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Site Details ATF Newsletters 
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2001 ATF NewslettersJan | Feb | March | April | May | June - July | Aug | Sept | Oct | Nov - Dec
Hi, I hope that you had a good summer. The running at the ATF is peaking up its pace again and in August we have some nice results to report: VISA, Charged Particle Optical Detector and a feasibility study on rough surface wake fields. Congratulations to Feng Zhou and his wife for their newborn daughter! Ilan Ben-Zvi. The VISA collaboration finished its last experiment run in the first week of June. VISA was scheduled to be dismantled to make way for removing the HGHG radiator undulator in August. ATF has decided to preserve and continue VISA experiment. We invited Aaron back for SASE FEL micro-bunching experiments. During the August run of the VISA experiments, we have achieved following results: 1. Two types of micro-bunching detectors were installed with help of Don Davis and John Skaritka, both successfully observed coherent transition radiation (CTR) due to the SASE electron beam micro-bunching. 2. Using the CTR foil perpendicular to the electron beam, we measured CTR energy 0.5 to 1 nJ for the fundamental. We also observed CTR and optical transition radiation (OTR) images of electron beam. 3. With a foil about 45 degree from electron beam, the second CTR detector allowed us to simultaneously measure both the CTR and SASE signals. In a first measurement of its kind, we experimentally observed SASE driven by the electron beam micro-bunching, and further demonstrated SASE saturation by observing the CTR saturation. 4. Second harmonic CTR was measured with CTR energy about 20 pJ. We also measured the second harmonic CTR as function of fundamental SASE, saturation was observed at the second harmonic. The first observation of a charged particle beam using the
electro-optic (EO) effect was made by our group a couple of years ago
with a continuous wave (CW) laser and the electron beam at ATF.
Since then we have improved the time resolution of our measurements
reaching 100ps, limited by the bandwidth of the detection electronics.
During our recent runs we used a streak camera and a pulsed laser
to improve the bandwidth and the photon statistics, respectively. We have achieved better than 10ps time resolution
in observing the electron beam provided by the ATF in single shot mode. From the preliminary analysis of the data: With an extinction of 1/5000, EO signals were observed with signal to noise ratios greater than 10:1 using a LiNO3 crystal in both single shot and averaging modes. See figure http://www.picosec.bnl.gov/ATF_reports/mvc-741f.jpg , the white trace which goes beyond the top scale is
the signal in single shot, using a 500 MHz oscilloscope and 1 GHz photodiode. After optimizing the signal it was necessary to switch from the multimode fiber, which had been used for good light coupling in order to assure good transmission of our signal to the control room, to a monomode fiber in order to minimize signal dispersion for pulse length and rise time measurements. The NSLS streak camera with the synchroscan feature was used for a final set of measurements. The synchroscan locks our trigger to the ATF source laser pulse thereby reducing trigger jitter in the signal averaging mode. An individual single shot file with a time resolution better than 10ps can be seen in http://www.picosec.bnl.gov/ATF_reports/atf_010829_3ns_4.pdf where a sharp rise time is observed and the (possibly) cavity modes. A signal in the averaging mode can be viewed in http://www.picosec.bnl.gov/ATF_reports/atf_010829_500ps_2.pdf with a time resolution better than 10ps.
At our run in September we will study the amplitude
of the observed frequencies by varying the electron beam parameters.
This program was supported by a three year LDRD which ends this
month. Most of the members
of the group can be seen in http://www.picosec.bnl.gov/ATF_reports/mvc-748f.jpg (all from BNL unless noted, seated: Brian Sheehy,
standing from left: Ludwik Kowalski (Montclair State University), Thomas
Tsang, Don Lazarus, Dimitrios Nikas, Yannis Semertzidis, and Triveni Srinivasan-Rao).
Missing from the photo are Vincent Castillo, Rich Larsen, Cenap
Ozben, and Arnold Stillman. We dedicate our measurements to the late Michael
Murtagh, past chairman of the BNL Physics Department., who had provided
the initial support to jump start the project. The design of modern FELs based on SASE calls for intense short electron beams with a small relative energy spread. A concern has been raised that the induced wakefields may increase the energy spread beyond the tolerable level. It has been pointed out that a major source of wakefield might be the roughness of the surface of the beam pipe in the FEL undulators. Some models have been developed to predict the effect, but they present the different results under the same conditions. Our experiment is to test these models. Three different beam pipes with 1 meter in length and 6 mm in ID are designed and fabricated. Their parameters are summarized as follows: 1. 1st beam pipe is "smooth". 2. 2nd beam pipe with many smaller bumps (the bump size is hemi-ellipsoid 0.6*0.3 mm, the packing factor for the bump along the beam pipe is 0.5) 3. 3rd beam pipe with many larger bumps (the bump is hemi-sphere, its radius is 0.6 mm, the packing factor is 0.5). The beam pipe is installed in between IQ2 and IQ3 in the beam line #2 at the ATF. In the 1st run, we tested the 1st beam pipe and measured the energy spread before and after the beam pipe. It is shown that the energy spread before the beam pipe is equal to the energy spread after the beam pipe under various beam conditions, i.e., different pulse length, different bunch charge. It is concluded that the 1st beam pipe has no contribution to the energy spread. In the 2nd run, the 2nd beam pipe is tested. Its wakefield induced energy spread is observed. The energy spreads with different bunch charge (pulse length constant, 6.3 ps with FWHM) and pulse length (charge is constant, 300 pC) are shown in Figure1 and Figure2, respectively. In the 3rd run, the 3rd beam pipe is measured. Its energy spread vs the pulse length at 200 pC is measured, as shown in Figure 3. From these measurements, the energy spread is strongly dependent on the bunch charge, and proportional to the inverse of the pulse length to the 1.2th-1.5th power. In all three figures, the model predictions are included. Please note that this is a zero-order analysis, since: 1) we did not fit the real pulse length to the models; 2) the resistive part is not included in the Stupakov model and 3) measurement errors are not included. In the further data analysis, these factors will be considered. In the following weeks, we will have some additional runs and try to collect more data.
1) I believe we have passed all the safety issues for the LACARA Magnet to be both tested and then installed at the experimental floor. I prepared a very short, but rich in content, description for every major piece of equipment required for the operation with the Magnet. As soon Xijie returns from his vacation I will give it to him. Andrew Ackerman promised to send me his final words this week. After getting it, we can test the Magnet, and then install it at the ATF floor. The Magnet itself is not coming soon. Everson Electric has some problems. I only have been told that probably within the coming month they will ready to ship it. 2) The final drawings for the Magnet Supporting Table have been made. The production of this Table is going to be started next week in the Yale University Workshops. I believe that the Table will be ready for installation roughly at the same time as we get the Magnet. Consequently, the Magnet will be installed on the Table and tested. 3) A few words about the Magnet test. following the delivery of the magnet we will connect all power/water supplies and call Everson to come for testing. At that time Andrew Ackerman and Chris Weilandics will come to measure the magnetic filed around the magnet. After the test is done we will pack the Magnet and wait for the opportunity to install it at the ATF experiment hall. 4) After some more discussion with Igor Pogorelsky I understood what we could have from the CO2 - laser in the nearest future. The Optics will be made for 5 joules and with the simplest focusing element (a mirror). There will be no telescope in the design. As Igor promised on Aug. 17 this year we can request the laser beam transversal size in the range 1-1.5'' and get it. It is more than enough for us. 5) There is some uncertainty about the placement of the Magnet in the second beam line. This is a subject for design and discussions with ATF management. Our suggestion is to install equipment for other experiments downstream of the LACARA experiment rather than upstream. We can turn off the magnet, and then it should not interfere with these experiments. 6) So far only 30 GW of the laser power is expected (then the energy gain is roughly 2 MeV), some simulations must be done to understand what the energy gain will be exactly, what the size of the electron beam has to be, etc. The outcome of these simulations will look very simple: 1) the currents in all the quadrupole coils along the electron path to the LACARA, 2) the real energy' gain for the given conditions. Vitaly Yakimenko taught me how to use the MAD8-code to simulate the delivery line. I have another code to simulate the LACARA interaction region. Now, the purpose is to match the outcome from one with the income to another not forgetting about the fringe B-field produced by the Magnet. A lot of details have to be checked. This work has been started.
Last Modified: December 3, 2007 |
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