Greetings all,

This week we had runs of the Smith Purcell experiment and HGHG, with a very brief STELLA and HGHG runs. Good progress in all experiments.

Ilan Ben-Zvi.

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Experiments

 

Smith-Purcell experiment (Reported by Harold Kirk)

 

 

July 27, 1999

Start of data taking at 2 pm.

Our alignment and initial beam tuning now proceeds in a relatively standard manner. Initial beam conditions:

Energy 45 MeV

Charge 500 pC

Pulse Length 6 ps

Beam spot size (Grating mid-point) 4 to 5 pixels rms in x (~ 60 microns).

Without removing the alignment mirror and turning on the e-beam we immediately see a 150 mV signal. The radiation is prompt and is superimposed (but with a longer decay time) on a prompt x-ray spike waveform. A sweep of the rotating stage on which the alignment mirror is mounted shows that this signal has a broad peak with a FWHM of 20 degrees. An analysis with polarizing filters reveals that this radiation is unpolarized. We believe this radiation to be scintillation from the glass of the alignment mirror.

A Faraday cup scan finds the e-beam/grating intercept position is 1.5 mm from the "usual" intercept position. Using the new e-beam/grating intercept position we find a 120 mV radiation signal coming from the grating. The radiation is strongly polarized with (base subtracted signals)

vertical orientation 7 mV

horizontal orientation 37 mV

A sweep of the grating rotating stage shows that the grating radiation has a much different behavior than the radiation which comes from the mirror. The grating radiation is found to be strongly polarized horizontally and the angular extent (of the stage on which the grating and alignment mirror is mounted) is reduced to to 4 degrees with a sharp cut-off corresponding to when the grating face closes to the e-beam.

July 28, 1999

Start of beam tuning: 9:30 am

Same startup procedure as July 27. Same e-beam conditions except that we try to place the e-beam more in line horizontally with the previous runs. A Faraday cup scan finds the e-beam/grating intercept to be at the more usual x = 39.8 mm. The radiation from the mirror has increased to 400 mV but the radiation from the grating has decreased to 65 mV.

Re-tuning the e-beam, we find that the signal of the radiation

from the grating has increased to 250 mV. The e-beam is now narrower horizontally than vertically so the beam conforms better to the plane of the grating.

x, u and v scans are taken (attached files):

Weekly_Reports/July_30_SP_xscan

 

Weekly_Reports/July_30_SP_uscan

 

Weekly_Reports/July_30_SP_vscan

 

A first attempt at dispersion is done by placing a prism immediately after the chamber exit ZnSe window followed by the off-axis parabolic mirror which focuses the collected radiation onto the HgCdTe detector. Alignment is done with the HeNe laser (630 nm). We observe a 30 mV signal when we move the grating into the e-beam. However, no structure is observed when we either rotate our internal collection mirror or simply translate linearly our HgCdTe detector along the expect line of dispersion. We next place a grating spectrometer (Arc SP-150) on the external breadboard and attempt to get radiation through the device. No signal is observed with any orientation of internal stages or the analyzing grating internal to the spectrometer.

We return to viewing the signal with no prism in place and try a series of filters. Results:

no filter 150 mV +/- 2% (with 3mV baseline)

Ge 44 mV

RG1000 77 mV

RG9 28 mV

sapphire 103 mV

polarizer(V) 19 mV

polarizer(H) 42 mV

No filter 130 mV (up to 160mV by adjusting vertical trim)

A global interpretation of these results is that roughly half of our radiation signal has wavelengths between 1 and 2 microns, ¼ lies below 1 micron and ¼ is above 2 microns. The beam is strongly horizontally polarized.

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HGHG Experiment (Reported by Li-Hua Yu)

Thursday (July30) we run SASE for the first time with the Pockels cell turned on, so there are double pulses from YAG laser, one for the RF gun, the other for the CO2 switching. This is one step further towards CO2 seeding.

The morning run showed the same large fluctuation even for spontaneous radiation as last week, this indicated an incorrect synchronization of the signal collection. By the afternoon this problem was resolved, YAG laser line was realigned with the Pockels cell on, and the RF gun cathode was laser cleaned, and in the evening the SASE run gave clean results. The charge was 0.9nC, the current 140 amp, the emittance 6.4 mm-mrad, the SASE/spontaneous Ratio was measured to be 11.5, as compared with theory’s 14.

Friday we again produced SASE, and we tried to use shunt bars to short circuit the Cornell Wiggler to measure radiation power as a function of wiggler distance, as this should provide a direct proof of the exponential growth. However, it turns out that the shunt bar gives too small a kick to reliably measure this function.

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STELLA Experiment (Reported by Karl Kusche)

1) Results of beam studies (7/27): This run had the goal of producing and measuring a small e-beam focus at the location of the ICA gas cell, which had been difficult to achieve with the IFEL wiggler in place. With the wiggler removed, Vitaly was able to quickly tune the line to something reasonable. Since our running time was very limited that morning, another run (next Monday) is indicated to further optimize the tune and finalize sigma values. Ping will provide simulation results to compare with Vitaly’s tune.

2) With 7 running days allocated for STELLA in August, it was decided to concentrate our efforts on improving the ICA portion of the experiment. Upon completion of beam studies next week, the ICA cell will be reinstalled and the wiggler will remain out. It is expected that the ICA effect will become more pronounced with an adjustment of the interaction length and/or shortening of the CO2 laser pulse.

3) To resolve the tuning problem mentioned earlier, the group is considering the construction of a permanent magnet wiggler to replace the IFEL wiggler. Its primary advantage will be a larger gap (~1cm bore, instead of ~3mm waveguide). In any event, it is not probable that any sort of prebuncher will be installed into beamline #1 until after the Compton Exp. runs in September.

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VISA Magnetic Measurements (Reported by George Rakowsky)

On Monday, one more set of interferometry measurements was done on Sections #1 and #2 in the 90 and 270 degree orientations. Repeatability, compared with previous measurements, was good. Closure errors, however, were still in the -30 to -50 micron range. The sign of the error has been consistently negative, i.e., the tooling ball offsets measured interferometrically all appear to be larger than the CMM measurements. This suggests a systematic error of some sort, although the source is not obvious. Recall that closure errors for 0 and 180 degree measurements were much better. If one splits the closure error, then we have fiducialization errors of 25 microns or less, which, as Robert Ruland points out, already meets our error budget. We temporarily suspended measurements to work with Robert and Brian Fuss on the trial assembly of VISA magnets into the vacuum tank and on trial alignment. We immediately encountered several difficulties. First, it became obvious that we could not safely insert the magnet sections by hand, without risk of damaging the tooling balls. Jeff made handles which straddled the magnet sections and acted as guides and feet, to protect the side and bottom tooling balls, while the section was being lowered into the tank. In spite of this, three tooling balls on Section #3 fell off (literally). They have been reglued temporarily, but their calibration has been lost. It is clear that ceramic balls glued to little posts will always be very vulnerable and a sturdier fiducial design is needed. Robert is working on it. Next, we found some more problems with the vertical kinematic adjusters.

These have been reworked for the time being, but will have to be replaced. Ben Poling at SLAC is working on a redesign. At this writing (Friday, 5:30 PM) The magnet sections have been installed in the tank and the lids are being put on. This will allow Brian to debug his alignment software. Tomorrow (Saturday) Jeff and Mike will come in to help Robert and Brian do an optical alignment, followed (hopefully) by tests of the interferometric alignment procedure on Monday.

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Facility

 

Technical Staff Tasks (reported by Mark Montemagno)

This week I completed the temperature stabilizer prototype for the RF phase/amplitude drivers. The initial testing shows that the temperature is held within 0.7 degrees C p-p. The mezzanine cycles greater than 3 degrees C p-p.

The safety devices for the terawatt laser have been installed. The devices are the blow-off tube and a pressure switch for the interlock system.

ATF Operations Schedule (Prepared by Xijie Wang)

 

August: schedules/Aug 98