2001 ATF Newsletters

Jan | Feb | March | April | May | June - July | Aug | Sept | Oct | Nov - Dec

Greetings everybody,

The MINOS detector group, (C. Velasarios, H. Ping, A.R. Erwin University of Wisconsin, J. McDonald, D. Naples University of Pittsburgh, B. Viren, M. Diwan Brookhaven National Laboratory and G. Tzanakos Athens University) completed a successful feasibility test at the ATF and are in the process of submitting a proposal for an ATF experiment. The report on their feasibility test is included in this Newsletter. To remind the readers what is a feasibility test: ATF Experiments are reviewed by our Steering Committee. The ATF Director may approve a short, low priority test to demonstrate the prospects of a new experiment. Following such a test the potential users can assess the value of the ATF for the experiment and the ATF can assess the suitability of the experiment to its resources and the other ongoing experiments. I am glad to note that the MINOS group has been highly professional and can fit into the ATF and its current complement of experiments very well.

The VISA Experiment completed its alignment and a strong SASE signal was immediately observed.

The terawatt laser has begun operations and a power level estimated at 0.1 TW (unoptimized) was seen immediately. We are looking forward to results following optimization. The Compton experiment is preparing for its next run which will be the first one to use the new laser.

The new miniature permanent magnet quadrupoles have been installed in the Compton chamber on Beam Line 1 and tested. A spot size estimated at 15 microns rms (with 0.5 nC charge!) was measured. An exact number has to await analysis. This is great news for various experiments that need very small beam sizes.

The new control computer of the ATF has arrived and is being tested by Bob Malone.

Professor Y. C. Huang of the National Tsinghua University in Taiwan is visiting the ATF with two of his students. Prof. Huang is the spokesperson of the Structure-based Laser Driven Acceleration in a Vacuum. AE27 and he is conducting a material laser damage tests of materials for his accelerating structure.

An Engineering Design Plan (EDP) for the ATF Beam Line 1 upgrade has been prepared by our mechanical engineers  Eugene Hu and John Skaritka. The modification is aimed at providing kinematic mounts and other adaptations to allow for a fast changeover of experiments on this busy and popular beamline. It will allow more experiments to use the beam line with minimal interference. The EDP is under review.

Kathy Loverro is now participates in the maintenance of the ATF web site. One of her new innovations is the Subject Index of the ATF Newsletter (work still under progress, but look at 

newsletters-subject).

Ilan Ben-Zvi.

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Preliminary Results from Initial Runs

Figure 1 shows the sketch of the MINOS chamber at the ATF.

Figure 1: MINOS1.jpg 

We have analyzed two runs that we determined to have excellent quality data. Run 205 appears to have data well below any kind of saturation for the two chambers, and run 206 appears to have data well above some saturation point for both chambers. The saturation could be a priori either electronic saturation or gas saturation. One other run far above saturation also might be marginally informative but is not studied here.

Fig. 2 shows data from these two runs for the 1.0 cm gap chamber and for the 0.5 cm gap chamber in separate plots. The Faraday cup charge is plotted along the x-axis. The total charge from all 4 instrumented strips is plotted along the y-axis after dividing by the average number of ion pairs expected for the gap width 1.

Figure 2: Minos2.gif

One might expect the slope of both of these curves to be 1.0 if the assumed number of ion pairs/cm is correct since the integration time is so long that both positive ions and electrons are completely collected if they do not recombine first. The chamber charge measured below the saturation points is somewhat greater than this expectation.

Fig. 3 shows the charge collected in each chamber strip summed over run 205. Each chamber is plotted separately. Most of the charge is collected by one strip in the 0.5 cm gap chamber. If we assume infinitely long strips and a Gaussian charge distribution centered on that strip, this implies an rms beam width of 0.7 cm and a beam "area" of A=1.5 cm²

Figure 3: Minos3.gif

There is evidence (see later discussion) that some of this width is due to motion of the beam center during the run.

Fig. 4 shows the sum of charge collected in each chamber strip for run 206 at higher beam intensity. The strip with the most charge (channel 10 or channel 20) in each chamber is operating within 0.1 volts of the output saturation for the SWIC electronics. This was because the ion chambers were running on a negative power supply, and the range of SWIC electronics is 10X smaller for that polarity. An attempt was made to switch polarity when this was discovered, but the accelerator shut down after a single run using the new voltage configuration.

Figure 4: Minos4.gif

The intersection of extrapolated slopes in Fig. 2 marks points of what is most likely electronic saturation. The slope changes at the intersection because the dominant charge collecting strip has electronically saturated, and only strips in the wings of the charge distribution continue to collect a reduced fraction of the total charge.

Fig. 5 shows the collected charge on each chamber strip as a function of the Faraday cup charge for run 206. Channels 10 and 20 are obviously saturated and show no further increase with increasing beam charge. Channel 16 essentially registers a pedestal value. (Note: Channel 16 has a 10,000 pF integrating capacitor instead of the 100 pF capacitors used for the other strips. This may account for some of the apparently unusual behavior we see at low Faraday cup values.)

Figure 5: Minos5.gif

The intersection points in Fig. 2 correspond to the same collected charge in each chamber, which would only be expected if the saturation were due to the same electronic limit. Unfortunately this does not rule out the onset of some gas saturation in the region of beam intensity where we have no data points. If the intersection of the slopes for the 0.5 cm gap chamber represents pure electronic saturation, then we can estimate the beam intensity of that point using the 1.5 cm² beam area from above. Using the Faraday cup value for beam at the intersection point, gives I = 0.8 10⁸ electrons per cm². The highest data point where charge collection is actually observed to be linear is about half that intensity, I '=0.4 10⁸ electrons/cm².

Using the charge collected by 4 strips, one can calculate 1-D average beam positions and standard deviations of charge distribution on a spill by spill basis. The standard deviations are plotted in Fig. 6 as a function of Faraday cup charge for the corresponding spill. The x and y widths are slightly different. If we calculate a beam area from A = pxy, where x and y are rms widths for the beam, we find a smaller beam size than the earlier estimate. A =  0.44 cm², about 3.4 times smaller than the earlier estimate.

Figure 6: Minos6.gif

Figure 7: Minos7.gif

Figure 8 Minos8.gif

This leads us to speculate that gas saturation in the 0.5 cm chamber does not set in before beam intensities of I =  1.4 10⁸ electrons per cm² or perhaps twice that.

Let us estimate at what intensity we might expect gas saturation effects to occur. If we use the area of 0.44 cm² with 500 volts across the 0.5 cm gap, we expect charge on the ion chamber plates to be 39 pC for this area. This would predict gas saturation problems begin when that many ions are released in a tube of gas having that area and a length of 0.5 cm. If each track forms 3.9 ion pairs, this implies saturation begins with a flux of about I = 1.4 10⁸ electrons per cm², which happens to be the highest flux we observed without saturation effects.

Conclusion

We probably barely missed seeing the onset of gas saturation effects because of the electronic saturation effects we encountered. We have now understood the apparatus with the two short test runs. In particular, we were very successful at collecting all of the charge and measuring it precisely in the Faraday cup even at small intensities. In the new proposed run we are confident that we can cover the entire dynamic range (few to 500 pC) over which these detectors are expected to operate and precisely measure gas saturation effects.

We are grateful to the assistance provided to us by the ATF personnel. Without their help, which was clearly beyond the call of duty, we could not have made sufficient progress to write our proposal.

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The last few weeks have been dedicated to moving the undulator (while in vacuum) to the correct position and a continuation of running the VISA system. Brendan and Heinz-Dieter came out during the movement and we believe that the undulator was moved to within 25um of the position specified. Also, the undulator appears not to have moved since the interferometer alignment in October and during the pumpdown. This gives us some confidence that the sections should be close to the correct aligned position.

A camera placed 3m past the end of the undulator showed that SASE lasing
had started while the repositioning process was taking place. Once the
re-alignment had finished we took orbit studies and quantified the signal
seen.

During orbit studies we noticed that there were inherent system fluctuations. This jitter allowed us not to better the horizontal orbit to <200microns peak to peak for multiple orbit studies at the same system settings. Still more studies need to be done, but the orbits taken seem not to be reproducible below this amplitude.

Detector energy vs. charge was also taken with a Joulemeter 3m from the end of the undulator. A peak SASE signal of 8.4nJ at around 450pC was measured.

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The new miniature permanent magnet quadrupoles have been installed in the Compton chamber on Beam Line 1 and tested. A spot size estimated at 15 microns rms (with 0.5 nC charge!) was measured. There was not enough time to analyze data at the time of this report. So it is very-very “fresh”. A 30 micron wire is shown for visual calibration. All images for the same magnification and almost same beam with 0.5 nC charge and 45 MeV energy. More data is coming after analysis.

beamsize.bmp

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 What if you suspect that an occurrence at the ATF has to be reported? This could be a safety violation, an incident involving possible exposure to ionizing radiation or laser light, fire, chemical spill, etc. The first action would be to apply the appropriate corrective action to contain hazards. But then, where would you look for an answer concerning reporting duties, even late at night? The answer is the BNL Occurrence Reporting System.

For Occurrence Reporting at BNL per DOE Order 232.1A, the Laboratory has an Occurrence Reporting and Processing System (ORPS) subject area https://sbms.bnl.gov/standard/20/2000t011.htm and Occurrence Reporting Program Description https://sbms.bnl.gov/program/pd05/pd05d011.htm.   

 Please familiarize yourself with the BNL Occurrence Reporting procedures. 

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Last Modified: December 3, 2007
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