Accelerator Test Facility

2) Modeling at STI - Loren Steinhauer and Wayne were able to simulate space charge effects through the beamline (after exiting the wiggler); the model’s results appear to predict Jan 7 results (energy spread 1-1.5% using ~42MW delivered) to the correct order of magnitude. We are planning to use the model to understand space charge limitations to basic experimental parameters (i.e. maximum e-beam charge to be used, etc.), Wayne expects the final result to define a limitation in the amount of charge we can use.

3) VOx imaging system has been completed for the ICA gas cell, which will be shipped back to ATF for installation during the next shutdown.

4) Wayne & Christian plan to come to the ATF during the last week of February to assist the local team during the several days of running to recommission the ICA system.

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VISA Experiment(REPORTED BY AARON TREMAINE)

We ran this last Tuesday. What we saw during the run:

A Molectron detector was set up at the FEL mirror of Pop-in 8. When the flipper was down (no radiation from inside the undulator to the detector) we saw 50mV of background. When up (radiation from undulator gets to detector), we saw 400mV or about 1.7nJ (from the manufacturer’s calibration). This radiation was linear with charge. Calculations show the expected amount of spontaneous emission expected for our angular and frequency bandwidth to the detector is about ~1.5nC. It appears that the measured energy is within error to the expected spontaneous.

BPMs:

Our main mission here is to engineer BPMs with OTR. We can see plain OTR with sensitive Cohus and the Hitachi can give a reasonable signal. Also, the Hitachi does give an image at 4 (OTR+ BPM optics), but this signal is weaker than all the rest. It looks like the way to go is to use OTR/BPM with more sensitive cameras, but more testing is to be done. One note: We must be cautious not to confuse spontaneous emission with OTR through the BPMs (cameras) which would be very sensitive to the energies above.

Magnets:

As discussed before, there is very little room between the steering magnet pole piece and the BPM optics. Removal of this pole piece leaves us with a C-magnet. The measured higher order effects from lack of symmetry gives a quadrupole field much less than a percent of the undulator field. It looks as though the near term solution is to run with a C-type steering magnet, but other solutions are being looked into. It may be down the line that 450 windings would not give the resolution needed, and may have to return to a lower winding. If it only takes ~150 windings to give the necessary steering, the steering magnet and BPM optics could co-exist. We are going to try this on one of the steering magnets. (We do have extra pole pieces).

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Nd:YAG Laser (REPORTED BY MARCUS BABZIEN)

Pulse shortening by saturable absorber: Testing of the saturable absorber shows good results so far. The first sample was received in September and measurements on the bleaching at high intensities were made. The material is LiF which is exposed to radiation to cause lattice dislocations that act as color centers. The color centers have broad absorption around 1 micron and relatively high cross section. The amount of pulse shortening depends upon how much energy is used for bleaching, but simulation shows using 1/3 the total pulse energy shortens the IR from 14 to 10ps FWHM, with the leading edge of the pulse experiencing all the shortening. The first test of the simulation was to use a short section of saturable absorber, only 25 out of 75 mm and measuring the changing transmission versus intensity. The only parameter required for comparison with the simulation is the unsaturated absorption, which depends upon the amount of radiation absorbed and hence the concentration of color centers and the length of crystal. The measured and simulated data are shown in the figure 1_21_Sat_absorber Except for low energies, where the measurement was dominated by noise, the agreement is very good. The measurement was performed with two pulses to determine if the recovery time is sufficient to shorten both of the pulses generated in the YAG system. The spacing between the CO2 slicing pulse and the gun pulse is 245 ns, and complete recovery was observed in this interval. More pieces are on order to provide the anticipated shortening, after which streak camera and electron beam measurements will be used to confirm the results.

Joulemeter: A more sensitive joulemeter probe was installed in the gun hutch to improve the signal to noise ratio at the low energies now delivered to the cathode. The background noise with no laser power is now about 1% for the few microjoule energies now encountered and depends strongly upon gain settings in the digitizer used. We can now accurately measure laser fluctuations in the gun hutch, as they are now considerably larger than the joulemeter noise.

Cathode monument: A new lens system was installed in the gun hutch to collect more light from the cathode monument. There is now enough image brightness to measure the beam profile at a few microjoules delivered energy.

New oscillator: Finally, the new oscillator from Time Bandwidth Products arrived before the end of the year. Brief measurements so far indicate over twice the power output of the current demo unit we are using. A few measurements of the power stability over two days indicate better than 2% p-p. Pulse duration, phase, and pointing stability measurements will follow.

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Technical Operations (REPORTED BY BILL CAHILL)

We have completed the cable labeling and assignments for the installed VISA wiring. These cables are now official and are entered in the cable log database. A major breakthrough in the Eurocrate power supply mystery. Marc and Aaron have been investigating the problem and it occurred that the voltage was the common denominator for all of the errors. Upon lowering the input voltage in steps, it was observed that the reason the units were behaving poorly was the supply voltage was too high. The design, we feel, called for 5vdc, we were operating at 10vdc. Upon reducing the supply voltage to 5vdc, the problem seems to be solved, although we still have occasional drift at the high end. We feel the drift is being caused be a thermal effect as we increase the current past 2.75 amps.. The fan pack cannot remove the heat fast enough and causes instability in the electronics. This was proved with “cool-spray”. We feel the system will not need this high current so all things being equal, it should work fine. We would prefer to operate at 10vdc in order to reduce line loss, the investigation continues!

A failure occurred in the CAMAC crate #5 power supply. A spare was installed and the network was brought back on line with minimum downtime. The mechanical section was plagued by the flu bug but we managed to install the laser filters on the CO2 vestibule windows. This task is in compliance with the laser safety requirements. Work is continuing with the design of the linac H2O temperature system and as always, the technical staff continues to support ATF control room operations on a daily basis.

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