLA Experiment (REPORTED BY KARL KUSCHE AND WAYNE KIMURA)

STELLA experiment ran for three days this week, attempting to improve upon the staging results from the previous runs. Dave Quimby and Christian Dilley from STI Optronics joined the local effort. Improvements to the system included the addition of a phase delay plate for finer and more accurate phase step sizes than possible with the stepping motor trombone delay line, as well as more tightly focusing the laser beam through wiggler #2 to increase transmission.

See a few of the many spectra taken during the run at:

6_9_STELLA_phasing.htm

Further data analysis will continue at STI during the week of the Advanced Accelerator Concepts Workshop 2000. Short-term work may include the replacement of the vacuum tube at wiggler #2 in order to improve laser mode and transmission through it. Otherwise, major design & modification work is expected for long-term continuation of staging experiments, which will require the TW CO2 laser and possibly a phase-feedback system.

Message from Wayne:

This past week completed another STELLA run in which we further demonstrated the ability to control the trapping and acceleration of the microbunch. Thus, we have successfully generated in a reliable and reproducible manner very short microbunches, of order 3 fs, which by adjusting the phase delay can be accelerated to nearly 5% energy gain (>2 MeV). We observe a clear peak of electrons within the energy spectrum that changes its energy position depending on the phase delay. This whole phasing process has shown to be stable on the few femtosecond level over periods of up to half an hour. This is rather remarkable given that the optical transport systems for the two IFELs are physically separated from each other and no active phase stabilization system is being used.

Thus, I'm pleased to announce that STELLA has successfully completed its major goal of demonstrating staging and acceleration of laser-produced microbunches. I want to congratulate the entire STELLA team for their hard work, dedication, and perseverance. Without exaggeration, the STELLA experiment is an extraordinarily complicated one and certainly the most difficult one I've ever worked on. We should not only be proud of our success, but also proud of our ingenuity in our ability to make fundamental changes in our approach during the past several months, i.e., switching to the two permanent-magnet IFELs. As they say in basketball, it's only the last few minutes that count. It took us over two years to prepare the STELLA experiment, but at the last minute we had to change our game plan.

And, it worked beautifully!

I want to thank the ATF staff for their tremendous support and help. More than once we have been saved by their special efforts to help us. I also want to thank the STI machine shop crew who so skillfully and quickly made many of the critical components for the experiment including the permanent-magnet wigglers (in record time at that!).

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Optical Detector Experiment(REPORTED BY YANNIS SEMERTZIDIS)

Recent measurements have led us to conclude that the poled fibers we tested lacked the necessary sensitivity required for charged particle beam instrumentation.

Several improvements in the setup for studying the electron beam induced electro-optical effect in crystals were undertaken. Recent measurements suffered from poor optical extinction due to inefficient coupling to the 35 m long single mode polarization maintaining fiber which had been run in order to transmit the signal carrying laser light from the setup in the beam line to the optical analysis instrumentation adjacent to the ATF Control Room. We successfully tested a new setup with local optical analysis in the beam vacuum with the output light coupled to a multimode fiber which transmitted the analyzed signal to the Control Room. The anticipated improvement was observed.

Laboratory measurements indicated that the decreased signal amplitudes observed with LiNO₃ since the failure of the original waveguide embedded modulator crystal were due to poor alignment of the r33 crystal axis relative to electric field direction of the electron beam. In the May runs the factor of ten improvement observed in the laboratory was reproduced with the ATF beam. This allowed us to perform a series of measurements characterizing the dependence of the signal sign and rise and decay times as well as amplitude on distance from the beam and its charge content. We observed electro-optical effect signals (e.g. see figure at http://www.picosec.bnl.gov/ITEMS/Fast-signal.ps ) with rise times of 60 ps as expected from the bandwidth limitation of the 7 GHz Tektronix oscilloscope used for data acquisition. We plan to reduce the rise time of the electro-optical signal with the streak camera in the June runs.

Inasmuch as the crystal used was 3 mm in height compared to the 50 micrometers of the wave guide embedded modulator crystal we were able to observe an unanticipated slowly decaying signal which was present when the beam was incident on the crystal at some distance from the polarized laser light. This signal changed sign with electric field direction in distinction to the light attenuation signal previously observed. This data, which suggests a bulk material effect, is being analyzed and the physical processes involved are under investigation.

A new beamline setup was constructed with a larger vacuum chamber containing the analysis elements including a polarizer, a quarter wave plate and a remotely controlled analyzer (polarizer). The new chamber has been successfully vacuum tested and will be subjected to outgassing measurements with all optical elements installed prior to placement in the beam line for our June runs.

Samples of a new electro-optical material

4-N,N-dimethylamio-4’-N’-methyl-stilbazolium tosylate (DAST) have been acquired. The electro-optical coefficient and dielectric constant of DAST should lead to a factor of 10 to 20 improvement in sensitivity over LiNO₃. However, it does have poor light transmission properties. We hope to test DAST in future runs.

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VISA Experiment (REPORTED BY AARON TREMAINE AND ALEX MUROKH)

For PAST couple of months, Alex has been working on the upgrade of BPM optics, and alignment laser systems. Last run was a “beta”-test for both.

A. BPM optics scheme remains the same, but uses new mirrors, lenses, and standard mounting components. The new set-up has 4 times bigger acceptance solid angle (factor of 2 gain in OTR intensity), has special arrangement for inserting filters and polarizers, is much faster to align; also the mirrors are used to filter out undulator radiation, with no appreciable attenuation of the OTR. All these improvements became possible after the realization, that we can remove the starboard side-poles from the steering magnets, and open up a space for optics.

Also the pop-in was added at port #1 (half the size of a standard BPM), and will be tested this Friday.

B. The laser system was upgraded during the shut-down:

1. The focusing scheme of the red laser had been changed from 1 lens to 3 lenses. As a result with less aberrations and better control, the spot size was decreased by factor of 2/3, almost reaching a fundamental minimum of sigma = 0.5 mm. The beam quality became sufficient to make a red laser - primary for BPMs (Just in time considering an IR laser failure).

2. The monitoring system was added, and prealigned, during laser alignment. Now we can align the laser at any time, without opening vacuum. Alignment down to 50 microns can be done in 15 minutes, thanks to new, big arm mirrors, made by Brendon (Robert Ruland’s team).

Better accuracy is achievable, but time consuming. Sub-50 microns accuracy is below kinematic limit of the mirrors, therefore optical flat was added and used for pure translation. With that set-up the alignment accuracy of 20 microns was achieved, which is a shot-to-shot fluctuations level.

3. The problem of ND-filters was not solved from the first attempt. I remind the problem: ND filters used to change the laser intensity, randomly (but repeatably) deflect beam by up to 500 microns. The replacement single plane variable filter that I bought was better, but still produced up to 100 microns errors.

Unexpected help came from changing the laser alignment procedure. Replacing the Quadrant detector with the miniature CCD camera (borrowed from Xijie) enabled us to align laser at exactly the same settings as used for the beam steering.

4. 800 nm laser is at Melles Griot. It is quite possible, that diode replacement will be necessary. I will know the exact delivery date by next week. With the monitoring set-up it can be installed and aligned, even when the system is under vacuum.

We are planning another shutdown from June 16-19. Robert and crew will come out and do an interferometric alignment. Also, we will do an alignment on the alignment lasers.

On our next run days we will try to do in depth trajectory studies and come to a conclusion of the undulator alignment. Unfortunately, we were not able to get any meaningful info from our last run days which were after a shutdown. We find it takes about a day to a day and a half to come back online after a shutdown.

The mounts for the mirrors and lenses for the transport is up. We need to install and align the mirrors to the FEL room. This is hard to do without the 800nm laser.

July and August we have several run days.

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Technical Operations (REPORTED BY BILL CAHILL)

The terawatt laser vessel has successfully passed all inspection criteria’s and has been reinstalled at the ATF. Don Davis and Bob Harrington have begun the tedious chore of cleaning the interior components in order to reassemble the system. The new chiller for the gun solenoid has been installed and is performing well. NSLS electricians were quite busy installing the power needed for the gun solenoid, the new linac closed loop water system, installation of a new power outlet for the UPS in the Main Control Room and the removal of unwanted conduit in the Experiment Hall. The CO2 gas manifold has been installed for the terawatt amplifier and we are waiting for NSLS technicians to plumb in the stainless lines. BNL custodians have started preparing the new offices in Bldg. 820-M for occupancy and the VISA group has begun to move in. Remaining staff will begin to merge in as the offices are finished. A general clean-up has been done ion the occasion of the annual users meeting next week. This statement does not imply that the ATF is untidy, merely that we want to look our best for company. As always, the ATF technical staff continues to support the operational program on a daily basis.

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ATF Engineering Meeting(REPORTED BY XIJIE WANG)

May 26:

ATF beam line vacuum: the vacuum in both beam line 1 and 3 were exposed to air without bake out for operation, this is not acceptable. All covers for the vacuum pump out ports are missing, this should be replaced (Harrington).
ATF will operate next Friday afternoon for beam studies (Vitaly)
Radiation survey: all components left in the experimental during the electron beam operation must be radiation check when they are taking out experimental hall.
Next Tuesday morning ATF clean up: all experiments should clean their areas, ATF personnel assignments: Bill Call in charge of all clean up and experimental hall. Marcus FEL room, Igor CO2 rooms, Malone and XJ control room, Vitaly mezzanines and Z-line area.
Quadrupole magnets exchange in STELLA beam line, both Vitaly and I felt that, we should make the three quadrupole magnets before the STELLA last acceleration stage a true triplet, it will reduce the space and make operation easier. Vitaly will contact Wayne on this matter.
New schedule for next two months now is on the web.
Have a nice holiday, see you at ATF user meeting.

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ISM Corner(ILAN BEN-ZVI)

We have a new responsibility at the ATF: our newly appointed "Vital Records Custodian" is Bob Harrington. Following the proper training he will make sure that all the vital records of the ATF are backed up regularly in a safe place.

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Last Modified: December 3, 2007
Please forward all questions about this site to: Vitaly Yakimenko