Accelerator Test Facility

Despite what we expected (optimum bunching at some intermediate laser power), there was no definitive correlation of delivered laser power to observed CTR signal. We believe that e-beam fluctuations are mostly responsible for this problem (CTR signal is very sensitive to the e-beam size and current). So, instead of relying on the CTR diagnostic to confirm microbunching, it will be used as a diagnostic secondary to the spectrometer. Thus, plans to install the gas cell and look for ICA & staging will proceed.

2) This past week and during the ATF shutdown, the following work has been or will be done for STELLA:

· Install ICA gas cell, align to beamline HeNe, reestablish vacuum, install

& test VOx diagnostic, align ICA optics

· Improvements to IFEL laser delivery system (HeNe focus, mirror mounts, enclosures, filter wheel & filters)

· GD2 bellows mounting assembly

· Remote control for encoders, filter wheels, joulemeter, and laser beam blocks.

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VISA Experiment (REPORTED BY AARON TREMAINE)

We saw good OTR signal through the BPM using the more sensitive COHU camera. The beam was propagated through the undulator and a reasonably shaped beam was seen at all pop-ins. Using polarizers and filters we were able to determine to our satisfaction what is seen in the cameras is OTR. 700pC of charge was sent into the undulator and we were getting transmissions of >90%.

We were able to transport >90% of the beam without using the undulator correcting coils. Then a “transport matrix” was made. The position of the electron beam centroid RELATIVE to the alignment laser at each pop-in was measured. This tells us what the undulator is doing to the electron beam since we know the alignment laser is a straight line and no correction is being used. One must keep in mind keep that the alignment laser is not at the magnetic center of the undulator.

There appears to be the betatron motion of the electron beam with large amplitude. We will get a good estimate after a calibration on the cameras (micron/pixel) is done. (A fairly illuminating plot of the workings of our undulator on the e-beam at this time)

One inconsistency. We were not able to reproduce the signal seen from last time at the detector- 400mV. At most we saw 20mV. This is puzzling since we were getting through a lot more charge and using the same setup.

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CO2 Laser (REPORTED BY IGOR POGORELSKY)

In the continuing work towards the operation of the terawatt laser, the

gigawatt pulse generated by the ATF CO2 laser was trimmed from FWHM 200 ps

to 25-30 ps using a combination of a plasma shutter and a saturable

absorber. The short pulse duration was measured with a single-shot

autocorrelator. Diagram of the ATF CO2 laser with the

trimming-autocorrelation setup is shown in Fig.1:

2_4_CO2_system

Later on, the short pulse will be sent to the booster amplifier to achieve the 1 TW peak power.

How does the pulse trimming work?

The plasma shutter is a short-focal-length telescope with a pinhole placed inside the adjustable-pressure gas cell. The leading portion of the laser pulse produces an optical spark at the pinhole. Overdense (above critical density, that is 1019 cm-3 for 10 micron radiation) plasma of the spark screens the tail of the pulse allowing only the pulse head to propagate through the pinhole. Reproducible short pulses have been obtained at a N2 gas pressure of 20 torr.

A saturable absorber cell is placed before the plasma shutter. This 1” thick device, filled with ~10 torr SF6, serves to increase the contrast by absorbing pre-pulses and background radiation. Concurrently, the saturable absorber helps to stabilize the trimming process by eliminating uncontrollable gas ionization due to parasitic and background radiation. Effects of the saturable absorber ant plasma shutter on the initially ~200 ps CO2 pulse is illustrated by Fig.2:

2_4_CO2_results

How does the autocorrelator work?

The first single-shot CO2 autocorrelator has been developed at the ATF in 1992-1993 by Igor Pogorelsky and Marcus Babzien. For a comprehensive description of this instrument ask Marcus about his MS.I. thesis (babzien@bnl.gov). The device was not in use since that time because its acceptance time range is a little short to measure the 200 ps ATF CO2 laser pulses. It is perfect though for the trimmed pulses. A principle schematic of the Marcus’ autocorrelator is shown in Fig.3:

2_4_CO2_SSAC

A diffraction grating, shown in the lower right corner, tilts the intensity front across the laser beam. The tilt angle is enhanced due to beam compression in the Brewster prism. After the cylindrical lens the beam splits and a variable optical delay is introduced in one leg. The line-focused beams with intensities I1~I2 intersect each other inside the nonlinear crystal. The crystal orientation is chosen to optimize the non-collinear second harmonic I*~ I1xI2 that propagates in a median direction. This permits a simple scribing out of the individual 2^nd harmonic components proportional to I12 and I22. Due to unequal number of reflections in the two legs, the intensity fronts of the intersecting beams cross each other. This results in a linearly variable delay introduced across the interaction region. The resulting transverse intensity modulation in the produced non-collinear beam becomes a signature of the autocorrelation function. We display the transverse profile of the non-collinear beam using a pyroelectric linear array or a pyroelectric camera and extract from the image the information about the time structure of the laser pulse.

What do we actually observe?

Above is a description of the basic principles, design and operation of the autocorrelator. We further illustrate these by Fig.4 that describes shows the 30 ps pulses are measured.

2_4_SSAC_shapes

We look at the cylindrically focused laser beams intersecting each other within the second harmonic crystal (SHC). Dark arias show crossed intensity profiles. When the plasma shutter is evacuated and no spark is observed, the transmitted laser pulse is still 200 ps long. In that case, the area in the crystal where the intensity of two beams combines to produce a non-collinear second harmonic is relatively large. We see on the pyroviewer a broad image (see Fig.4A). The same situation is when we have a gas-filled shutter and a strong attenuator upstream that reduces the laser flux below the optical breakdown threshold. However, when we put the same attenuator after the plasma shutter, we see a spark inside the shutter and short image of the non-collinear second harmonic on a pyroviewer (see Fig.4B). Next, we change optical delay in one of the autocorrelator legs with a micrometer translation stage and observe a shift of the image (dashed lines on Fig. 4B). This permits us to calibrate the transverse coordinate x against the time and to measure a temporal width of the image. The typical number is 40 ps. Understanding that the width of the autocorrelation function is about 1.5 bigger than the FWHM of the light pulse we estimate the duration of the trimmed laser pulse between 25 to 30 ps.

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Mechanical Group (REPORTED BY JOHN SKARITKA)

1. STELLA -The CO2 beamline enclosure has been modified at the request of the experiment, and the down coming beam safety enclosure has been fabricated.

2. TW LASER- In response to the NSLS safety committee recommendations, the TW laser vessel was made ready for shipment to central shops for further inspection.

3.TW LASER- Mechanical work has started on the fabrication of parts and assembly of the TW laser breadboard enclosure.

4. YAG LASER- Design of a beam line safety enclosure was completed and materials have been ordered.

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Technical Operations (REPORTED BY BILL CAHILL)

The groundhog has seen his shadow and we look forward to six more weeks of winter. Marc Montemagno has been completing the ordering phase of the new linac water temperature control system. The orders have been placed and preparation is being made to schedule the changeover. Due to the lack of parts, the initial work scheduled for next week had to be cancelled. This will not impact the overall plan. The COHU camera assembly for Beam Profile Monitor # 5 has been assembled and will be installed next week. Bob Harrington has been occupied with the terawatt laser in retrieving the blow-off pipe assembly and assisting with the removal of components involving the move of the discharge cell to central shops. The NSLS Mechanical Group has come to our aid in disconnecting all components on the vessel. Plant Engineering has been notified and the cell should be on its way early next week. Technical assistance was offered to the HGHG Experiment and minor problems with the CAMAC control system were quickly taken care of. Of course, as usual, the technical staff continues to support the operations of the facility on a daily basis.

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Friday Engineering Meeting (REPORTED BY XIJIE WANG)

The following subjects were discussed at this Friday’s ATF Engineering Meeting:

1. ATF shut down job and responsibilities list:

A. “Must do” list for this coming shutdown:

1)   Install new oscillator starting on Monday Feb.7, ready for beam testing in the week of Feb.14. (Babzien).

2)   Install new AC for ATF experimental hall starting on Feb.8 (Cahill).

3)   Facility safety maintenance and clean up. All experiments must make sure that cables in use are labeled. Persons with beam line / area responsibility are: Cahill for all ATF. Babzien for FEL room. Kusche for beam line #1, Yu for beam line #2, Tremaine for beam line #3, Malone for control room.

4) ATF radiation certification will be done on Thursday 10 (Dickinson, Cahill).

B. “Should do” list:

1)   Install cavity monitor on Monday of Feb.7 (Yakimenko, Harrington)

2)   Install new camera for BPM on Monday Feb.14. (Wang, Harrington).

3)   Improving gun cooling on Monday Feb.14 (Wang, Harrington).

4)   Improving cooling of CAMAC (Montemagno).

2. STELLA experiment: install ICA cell. No survey help requested.

3. VISA experiment: resurvey the undulator, survey help needed on Monday Feb.14 (Skaritka). Exclusive access during the final alignment starts Thursday of Feb. 17.

4. TW CO2 laser: TW discharge cell will be moved out next week, Pogorelsky and Skaritka will follow the progress of the work in central shops.

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