2001 ATF Newsletters

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

Greetings,

In this update we see some of the facility development work as well as some accelerator physics. The ATF new computer control system make great strides towards operations. The CO2 laser and capillary plasma channel promise to shape up the future of x-ray production and laser acceleration. 

Photoinjector R&D continues at the ATF as usual. Results from experimental work-in-progress show the effects of laser uniformity on the emittance of the photoinjector.

Ilan Ben-Zvi.

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 The long awaited 100 MHz Ethernet CAMAC crate controllers have arrived, replacing the 10 MHz units which were on loan from the manufacturer pending final delivery of the 100 MHz components.  Initial tests uncovered a communications glitch which ocurred 5 times in 2^24 iterations.  It is suspected that a network switch, which was not properly filtering traffic from the BNL campus network, caused this problem.  BNL’s Information Technology Division will be reprogramming the switch to isolate the CAMAC communications.  Otherwise, the performance has been excellent and we feel comfortable with this new communication hardware which will eventually replace the CAMAC serial highway and associated controllers.

As with the 10 MHz controllers, the new units are presently operating as auxiliary crate controllers, sharing the CAMAC crates with the old VAX/serial highway system.  This shared operational mode will continue until all VAX software has been ported successfully to the new system and the VAX retired.

·        The new IBM control system host has been upgraded to the latest Linux kernel, version 2.4.3.  The new kernel more reliably supports multiple CPUs and large memory systems.  Although things are now well-behaved and stable, the upgrade required a number of telephone calls and e-mails to both IBM and Red Hat to iron out configuration, compiler and RAID subsystem issues.

·        First server program successful: The server program which handles Darlington magnet power supplies has been completed and tested successfully.  This is an important event since it marks the first integrated test of separate components (operator displays, databases, low level drivers and new CAMAC controllers) working together in the new control system.  Many of the software libraries needed for this server are common to other programs yet to be ported to the new system, so the success of this first program gives confidence about the work remaining to be done.  (Note: Since the data communication rate is relatively slow for magnet power supplies, the Ethernet switch problem described above was not an issue during this test.)

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The ATF continues with the upgrade of the facility, in particular its high-power CO2 laser, in order to better serve the more demanding users' experiments. An important step in this process was a meeting of Laser Advisory Panel, which took place  on September 10-11, 2001. The Laser Advisory Panel considered the possibilities and issued recommendations on the technical issues and the most expedient way for completing the ATF CO2 laser upgrade to the 1 TW level.

Panel members:
Paul Corkum (NRC)
Sergei Tochitsky (UCLA)
Keith Darabos (Spectra Gases)
Triveni Rao (BNL)
Lowrens Botha (SDI)
Igor Meshkovsky (Optoel)
Igor Pavlichine (Optoel)

Meeting attendees:
Igor Pogorelsky (ATF) - meeting coordinator
Tomas Marshall (Columbia Univ.)
Sergei Schelkunov (Columbia Univ.)
Xiji Wang (ATF)
Marcus Babzien (ATF)
Vitaly Yakimenko (ATF)

The list of principal recommendations includes the following:
· Build a short-pulse second harmonic generator to produce 0.53 micron pulses down to 3 ps duration using the relatively long 15 ps ATF YAG laser.
· Build a Kerr-switch (CS2 cell between crossed polarizers) controlled by the YAG second harmonic, to produce 10 micron seed pulses shorter than 5 ps. This method will either replace or work in combination with the present semiconductor optical switch.
· Replace the present ATF's 3-atm preamplifier with a 10-atm preamplifier laser to overcome the 150 ps limit of the current system.
· Improve diagnostics. performance of the big-aperture booster amplifier.
· Improve the performance of the large-aperture booster amplifier. 

All the above recommendations are built into the ATF implementation plan that is scheduled for completion in summer 2002.

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High-power laser guiding in plasma channels is a widely recognized way towards next-generation laser-driven electron accelerators where electrons move collinearly with the laser beam. When the motion is changed to counter-propagation, laser and electron beams produce intense monochromatic x-rays via Thomson scattering. This process can benefit from plasma channel as well as allow the efficient use of relatively long laser pulses.
The ATF,  jointly with the Tokyo Metropolitan University (TMU) and the Hebrew University (Jerusalem) initiated a plasma channel study aimed at providing current and future users with this state-of-the-art technique. The technique which we adopted is the capillary discharge plasma channel.

Several problems must to be solved. These include:
· Finding conditions for channeling of intense 10 microns laser radiation, which is the hallmark of the ATF laser technology. (Previous demonstrations of the capillary channeling used short wavelength lasers (~1 micron).
· Placing the plasma channel in the electron beam line (never done previously).
· Making the plasma channel technology a robust and reliable user-friendly tool. 

Solutions to these problems are still ahead. However, important steps were already taken in this direction. The technology of laser channeling in capillary discharge developed by the Hebrew University has been implemented in a new Thomson scattering cell that has been designed and tested with a help of Optoel-Intex Co. (Russia). 

Figure 1 shows a 3-cm long capillary cylinder with the electrical components forming the discharge circuit assembled at the tip of translation/tilt manipulator.

The assembly was placed in vacuum inside the Thomson cell shown in Figure 2. Electrical feed-throughs and manipulators on the top flange control the capillary. A parabolic mirror that focuses the laser beam into the capillary is mounted inside the cell on the front left flange that has a translation manipulator and motor-mike adjustment feed-throughs on top.

Preliminary optical tests performed with a Nd:YAG laser (8 mJ, 200 ps) demonstrate proper functioning of the device. Figure 3 shows the laser beam profile observed after the 1 mm diameter capillary without discharge (left picture) and at a proper timing after discharge (right). The effect of the channeling is clearly seen by the fact that the laser spot-size is reduced significantly.

The next step of the research program is demonstration of the energetic CO2 pulse transmission though the channel and a test of a pellicle divider that will separate the discharge cell from the ultra-high vacuum linac beamline. Upon completion of these tests, the new device will be ready for installation into the ATF beamline for the Thomson-in-capillary experiment.
 

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A study is under way to measure experimentally the emittance of a photoinjector as a function of the uniformity of the laser distribution on the cathode. In this study we generate various laser intensity distributions by masking part of the laser light, measure the laser distribution and the emittance under various conditions and compare the results to simulation done with the PARMELA code. This study is at a very preliminary stage, but it is worth looking at some of the results.

Some preliminary results can be seen in the figures below. 

Figure 1 shows the measured values of five laser intensity profiles (shown here as a function of radius, the distribution has a cylindrical symmetry). 

Figure 2 shows the emittance for these five distributions holding the total gun charge constant at 0.5 nC. The agreement with the simulation results is satisfactory.

Figure 3 shows the emittance as a function of charge for one of the laser types (profile 5). The agreement with simulation is good, as above, except that PARMELA seem to overestimate the emittance when it is very small.

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

One of ten national laboratories overseen and primarily funded by the Office of Science of the U.S. Department of Energy (DOE), Brookhaven National Laboratory conducts research in the physical, biomedical, and environmental sciences, as well as in energy technologies and national security. Brookhaven Lab also builds and operates major scientific facilities available to university, industry and government researchers. Brookhaven is operated and managed for DOE's Office of Science by Brookhaven Science Associates, a limited-liability company founded by the Research Foundation of State University of New York on behalf of Stony Brook University, the largest academic user of Laboratory facilities, and Battelle, a nonprofit, applied science and technology organization.

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