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Site Details ATF Newsletters 
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2003 ATF NewslettersJan. - March | April - May
Greetings, In this ATF Update you may read about the progress of two of the ATF experiment, progress in the facility electron beam optics, laser R&D and of course the ES&H corner. A joyous occasion for all of us at the ATF: Marcus Babzien married Theresa DeRienzis on Friday, March 14. The ceremony was held at St. Patrick Church in Smithtown, New York, followed by a reception for the family at the Three Village Inn in Stony Brook. The couple spent their honeymoon on Elbow Cay in the Bahamas. Congratulations, Marcus and Theresa! Ilan Ben-Zvi VISA II will perform an experimental study of the physics of a chirped beam, SASE FEL using the upgraded facilities at the ATF. Upcoming experiments include an ultra short gain-length experiment, where bunch compression along the new (about to be installed) compression chicane and the dispersive line will allow for deep saturation wavelength studies. Chirped beam experiments will also be studied, where a time-frequency correlation on the FEL output is imparted by a time-energy correlation in the beam. Subsequent compression of the FEL light using gratings is expected to shorten the FEL output pulse. Since December 2002, we have had 3 runs dedicated to VISA. The main goal of these runs was to bring the system back up to VISA operating conditions, which embodied many tasks. 1.After a careful optical undulator alignment inspection, an offset of about 120 microns between the first 2 (of 4) undulator sections is perceptible. This offset must be verified by a second method, namely measuring the electron beam trajectory through the undulator, before any actions relating to moving and re-aligning the undulator occur. 2. Undulator beam diagnostics were restored to operating conditions. 3. A methodical understanding of the electron beam dynamics along the newly upgraded H-Line was achieved. After the high-quality improvements were made, calibrations were straightforward and provided a systemic method to determine beam envelope sizes. These measurements were compared to simulation and the results compared well (see figures below). A thorough understanding of the beta functions along the H-Line is necessary for matching purposes at the entrance of the undulator. A similar upgrade will occur along the dispersive F-Line and Beamline 3 with the support of VISA. 4. Preliminary electron beam trajectory measurements were made through the undulator. Using an alignment He-Ne laser as a reference, we measured the electron beam size at 8 different positions along the 4 meter undulator. On the figures below, the data collected is plotted and compared to the trajectory expected by simulations (solid line). Measurements of beam position along the ATF and VISA undulator compared to simulation. 5. As a spin-off of our trajectory measurement, we achieved low-gain lasing. We roughly estimate that there was a gain of under 1000. Based on simulation studies using ELEGANT, we concluded that the addition of 2 sextupole magnets along the F-Line will aid in minimizing dispersion and linearizing Beamline 3, the matching line before the undulator entrance. The addition of sextupoles will take place concurrently with the upgrade of the F-Line and Beamline 3.
Electro-Optical Fast Detector Experiment The objective of this run was to observe more single shot EO signals on the streak camera. This is the first run at beamline IIIb after months of ATF shut down run. Stripline signal from a new location and/or difference cable length was provide by Vitaly. Also in this run we have replaced the laser source by a single longitudinal mode HeHe. The laser noise is significantly reduced, see Figure 1(Laser noise – old (ours) and new (Peter’s)), allowing us to collect data whenever electron beam is available. Before beam started, the minimum dc light level (destructive interference) from the dark port is as low as 12 mV (into high Z), while the maximum light level (constructive interference) is as high as 950 mV. The spatial beam profiles of the reference, EO, and the combined EO+ref are good.. Like all previous experiments, the beam profile degraded with increasing exposure time to the electron beam field. As a consequence the dc light level increased to ~30 mV at the end of the run. The beam charge is ~0.5 nC, electron beam stability was poor and beam size was large, see Figure 2(electron beam profile). Shot-to-shot beam position varied by as much as one beam diameter. After beam tuning EO signal was obtained on the oscilloscope, see Figure 3( Data, oscilloscope traces). It was found that the new stripline signal arrived ~5 ns earlier than all other previous experiments. Since our EO detection and the trigger to the streak camera rely entirely on the arrival of the stripline signal. A timing calibration was performed using picosecond laser diodes. Trigger signal to the streak camera was located to better than 50 ps – see Figure 4. We then sent the EO signal onto the streak camera. 60 single shot data were collected in one set of measurement. A total of 8 data sets were accumulated. In between data sets, several beam tuning were also performed to stabilize the electron beam. Figure 4(Streak camera, test data and EO data #20) shows a representative EO signal collected on the streak camera. The scan time of the streak camera was set at 1.5 ns full range. EO signal appeared to have a duration of <25 ps limited by the resolution of the streak camera at the 1.5 ns scan range and the low EO signal to noise ratio. Since we relied on the gating of the streak camera to gate out significant amount of the dc light that is always accompanying the EO signal while the dynamic range of the signal is low at high PMT gain, significant random photon statistic noise is observed on the streak camera. At beamline IIIb, we constantly experience large electron beam instability and the beam profile cannot be reduce to less than mm size, it was suggested that we move our experiment to location at beamline I or II. In the next runs we’ll redesign our EO setup to be place inside the Compton cell of either beamline I or II. Also a new ZnTe EO crystal will be used as our fast EO detector. A preliminary, brief beam study with the new BPM and H-line upgrade was made. The emittance measured as a function of charge for two laser spot sizes (1 and 2 mm) is shown in the linked figure(Preliminary emittance measurements in the upgraded ATF H-line). A few trends can be noted nevertheless:
The later can be explained by the large current in the steering magnets between the gun and the linac, which are needed in order to compensate gun misalignment or solenoid field error. Further studies will be done in order to better understand limiting factors for small emittance beams.
Due to the tremendous
success of the new BPMs,
The upgrade of the ATF CO2 laser system to a few-picosecond pulse at Terawatt peak power is a the final stage.
The first user’s experiments using several Joules of the Terawatt laser energy are planned for May-June. The new ATF capabilities will allow the order of magnitude gain in laser acceleration experiments, such as STELLA, or in X-ray production via Thomson scattering, observation of the nonlinear Thomson scattering, etc. In addition, a high repetition rate of the new preamplifier allows delivering multi-Gigawatt laser pulses at the repetition rate of the ATF linac. This will be very handy in order to speed up electron/laser synchronization and co-alignment or producing fancy tricks like STELLA-type 6-fs micro-bunching of each electron bunch driven by the linac.
Coordination between the ATF and the Physics Environment, Safety and Heath (ESH) committee continues to improve, with significant progress made in many areas: 1) ATF experiments are now using the new Physics experimental safety review (ESR) form. The Physics ESH committee has reviewed and approved several of our ESR's recently without any major issues or delays. Presentation were given to familiarize the committee with ATF beamline operations; 2) The ATF Conduct of Operations (COO) has been revised, as per the Laboratory ESH Committee (LESHC) requirements in USI #3, and is currently under final review by the Physics ESH committee (PESHC). These guidelines set forth the procedures we must follow in support of linac operations. The ATF SAD will be the next major document update; 3) The ATF chicane upgrade proposal was submitted to the PESHC, with a presentation given by Ken Batchelor on March 28. Approval is forthcoming (pending determination of need for LESHC involvement), with the expectation that fault studies following installation will support calculations. The CAD interlock group is presently developing an interlock system for the chicane, which may require a separate review by CAD. A work planning meeting will be held to identify & address all work permit and ESH issues for the phase 1 installation in mid-April 4) The experimental hall and CO2 laser rooms experienced a flood at the beginning of March, following a long stretch of snow and heavy rain that clogged the drain in the northwest parking area. Damage to electrical equipment was minimal, but the floors sustained water logging. We used this opportunity to remove the remnants of the carpeting in the hall and FEL rooms. Cleaning and painting are scheduled next. Following the flood, all loose lead bricks were removed from the experimental hall. Painted bricks are now available to experimenters for short term use in the hall. 5) In response to the flooding incident, all staff and experimenters are requested to make sure all electrical devices (equipment, wiring, multi-outlet extensions, etc.) are at least 2 inches off the floor or in appropriate containment so if the flooding ever occurs again, the chance for a hazardous situation is greatly reduced.
Last Modified: December 3, 2007 |
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