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Site Details ATF Newsletters 
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2002 ATF NewslettersJan | Feb - March | April - June | July - Sept | Oct - Dec |
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Contents |
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| 1.
Introduction |
5. Fast Detector Run |
| 2. Emittance of Photoinjector | 6. Lessons Learned |
| 3. Yag Laser Pulse Compression | |
| 4. STELLA II | |
Friends,
The Accelerator Test Facility will be moving soon (organizationally, not physically) from the NSLS Department to the Physics Department. This move reflects the very wide area that ATF R&D covers, from High-Energy Physics and Basic Energy Sciences to Nuclear Physics and basic accelerator and source research. In the highly diverse Physics Department the ATF will have a good home, with natural connections to both HE-NP and BES missions and funding. I consider this a significant step forward in the evolution of the ATF as one of the ten Research Facilities at BNL.
In this issue of the ATF Update, read about the effect of laser uniformity on the emittance of a photoinjector, Nd:YAG laser pulse compression, progress on the STELLA II experiment, results from a recent optical detector experiment run and, most important for ATF staff and users, in the SAFETY AND LESSONS LEARNED department, Disposal of Waste Metals from "Activation Areas".
Ilan
In a recent run we studied the emittance growth due to the non-cylindrical non-uniform laser beams. For this purpose, we produced masks that create such laser profiles. Six masks may be mounted on one plate. The first mask is simply blank glass with no distortion and defines a 100% laser transmission. The next 5 masks have a rectangular grid pattern in which alternate squares have a reduced transmission. The variable parameter is the degree of attenuation of the reduced transmission. This creates a non-uniformity, which is completely characterized by the transmission through the mask, an easily measured quantity. Thus the highest non-uniformity is at 50% transmission, when every other square is totally blanked. Thus we have the following masks: “100%”, “90%”, “80%”, “70%”, “60%” and “50%”. The laser beam images for these masks are shown in Figure 1.
As these masks were alternated during the measurements, the total laser energy was adjusted in order to keep the bunch charge constant at 0.46 nC. The results are shown in Figure 2. We observe that the emittance is essentially linear with the laser transmissions through the masks. With the mask of “50%” transmission (full modulation of the intensity), the emittance is increased by about a factor of two. For a slight laser modulation, e.g., “90%” transmission, the emittance is increased by about 30%. One must remember that the base uniformity (Type 1) is not perfectly uniform. That means that our best measured emittance has some contribution from non-uniformity. Using the measurements, the measured laser residual non-uniformity (<20%) and quantum efficiency non-uniformity (<10%), we estimate the base emittance (for a perfectly uniform laser) to be about 1 mm×mrad at 0.5 nC. This value is close to the best-measured emittance of 0.8 mm×mrad at the same charge at the ATF, in which the uniformity (and other parameters) where carefully optimized.
An important step has been achieved on the journey to the ATF CO2 laser short pulse upgrade. To get a short CO2 pulse, we plan to use the YAG laser and a Kerr switch to slice the long CO2 pulse, and a pulse length shorter than 5 ps is required.
Recently, the existing 14ps YAG laser pulse at ATF has been successfully compressed to less than 5 ps. Using standard type II second-harmonic (SH) generation in a long crystal, but with a delay between the two orthogonally polarized fundamental components, can lead to effective pulse shortening. For the relatively long 14 ps pulse length of the 1.06 micron input, high pulse energies are required in this technique. It is challenging to use the ~7 mJ energy now available from the YAG laser. This challenge is met by using a large KD*P crystal which is 20mm x 20mm x 80mm in size.
Figure1 is a curve of second harmonic conversion efficiency vs fundamental pulse energy. Saturation of the SH shows that fundamental depletion, a necessary condition for compression, is being achieved. The first pulse duration measurements in this study indicate that modest compression can be achieved. A single-shot autocorrelator has been purchased and is being modified to achieve the required sensitivity for these measurements. However, the initial data shows that a pulse duration of 3-4 ps has been reached.
In parallel, a streak camera was used to speed up progress toward generating short CO2 pulses. Thanks to the cooperation of the NSLS, a synchro-scan streak camera with sub-ps resolution was loaned to ATF for a few days. With the expert help of Brian Sheehy, measurements of the pulse duration were obtained quickly. The camera was used at short integration times to look at a small portion of the core of the second harmonic. These showed pulse durations as low as 2.5-4.5 ps. Next, the entire beam was sent into the camera to study the effect of the different intensity levels across the Gaussian profile on the compression.
Figure 2 is a raw streak camera image after a 6 minute integration. The vertical direction is proportional to time, and the horizontal direction is the slice through the beam profile.
Figure 3 is the enhanced streak camera image. Each pixel was smoothed over three adjacent pixels to reduce the noise level. The time scale is 0.2 ps/pixel, so this resulted in negligible loss of temporal resolution. Next, more horizontal smoothing was applied, and each pixel column was normalized to the same peak height.
Figure 4 is the fitted pulse length as a function of pixel column, scaled to the actual beam spot size. The minimum pulse duration in the center of the beam is close to that observed in the previous two measurements, but the increase in pulse duration toward the edges is noticeable, indicating that only the central 1/3 of the beam is usable. Estimates of the energy contained in this part of the beam are 10-30 microjoule.
The curvature of the image is not understood, but because of the complexity of the compression mechanism, it is not clear if it corresponds to a real arrival time delay at the edges, or distortion in the streak camera optics. Further work will emphasize better optimization of conversion efficiency so that the energy requirements of the Kerr cell are satisfied. The Kerr cell is under preparation, and later the green pulse will be transported to the FEL room for testing the Kerr switch.
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Progess on STELLA II,
AE20(REPORTED BY KARL KUSCHE)
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With the help of STI Optronics' efficient design and machining teams, and Christian Dilley from STI during the week of March 18, the local STELLA team was able to install and align the new laser transport system on beamline #1 in a relatively short period of time. We were also able to correct the alignment of the magnet component spool pieces, which was discovered during the previous beam studies. In order to accommodate the higher CO2 laser power needed for the second phase of STELLA (>10GW), the amplified beam requires expansion and thus larger optics. As is now obvious in the Experimental Hall, two 8-inch mirrors are mounted on the ceiling above the Smith-Purcell chamber, whose lid was modified to accept a 6-inch NaCl window. The beam is brought down to an off-axis parabolic mirror (f cm) assembly in vacuum and sent along the e-beam axis to a waist at the wiggler location (Rayleigh range 17cm). To test the system, we delivered 3-5J (assume 180psec) with no observable damage to optics nor vacuum excursions. We did notice, however, air breakdown in the space between the 8-inch mirrors. This was not unexpected as our simulations predicted that the backward reflection from the NaCl optics would focus in this vicinity. Unfortunately, two vacuum leaks in the new chamber lid prevented us from running e-beam into beamline #1. The first, in the lid's large viewport, has been temporarily fixed. The second, in the large aperture spool piece, may be the result of a manufacturer's defect in the flange weld. We are currently working with the NSLS vacuum group to solve this in time for the next run during the week of April 8.
The objective of this run was to observe an Electro-Optical (EO) signal using a low power HeNe laser. This signal allows one to deduce the electron beam induced beam field on the crystal, and the number of EO signal photons. With this information and our lab test results we can then determine whether our next EO Mach-Zehnder (MZ) setup would be able to produce adequate EO signal photons above the dc background for our streak camera detection.
On March 14 run, our run started at about 2 pm. We spent most of our time steering the electron beam to the right location. The electron beam charge was about 0.4-0.5 nC on March 14 and 0.5-0.6 nC on March 15. The electron beam size was about 1 mm and it was located about 0.5-1 mm above the optical beam. The laser source was a 1 mW HeNe laser. We learned that the electron beam must steer clear of the sample holder and not scrape against any beam pipe upstream or down stream, otherwise the e-beam induced optical noise overwhelm the Electro-Optical (EO) signal.
On March 14, negative EO signal of 0.5 mV (into 50 ohm) was observed. The EO signal arrived 192 ns after the stripline signal. The dc light level varied slowly from 135 mV to 500 mV (into high impedance). The raw EO data does not show a noticeable signal until the background (i.e. EO signal w/o electron beam) was subtracted and the signal averaged.
On March 15, a half-wave plate was inserted and dc light level was adjusted a couple times. Positive EO signal of 0.6 mV (into 50 ohm) was observed. At one time EO signal was noticeable in single shot (even before background subtraction). The dc light level was as low as 35 mV (into high impedance). On this run the EO signal cable was accidentally increased by ~2 ns. The noise that appeared on the EO raw signal traces is inherent to the HeNe laser, which is one of the reasons we need background subtraction. The signal is of EO origin because (a) it arrives at the right time with the opacity signal, (b) we can manipulate the polarity of the signal, and (c) the temporal width of the signal is in fact detector limited to ~0.5 ns.
We did send the EO signal to the streak camera at the end of the experiment, but did not observe any noticeable EO signal. It was determined that the dc/EO ratio might not be low enough. However, our next run using the MZ setup would produce a much lower dc/EO ratio for the streak camera measurement.
Here are some parameters deduced from the experiment. Assuming EO signal broadened to 15 ps, limited by the fiber dispersion, and our 1-GHz detector has a bandwidth of 350 ps. The real EO signal amplitude would be 23 times higher than what is shown on the scope, it would then become ~80 mV (0.5x2x3.5x23) riding on a dc background of ~40 mV. The dc/EO ratio of 1:2 is adequate for the streak camera measurement. Based on the lab run results, 80 mV of EO signal corresponds to 71 microWatt peak power. That is, 3350 EO signal photons in 15 ps duration. Our streak camera needs a minimum of ~1000 signal photons. It is possible for the ILL streak camera to pick up the EO signal. Comparing the lab results, 80 mV of EO signal corresponds to an electron beam field of ~6 kV/cm. When the dielectric constant of the crystal (43) is taken into account, the actual beam field would be 258 kV/cm.
Figure 1 shows the photograph of the electrooptic crystal and alignment flags mounted onto the vacuum flange. This section is in the vacuum while the laser, polarizing optics and detectors are in air for ease of operation.
Figure 2 displays the timing of the stripline signal (stripline), signal from a vcsel located upstream of the interaction point (Vcsel), electro-optic signal when the e beam hits the crystal directly (hit), raw electro-optic signal when the electron beam is near the crystal (EO raw), background noise (EO bg) and the signal when the noise is subtracted from the signal (EO sub). The stripline signal provides information on both the timing and the magnitude of the electron beam as well as trigger signal for the oscilloscope. When the electron beam passes near the Vcsel, the electric field induced by the electron beam influences the Vcsel to lase. This signal provides an indication of both the bunch length and the timing of the electron beam. When the electron beam hits the crystal directly, the crystal becomes opaque to the incident laser beam. This provides clear indication of both the arrival time of the electron beam at the crystal and the location of the electron beam with respect to the crystal.
Figure 3 and Figure 4 show clearly the change in the polarity of the signal when the location of the electron beam and hence the direction of the electric field in reference to the crystal was reversed. The rise time of the signal and the dependence of the polarity on the e beam position confirm that the signal indeed is e-o in nature.
Disposal of Waste Metals from "Activation Areas" (To be presented to and discussed with NSLS Supervisors, who in turn will hold toolbox sessions with their line staff members.) NSLS, ATF and SDL accelerator vault and ring doorways are posted as follows: Caution Radioactive Material Area Activation Check Required Items physically attached to the electron beam chambers and electron beam stops during operation must be checked for induced radioactivity before they are released from this area. [Contact Rudy Zantopp, x5565, or Marlon McAvoy, x6389, to conduct radiation surveys]
Surveys Required A posted table and a roll of yellow radiation tape will be near each "Activation Area" primary exit for metal items that require surveying before disposal. Place all such items on this table and label them with the tape. Radiological Control Technicians will survey each item, remove the tape if the item is activation-free, and place the item into an dedicated "Suspect Metal" waste container. Typical items to be surveyed would be metal hose fittings attached to the electron beam chamber, pipe section copper gaskets, RF connectors on PUEs, heating tapes and temperature sensors on the beam chambers, and any clamps or other fittings and hardware that held the above items in place on the chamber (Note: hoses, cables and wires connected to the listed items should be cut off and disposed of as regular trash or metal scrap). Vacuum components that require a bleedup before removal and any items removed from the electron beam chamber that will not be discarded should not be left on the table. These should be surveyed in place by the RCTs prior to release. If there is doubt about an item, contact John Aloi (x7018), Andrew Ackerman (x5431), or Nick Gmür (x2490). Items at or near electron loss points require additional scrutiny.
Surveys Not Required Items that do not need to be surveyed prior to release include components from front ends and beamlines that see only synchrotron radiation such as water-cooled masks, vacuum valves, shutters, and the hose fittings and connectors attached to them . These objects are not subject to being struck by high energy electrons and activated. Likewise, other material from the accelerator enclosures such as cabling from cable trays, rack-mounted electronic chassis, and plastic water hoses are not subject to activation and do not require a survey.
Suspect Item/Storage For those metal items that were surveyed and found to be activation-free, DOE has imposed a temporary disposal moratorium requiring all DOE sites to consider metal used in Radiological Areas as "suspect" for the purpose of recycling. If you plan to keep the item for future use, or if you have items that were released from activation areas in the past, each item should be tagged indicating "Suspect Metal", origin, and date of survey/release. It is important that we know each item's history and that we do not let these items be released for general use outside BNL.
Suspect Item/Disposal If you plan to dispose of a surveyed "Suspect Metal" item as recycled metal waste, contact Bob Kiss (x4926, pager 5827) to pick up the item and place it in the barrel labeled specifically for "Suspect Metal Waste" inside the caged metal waste area east of Bldg. 725. DON'T LEAVE SUSPECT METAL WASTE THAT CUSTODIANS MIGHT DISPOSE OF!
Last Modified: December 3, 2007
Please forward all questions about this site to:
Vitaly Yakimenko
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