2001 ATF Newsletters

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

Hi,

Head-up for all ATF users / potential users: The ATF Program Committee and Users Meeting will take place at the end of January or early February. We will advertise the exact date as soon as it is decided. This is the opportunity to present new proposals for ATF experiments to our committee.

Last month Igor Pogorelsky reported on the newly arrived capillary plasma laser channel. The progress has been rapid, thanks for the teamwork of Igor and his coworkers,  Boris Grinberg, Tetsuro Kumita and Igor Pavlichin. In this update we see results (very preliminary) which already demonstrate laser channeling with good energy. To my knowledge this is the first channeling of a CO2 laser. Laser channeling is considered as a key process towards laser plasma wakefield acceleration over significant distances. This work is done as part of the Compton Scattering experiment (AE22), done in collaboration with Tokyo Metropolitan University, Waseda University and KEK.

Other items reported are progress on the STELLA II experiment (Wayne Kimura and Karl Kusche ), a report on the photocathode laser development (Marcus Babzien) and a report on ATF operations in October (Xijie Wang).

From time to time I add new names to the ATF Update mailing list, such as prospective users, collaborators or for other reasons. If you receive this email for the first time, I apologize for not having added you earlier. If anybody wishes to be removed from the ATF Update mailing list, add his name or a colleague's name, please drop me an email at ilan@bnl.gov and I will be happy to comply.

Last but definitely not least, we have the Safety and Lessons Learned section shedding light on the significance of the new ATF's Accelerator Safety Envelope, written by Nick Gmür. 

Ilan Ben-Zvi.

Back to Top

Report on CO2 laser channeling tests of October 2001.

Participants: I. Pogorelsky - BNL (USA) I. Pavlichine - Optoel-Intex Co. (St.Petersburg, Russia) B. Greenberg - Hebrew Univ. (Jerusalem, Israel) T. Kumita - Tokyo Metropolitan Univ. (Japan)

In the previous ATF Newsletter of September 2001 we gave an account of the goals and status of the ATF study of the CO2 laser channeling in a plasma channel. Since then, we achieved the first demonstration of picosecond CO2 laser pulse channeling in plasma that we report here. For plasma channel formation, we use the capillary discharge technique developed in the Hebrew Univ. (Jerusalem). Two sections (ignition and main discharge) Polypropilene capillaries, 1 mm inner diameter and 18 mm long, are positioned in a vacuum chamber (3x10^-5 torr) with a translation-tilt manipulator. Up to 14 kV pulsed voltages are applied to the capillary electrodes according to a principle electrical diagram shown in Fig.1

Figure 1

The 200 ps 100 mJ CO2 laser pulse was focused at the entrance of the capillary with a lens of 20 cm focal length (F#=12). Another lens imaged the output laser beam with a x7 magnification to the Electrophysics pyroelectric video-camera. Intensity profiles are grabbed with a Spiricon laser beam analyzer. Translation of the imaging lens allows us to observe the cross-section of the laser beam at different depth around the focus inside the vacuum cell. The intensity distributions are shown in

 Figure 2

Fig 2a and Figure 2b are obtained in "free space" (capillary retracted). Fig. 2a is taken at the focal point. Fig. 2b is taken at 18 mm distance downstream from the focus. Note that in order to stay within a linear response of the camera and frame-grabber, images 2a and 2 b are obtained at a different attenuation. This permits measurement of the beam size that, as we see, expands approximately 6 times in diameter between the observation points that are spaced by ~6ZR (where ZR is the Rayleigh distance) When we insert a capillary with its entrance tip set at the focal point and establish proper discharge conditions (electrical current and timing) the observed intensity changes from 2b to 2c without refocusing of the imaging lens. Comparison of the image at the plasma channel exit (Figure 2c) with the distribution in the free space at the equivalent distance from the focus (Fig. 2b) demonstrates an evident optical guiding effect (optical attenuation has not been changed). The best channeling condition is obtained at 12-14 kV voltage applied to the capillary, peak current ~400 A, laser pulse is delayed by ~120 ns (3/4 of the current period) relative to the 1st current peak. Note, that the laser intensity observed at the capillary exit without discharge (not shown) is distributed over the 1 mm capillary cross-section and is strongly and rather irregularly modulated due to reflection and diffraction on the inner capillary wall.

The  laser channeling results reported here have been collected over just 1 hour of experiment run (~100 laser shots) and shall be considered as preliminary and qualitative. Laser alignment and capillary conditions still need to be optimized and accurate measurements of the guided beam size and energy are to be done. We decided to report these rough results now for the following reasons: a) it is a time for the ATF monthly update, and b)  because of importance and novelty of the results. This seems to be the first experimental evidence for guiding a 10 micron laser beam in a plasma channel. Previous demonstrations have been done with 1 micron and shorter-wavelength lasers. The electron plasma density measured in the previous demonstrations was at ~10¹⁹ cm^-3 and above. This number corresponds to the critical plasma density for the 10 mm radiation. Our observations, being of a special importance for the CO2 laser applications, also provides a confirmation for establishing a plasma channel in the range of 10¹⁷-10¹⁸cm^-3. These conditions are of interest for such important application of guided laser beams (including solid state lasers as well) as laser wakefield acceleration. As has been stated earlier (see September ATF Newsletter), in addition to better characterizing and optimizing the CO2 laser guiding conditions, our plans include demonstration of efficient transmission of several Joules of laser energy through a plasma channel and adopting the capillary discharge technique for the Thomson scattering experiment.

Back to Top

Progress report on the installation of STELLA-II on beamline #1: Installation & survey of the new STELLA-II magnet assembly baseplate is complete with pumpdown/bakeout now in progress. Initial runs will be first to commission the newly-configured beamline without undulator magnets. Next the untapered undulator will be operated by itself to reestablish proper spatial and temporal overlap of the e-beam and laser beam. The untapered undulator will then be exchanged with a 5% gap-tapered (3.2% energy tapered) undulator and the tapered undulator will be run by itself. This will allow us to test our ability to operate the tapered undulator, which is more sensitive to proper matching of the e-beam energy for a given laser power. The 5% gap-taper will also be one of the highest tapers tried in an IFEL. The STI undulators are capable of a maximum gap taper of over 18%. Installation and testing of the buncher and chicane will also be done individually before the fully integrated STELLA-II experiment is operated together.

Back to Top

During the last shutdown period (10/22-10/27), significant effort was made in understanding the limitations encountered when using so called "Gaussian" or graded reflectivity mirrors (GRM), for transverse profile shaping in the system. Thus far, efforts to achieve an ideal top-hat profile have been limited by the manufacturing problems associated with these mirrors. Figure 1(Desired (- -), and measured (o) radial reflectance profiles of GRM.) shows the difference between the desired ideal profile which in simulations gives close to a flat intensity profile in the central part of the YAG beam.

The first problem with the actual mirror profile is that the magnitude of the reflection at peak is larger than requested, and the other is that the transition from high to low reflectivity is much sharper than the desired n=4 supergaussian. The specification was for a closer tolerance to the desired profile. However, long delivery delays from the vendor (Laser Components, Inc.) and few alternatives make it unlikely that a better match can be obtained. Nevertheless, the mirror was previously used in the configuration in Figure 2(Optical configuration with GRM in place.)  to demonstrate good beam shaping. By using the GRM in transmission as close to the amplifier as possible, some power is lost to reflection, and the transmitted beam has significant energy in the TEM 01 mode, with a local minimum on axis. By minimizing the propagation distance after the GRM, the development of a sharp diffraction peak in the center of the beam is avoided. The total energy delivered can be restored to the same level as before, since our limitation is damage threshold, not gain.

In order to achieve a top-hat distribution, the contribution of the TEM01 mode must be reduced by spatial filtering. Since higher-order modes have larger divergence, they are preferentially attenuated by the pinhole at the waist position. By changing the pinhole size from 1.8 down to 0.9 mm diameter, the beam profile in IR goes from the "donut" shape of Figure 3a to the more tophat profile of Figure 3b.

However below 1.2 mm, the shot-to-shot energy jitter increases above 20% p-p, and profile stability severely degrades as well.  These issues were addressed in the past two years when efforts were made to understand and minimize the causes of jitter in the system.  Pointing jitter in the CW beam drops significantly above 1mm pinhole diameter, and the additional thermal lens instability from flashlamp pumping requires a minimum pinhole size of 1.2 mm in order to achieve less than 10% p-p energy jitter on the cathode.

For comparison, the expanded UV beam profiles are shown before and after the installation of the GRM in Figure 5.  Taking slices through the images and measuring the beam flatness with a standard algorithm, the UV modulation from edge to edge is approximately 30% worse than without the GRM.  This is the major problem currently.

With more running time, it will be seen if this configuration is more sensitive to long term drift.  From the past few days of machine operation, measured shot-to-shot stability appears slightly worse now, but is still less than or equal to 10% p-p on the cathode.  Pointing stability looks essentially the same.

The IR beam currently is more uniform than without the GRM, and this will provide benefits when introducing other improvements in the system such as saturable absorbers for pulse front sharpening, or the pulse compression frequency doubling scheme for driving the CO2 system.  Further improvement in the profile on the cathode could be obtained through two different methods.  First, a passive device utilizing birefringent lens pairs can be assembled and tested to give the very small variable correction to the radial profile required in the IR beam.  This technique has been demonstrated on high-energy pulsed lasers previously and requires some investigation into vendor manufacturing capabilities.  Second, an active mirror system could be used in the second harmonic to adjust the beam profile and also make correction for higher spatial frequency noise introduced in the UV beam.  This technique requires some simulation to confirm functioning and is more expensive (~$40k), but is far more capable.  The two methods could also be combined.

Back to Top

In the month of October, ATF provided beam time for the following experiments: 

1. Dielectric wake field experiment. 2. Surface roughness wake field feasibility study. 3. Feasibility studies for a vacuum acceleration experiment.

ATF was shut down for the week of October 22 for scheduled maintenance. During the shutdown we did miscellaneous facility maintenance jobs, new computer control system tests and installation of three new experiments. These experiments are STELLA II (AE20), MINOS detector development (AE28) and dielectric wake field experiment (AE19). Thanks to the help of the NSLS mechanical section, all shutdown jobs were successfully completed on time.

Back to Top

SUBJECT:  Explanation of new ATF ASE - dated August 8, 2001

             Ilan has asked me to elaborate a bit on the new ATF Accelerator Safety Envelope (ASE), dated August 8, 2001 and approved by DOE-BAO on September 25, 2001.

            The ASE is the highest level facility authorization document for the ATF.  It has been reviewed and approved by the NSLS ES&H Committee, the BNL ES&H Committee, Tom Sheridan and the DOE Brookhaven Area Office (BAO).  It is very much your “contract” for operation with DOE-BAO.  The ATF will be held strictly to the requirements of its ASE.  It sets operational limits based on the hazards analyzed in the ATF Safety Assessment Document (SAD), your second facility authorization document.  The SAD provides a complete description of the ATF and the hazards associated with operation.  We are expected to keep the SAD up to date.  A web link is given below:

ATF Accelerator Safety Envelope and ATF Safety Assessment Document

            The NSLS, ATF and SDL ASE’s now all have the same format and are approved by DOE-BAO; an effort that has taken about a year to accomplish.  Section 5 in each ASE is unique because each machine has different capabilities, but the facilities all follow the same procedures and limits given in Sections 1-4.

            Section 1 of the ASE sets out the requirements for operating under this ASE, what happens should the ASE limits be exceeded or if procedures are not followed; also how to deal with hazards not already accounted for in the ASE or SAD.

           Section 2 defines hard limits for personal radiation doses.  There should be no reason for any ATF or non-ATF personnel to even come close to these values.

            Section 3 and 4 define engineered safety systems (interlocks, monitors) and administrative controls (procedures, etc.) and are linked, where possible, to guidance documents.

            Finally, Section 5 defines the ATF operating envelope.  These values must be taken seriously.   Administrative or operational controls such as lower, buffer values need to be in place so that these values are not exceeded.  As stated in the text, exceeding a value is not a violation of the ASE itself, but the event itself should be examined and corrective actions implemented to prevent recurrence.

Original e-mail from Ilan Ben-Zvi to ATF staff members

Sent: 10/22/01 6:46 PM

Subject: Accelerator Safety Envelope

Importance: High

All,

Please make sure that you read the ATF Accelerator Safety Envelope and then send me an email confirming that you have read and understood the document. It can be found on the ATF web site under ES&H. The direct link is  

/Safety/ATFASE2.pdf

I want to emphasize that violation of the ASE will be very bad to the ATF's experimental program (which may be shut down for a prolonged time) and our good reputation as a facility that follows closely safety regulations. In addition this will affect the performance appraisal of the person(s) responsible for the violation.

Operators and duty operators should pay close attention to section 4.1,5.2 and 5.3.

Anybody working in the experiment hall should pay close attention to section 4.2.

The fact that I am pointing out these sections must not be understood as if other sections are less important!

Thank you for your cooperation.

Ilan Ben-Zvi

Head, ATF.

Back to Top

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 Stony Brook University, the largest academic user of Laboratory facilities, and Battelle, a nonprofit, applied science and technology organization.

Privacy and Security Notice  | Contact Web Services for help