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Site Details ATF Newsletters 
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2003 ATF NewslettersJan. - March | April - May
Dear Readers, CALL FOR PROPOSALS: This coming fall the ATF will hold its Users' Meeting and Experiment Program Advisory Committee (EPAC). This is a call for proposals for new ATF experiments to be reviewed by our EPAC. The ATF web pages contain much information about the services of the ATF for users and how to submit a proposal. As always I am at your disposal for any question. (ilan@bnl.gov, (631) 344-5143). What is new at the BNL Accelerator Test Facility? A first of its kind result in laser acceleration: The staged-laser accelerators experiment STELLA II achieved its goal to demonstrate high-quality, mono-energetic beam produced by a laser accelerator. Congratulations to Dr. Wayne Kimura, the experiment's spokesperson and to the whole STELLA II team. You can read also about the steady progress made by the VISA Free-Electron Laser experiment. We welcome new people to the ATF: Kelly Bergesen is a visiting graphics design student from Long Island University's Southampton campus. She is working with Bob Malone. Todd Corwin is a mechanical / electrical technician with a rich experience in an array of subjects. He transferred to the ATF Group from another High-Energy Physics group in the Physics Department. Michael Polyakov is an undergraduate student from Carnegie Mellon, who will be working with Karl/Marcus/Igor on a variety of CO2 laser system improvements as part of the Summer Undergraduate Laboratory Internship ("SULI") program through August. Takahiro Watanabe is a Research Associate. He comes to us after finishing an award winning Ph.D. thesis at Prof. M. Uesaka's laboratory at the University of Tokyo. He will be working in accelerator operations and various experiments. Fanshi Zhao is a Stony Brook University graduate student in the electronic engineering department. He will be working with Bob Malone on control systems. If you take a look at the ATF web page, you will see that it has a fresh look, the art-work of Kelly Bergesen. On the personal aspect of our staff: A boat with 5 persons overturned in the rough waters of Moriches Inlet on Sunday, May 18, 2003. A mother and her son were rescued alive, but the father, his teenage daughter and his friend were pulled from the cold waters in cardiac arrest and delivered to the Moriches Coast Guard station by boat. Photos in an article in the New York Daily News show rescuers from East Moriches Volunteer Ambulance, including Paramedic Karl Kusche and his wife Cathy, providing Advanced Life Support to the two male victims on the dock and loading them into awaiting ambulances. While at work, Karl is the ATF's ES&H Officer and works on STELLA and other experiments. In case you were wondering: Both Karl and his wife Cathy have been active volunteers in several local fire departments and ambulance corps for 20 years. They were trained as paramedics at Stony Brook's University Medical Center and in New York City. Much of their spare time is spent together responding to alarms and satisfying annual training requirements. They also served at Ground Zero in the aftermath of September 11, 2001. Ilan Ben-Zvi
During May 2003, the STELLA-II experiment successfully demonstrated for the first time monoenergetic laser acceleration of laser-generated microbunches and high efficiency trapping of the accelerated electrons. We demonstrated 16% energy gain (>7 MeV) with an energy spread of 0.84% (FWHM) and a trapping efficiency of ~14%. We also demonstrated even larger trapping efficiency of ~80% with an energy spread of 2-3%. To briefly review the experiment, Fig. 1 shows the STELLA-II layout. The ATF TW laser beam modulates the energy of the e-beam passing through the first inverse free electron laser (IFEL), which consists of a permanent-magnet, untapered undulator. These electrons pass through a hybrid permanent-magnet (PM)/electro-magnet (EM) chicane. Immediately downstream of the chicane is the second IFEL, which is a PM undulator with an 11% gap taper. This second IFEL is driven by the same TW laser beam and serves as the accelerator. The PM field strength of the chicane has been preset to cause the electrons to bunch at the entrance to the second IFEL. The EM field is used to adjust the phase of the microbunches with respect to the laser field inside the second IFEL. Depending on the laser power being delivered to the experiment, the laser intensity driving the second IFEL varies and affects the trapping and acceleration process. (Varying laser power also affects the modulation by the first IFEL.) If the laser power is below the minimum needed to drive the tapered undulator, then poor trapping and acceleration occurs. At minimum threshold, the microbunch electrons are only just trapped and accelerated resulting in a narrow energy spread. However, because the laser intensity is at threshold, trapping by the tapered undulator is limited to a relatively small energy and phase extent. This can lead to reduced trapping efficiency. If the laser intensity is well above threshold, then the potential well of the tapered undulator expands in both energy and phase extent, which enables trapping of a larger fraction of electrons. Generally this also results in a wider energy spread of the trapped and accelerated electrons. Our experimental data shows evidence of both these situations. Figure 2 is the electron energy spectrum for the first case of narrow energy spread with relatively low trapping efficiency. The spectrum is produced by averaging over 20 adjacent raster lines in the spectrometer camera image to help reduce noise. A narrow peak of accelerated electrons with an energy width of 0.84% (FWHM) can be clearly seen at 16% energy gain. Simple integration of the area under the spectrum profile indicates ~14% of the electrons are under this peak. The two dips indicated as fiducial lines are caused by scribe marks on the phosphor screen viewed by the spectrometer camera. Figure 3 gives the electron energy spectrum for the second case of high trapping efficiency with relatively wide energy spread. Note that the accelerated peak of electrons is now wider [~3% (FWHM)] and has structure within it. We believe this structure is due to rotation in energy/phase space of the trapped electrons. An integration of the profile yields 80% of the electrons within this accelerated peak. Varying the chicane EM field causes the microbunch peak to shift within the spectrum. At a phase delay 180-degrees away from the maximum acceleration seen in Figs. 2 and 3, we observed loss of acceleration as expected. Analysis of the May 2003 results is still occurring. In particular, comparisons with the model will be made to better understand details seen in the data spectra.
In this period VISA achieved the first sustainable lasing after the renewal of the effort. Here is the summary of the results. 1. Beam tuning Following the laser cathode cleaning, the new tune was tested. After small modifications, lasing was observed and optimized. Beam measurements while lasing showed the following envelope behavior: The dots are measurements, and the lines are simulations. Below is the model used for the simulations, including quad settings: Figure 2. Simulation model screens. The agreement is rather good. In the horizontal plane, the average discrepancy between theory and simulations is under 40 microns throughout the length of the beamline. In addition, for the first time ever in VISA history, no dispersion could be observed in the VISA matching section. Both observations and gain measurements tell us that we are on the right track. However, there are discrepancies. First of all, when the lasing is peaked, according to the model the beam is not matched. In the horizontal plane the mismatch is small and can be easily interpreted as an imperfection of the undulator entrance model. Vertically, on the other hand, there is a strong discrepancy from the middle of the dispersive section all the way into the undulator. Further studies are required to understand what causes such behavior. Possible candidates are vertical focusing in the dipoles, wrong dimensions in the model, nonlinearities (and/or scraping) due to the large vertical beam size, etc. For the next run: We have yet another tune to try which kills dispersion but keeps the beam size smaller vertically in the dispersive section. Using this tune would be an important test in trying to find the cause of the model/measurements discrepancy shown above. 2. Lasing The observed FEL performance was rather unusual in a good way. The gain was not very sensitive to RF jitter and phase drifts, and generally lasing could last for hours without system readjustments. This is a clear indication that there is no compression, or that any other phase sensitive processes are involved. The stability in the system is rather incredible. From statistical analysis one can conclude that there are about 20 independent modes (see plot below), when only 10 spikes are expected longitudinally. Figure 3. FEL energy statistical analysis. For the next run: Get more data. Spectrum and gain length measurements will help to determine if this observation is real and reflects lack of full transverse coherence. 3. Compressor Status The ATF-UCLA compressor collaboration is in its final pre-installation phase and will be ready for the tentative July 14th shutdown. The last pieces of hardware are being sent to the ATF including the central BPM diagnostics, which integrates an actuation system engineered by Martin Woodle "ATF". A final check of the magnet and vacuum stand hardware has been performed with no significant issues detected. The electrical system has been specified and the supporting power supplies and relay electronics will be built shortly by Tony Rodrigues "ATF". A degaussing algorithm has been developed with Vitaly Yakimenko "ATF". Noise in the electrical system has been examined and Carlo Vicario "University of Rome" and Takahiro Watanabe "ATF" are working on the filtering system. Most importantly, Martin Woodle "ATF", has also completed the shutdown schedule which will tentatively be performed from July 14th through August 23. Additionally, examination of the CSR signal and associated bolometer diagnostics along with further compressor performance and compressed beam transport studies are being carried out in preparation for the anticipated experimental work. On the topic of co-operative education, LIU Southampton has named BNL its co-operative education Employer of the Year for 2003. BNL's Office of Educational Programs received the award on behalf of the Laboratory at a gathering with keynote speaker Congressman Tim Bishop. The ATF is pleased to be part of this program which again underscores our history of commitment to educational opportunities for students. 1) ATF Safety Approval Document (SAD) update is in progress; 2) ATF passed another quarterly Tier I safety inspection without incident; 3) A revised policy for linac operators handling ATF interlock keys has been instituted. Electronic means of control & audit are being investigated to improve logging accuracy and reduce paperwork; 4) Painting & cleaning of the experimental hall is complete. Everyone is asked to please keep your areas clean and put away unused equipment. Yellow lines now mark the accessways to exits and interlock stations - please keep equipment out of those areas as much as possible; 5) In response to recent incidents, the air conditioning system that serves the hall and CO2 laser rooms has had new parts installed to improve temperature stability. There still exists an imbalance in cooling/heating capacity between the C1/C2 rooms and the EH/FEL areas. To correct this, we are considering additional vents and independent reheats & control zones; 6) The CAD interlock group is currently installing components for the chicane radiation interlock system, as well as progressing toward the migration to a modern logic-based interlock system for the entire facility.
Last Modified: December 3, 2007 |
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