2001 ATF Newsletters

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

Hello readers,

You may have been wondering for the few recent changes in the heading of the ATF Newsletter. The reason is very simple: I am trying to get around some email filters that reject the email because its header contains certain words, such as 'newsletter'...

The ATF is holding the brightness record for photoinjectors. The bar has been moved considerably higher this month by ATF beam physicists: See Vitaly Yakimenko's report in this ATF Update of a sub-micron emittance measurement with the hefty charge of 0.5 nC. This very high brightness explains the very small beam sizes measured on Beam Line 1 and the saturation of VISA with a gain length of 18 cm measured on Beam Line 3.

See also reports on the MINOS Beam Detectors Experiment (AE28), VISA (AE24) and the Compton Experiment (AE22). In particular, notice that the ATF's Compton experiment, which held the record of picosecond hard X-ray photon per pulse production (by a few orders of magnitude, I might add), increased its achievement by a factor of 10 thanks to the increased power from the ATF's CO2 laser.

Following the recent publication of the experimental results of the STELLA experiment in Physical Review Letters, Physical Review Letters 86, 4041-4043 (30 April 2001),  The STELLA experiment got the attention of Nature. It appeared as a research highlight on home page of the Nature Physics Portal. Since the Portal's Home Page gets updated frequently, I saved the article (with permission of Nature's North American Editor, Dr. Laura Garwin) here: Nature Physics Portal Article for your browsing convenience.

The ATF’s innovative use of Mathcad as on-line software tool for control and analysis has been featured on the Mathcad company's home page as a highlight, and also made "featured User" in their email newsletter. See the saved version of  MathCAD's home page. The Mathcad links point to the front-page article in the April Issue of Scientific Computing and Instrumentation (see our March Newsletter).

Next month we are hosting the 21st ICFA Beam Dynamics Workshop on Laser-Beam Interactions, (June 11-15, 2001 at BNL and Stony Brook). This exciting event, chaired by Professor Tachishige Hirose of Tokyo Metropolitan University and Ilan Ben-Zvi is closely related to many of the research activities conducted at the ATF. The excellent scientific program of the workshop was prepared by an international program committee chaired by Igor Pogorelsky, and a tremendous amount of work is being done by the Local Organizing Committee chaired by Marcus Babzien. 

ATF staff and users should carefully look over the ATF's new Accelerator Safety Envelope included in this report.

Last but not least: The ATF staff has been joined by Tony Rodrigues (replacing our electronic technician Mark Montemagno who is now working in the Instrumentation Division with Triveni Srinivasan-Rao). Welcome to the ATF family, Tony!

Have a great summer!

Ilan Ben-Zvi.

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A few recent experiments suggested indirectly that the emittance of the electron beam at the ATF is of the order of 1 micrometers (or less) for a charge of 0.5 nC.

The recent successful campaign for emittance improvement was started with the realignment of quadrupoles in the beginning of H line. That allowed us to send the beam closer to the linac axis and thus reduced wakes in the linac. Other steps were taken to improve the roundness of the drive laser and its homogeneity through the replacement of some damaged optical components. The final step in this series of improvements (that led to the saturation of VISA) was an increase of the RF gun gradient.

The linked figure shows the electron beam profile in the the H line monitors (and one F line monitor, with the energy spread showing due to dispersion) following the recent improvement in the laser transport.

The first experiment that suggested that we have an improved emittance was the production and measurement of an ultra small beam size on Beam Line 1. The recently installed in-vacuum permanent-magnet quadruples allowed us to squeeze the beta function down to the record small value of 1 cm. With that, the chromatic effects limited beam size was measured as 14 um for 0.5 nC beam. The final beam size of 10 um was estimated after subtracting broadening due to the optical resolution. One could easily calculate that this set of numbers corresponds to 1 um emittance.

The second experiment to indicate an extremely small emittance was VISA. The slice beam emittance had to be 0.7 um to explain the obtained gain length and saturation measured on Beam Line 3. 

The usual method of emittance measurements (quadrupole scan) has natural limitation at approximately 2 um for the typical transport setup at the ATF. This can be simply explained by the limited resolution of the phosphor screen measured in the small beam experiment mentioned above. This limitation was not noticeable until the recent improvement of the ATF emittance.

To overcome this difficulty, we adopted a fitting procedure instead of the usual quadrupole scan to measure ultra small emittances. The beam sizes measured on the four beam profile monitors along H beam line together with beam line layout and real quadrupole settings are used to fit emittances and optical functions.

This technique was developed and first applied at the ATF for beam transport through small aperture during successful Inverse Cerenkov Acceleration experiment. The accuracy of transport was further improved during tomographic phase-space measurement.

With this new technique, and following the above mentioned improvements, a record emittance of 0.8 um at a bunch charge of 0.5 nC was successfully measured for a tune in which the quadrupoles were set to produce beam sizes larger than 400 microns on all four monitors. Please note that this is INTEGRATED EMITTANCE. The slice emittance is expected to be smaller than this, but has not been measured at this time.

The results of the measurement are shown in the linked figure. The horizontal axis shows distance along H line in meters and the vertical axis corresponds to beam size (RMS sigma) in microns. Red x’s represent measured value on the BPMs for the horizontal plane and the solid line corresponds to the fitted beam envelope. Green color is used correspondingly for the vertical plane.

The special tune was necessary since the regular H line quadrupole setting produces a waist at HPOP-UP2 (approx. 6.5 m from beginning).  The fitted emittance in this case is considerably higher (1.4 –1.8 um) due to the limited resolution of the monitors , approximately 50um. 

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Last week was the VISA meeting here at BNL. The talks went well. The meeting emphasized the point that we need to have a reliable number for the current. Presently our measured current out of the linac is about 70A with which simulations give an emittance of <.8mm-mrad. Obviously, it would be nice to have the bunch length measured immediately before the undulator and discussions of how to best do this are in progress.  

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MINOS test ran on May 22-23. We took data using a 0.5 cm gap ceramic pad chamber and a 0.25 cm gap strip chamber. From our run in April we had leared that we needed to pay attention to the position of the beam in the Faraday cup as well as other run conditions, such as gate widths, noise levels, etc. We also learned that we needed a lot of data in the intensity range of 1-30 pc.
In the beginning of the run we learned that the wrong gas mixture was delivered to us by Proxair. This was quickly fixed. We took data with pure He as well as He(98%)H2(2%) mixture. Online analysis of the data shows that the 0.5 cm chamber starts deviating from linear behaviour at about 30 pc. The deviation is at higher intensity for the mixed gas. An interesting effect occurs with pure gas at about 200 pc. There appears to be some multiplication in the gas. This multiplication is not present in the mixed gas. This and other effects will be precisely measured when we complete our analysis.
We are grateful to the help and cooperation from the ATF crew.

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A new ATF booster CO2 laser amplifier has been used for the first time in Thomson scattering experiment. 3J of laser energy have been transmitted through the interaction point. That is 15 times higher then in the previous 1999 run. A higher laser flux resulted in nearly proportional increase of the x-ray yield setting a new world record for relativistic Thomson scattering experiments.
· Other conditions of the experiment:
electron energy 60 MeV
electron bunch charge 0.4 nC
electron bunch length 5 ps
electron focus sigma =37 micrometers,
laser wavelength 10.6 um
laser pulse length ~180 ps FWHM
laser focus ~ sigma =56 um
x-ray cut-off energy is 6.5 keV.

The spot size of the e-beam is calculated from transmission through the 150 microns pinhole. Spot size of the laser is measured by transverse scanning of the e-beam.
Collimated Thomson x-rays were observed on a phosphor screen and Kodak luminescent screen.
The maximum output of a Si detector, which is used for absolute measurement of the x-ray yield, was 2.4V.
Energy deposition on the detector is calculated as follows:
Stored charge is 2350pF x 2.4V = 5.6nC = 3.5x10^10 electrons.
Photon energy required to create an e-hole pair is 3.67eV.
Total energy deposition is 3.5x10^10 x 3.67eV = 1.3x10^11 eV.
Since 95% of X-ray energy is absorbed in a 160 um thick silicon, total delivered by the X-ray signal is estimated 1.4x10^11 eV.
To determine the number of x-ray photons, we need to compare this quantity with a CAIN computer simulation. The linked figure shows the full spectrum of Thomson scattering and its portion (in red) transmitted to the detector through a 250 micron thick Be vacuum window and 22 cm of air. This attenuation leaves 5.9x10^7 x-ray photons out of the total 4.0x10^8 photons produced at the interaction point. This corresponds to energy deposition on the detector 3.3x10^11 eV (simulation). Experimentally obtained energy 1.4x10^11 eV is 42% of the value expected by simulation. Therefore, total number of X-ray photons is 1.7x10^8/pulse, which is about seven times larger than the quantity in the 1999 experiment and a new world record. 

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ATF supported six experiments in the last five months. These are the VISA FEL (AE24), the Linear Collider Cavity BPM (AE16), the Charged Particle Optical Detector (AE23), the MINOS detector development (AE28), the Compton (AE22) and the Structure based laser accelerator (AE27) experiments. In the coming ATF shutdown (June 11 to July 23, 2001), several major facility improvements (new linac water system and klystron charging power supply protection system) and new experiments will be installed. Tests will be performed on our new computer control system during the shutdown.

1. Since January 2001, ATF has provided electron beam with normalized emittance about 1 mm-mrad for charge from 200 pC to 500 pC. We have been continually working on laser transverse beam profile improvement, magnet alignment and other projects to improve the electron beam quality.
2. As temperatures warm up, the ATF linac water system is not adequately stable for high power operation, specially for the VISA experiment. We are now trying several temporary solutions. One is to reduce the linac RF pulse length, and another is to mix cold water with linac water system. We have made some progress in the last week.
3. Linac low level RF system was tuned  up  last to insure that the ATF can produce a 71 MeV electron beam for VISA with relatively low charging power supply voltage operation.
4. The ATF had two Tier I safety tours last month. We addressed almost all identified items with the help of the NSLS safety personnel, which we gratefully recognize.

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Those individuals who have a need to enter the interlocked accelerator areas will notice a change in the radiological posting. Until recently these areas were posted using signs with the following wording:

· CAUTION
· Activation check required
· Items present in {designated area} enclosure during operations must be checked for induced radioactivity before they are released from this area.

This wording has been changed somewhat to read:

· CAUTION
· Radioactive Material Area
· Activation check required
· Items physically attached to the electron beam chambers and electron beam stops during operations must be checked for induced radioactivity before they are released from this area.

A previous version of the Radiological Control Manual (RCM) did not require those areas containing potentially radioactive (from activation) to be posted as Radioactive Material Areas provided they were located in a Controlled Area and the area exit required an activation check. The latest version of the RCM requires these areas to be designated as Radioactive Material Areas, hence the additional wording. Aside from this wording change there are no differences in training or entry exit requirements for these areas.

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Attention all ATF staff and operators. The new Accelerator Safety Envelope (ASE) of the ATF has been approved. Please study the ASE below - it is an extremely important document taken very seriously by the authorities!

ACCELERATOR SAFETY ENVELOPE

1. Introduction

This Accelerator Safety Envelope (ASE) governs the operation of the Accelerator Test Facility (ATF), including the gun, linear accelerator, transport lines, beamlines and beam stops.

Violation of this ASE’s Limits listed in Section 2 requires an immediate halt of accelerator operations and notification of Department of Energy-Brookhaven Area Office (DOE-BAO), Brookhaven National Laboratory (BNL), ATF and National Synchrotron Light Source (NSLS) managements.  Reviews will be undertaken and corrective actions developed, scheduled and tracked until all actions have been completed.   Notification of BNL and DOE management and the approval of the NSLS/ATF managements are required to return to accelerator operation.

Sections 3 and 4 require the existence of a number of programs that ensure that the hazard evaluations of the SAD are maintained intact and controlled.  Violation of a programmatic requirement listed in Sections 3 and 4 requires an immediate halt of the specific program activity and notification of DOE-BAO, BNL ATF and NSLS managements.  Reviews will be undertaken and corrective actions developed, scheduled and tracked until all actions have been completed.   Notification of BNL and DOE management and the approval of the NSLS/ATF managements are required to restart the activity.

No activity or facility modification may compromise the Safety Analysis Document (SAD) or the ASE. Proposed changes are to be screened for hazards that lie outside the bounds of those considered in the SAD and in the development of the ASE, by implementing the Unreviewed Safety Issue Process.  The USI process may result in rewriting portions of the SAD and modifying the ASE. Such revisions require applicable review and approval. Reportable events may also cause the USI process to be initiated.

This document, as well as the companion ATF Safety Assessment Document listed on the cover page, is subject to change control managed by the NSLS Department according to the Internal Controlled Documents Subject Area.

2. Safety Envelope Limits

The operation of the ATF, including the linear accelerator, transport lines, beamlines and beam stops must be carried out in a manner that ensures that the following safety envelope limits are not exceeded:

·Less than 25 mrem in one year to individuals in other BNL Departments or Divisions adjacent to an accelerator facility.

· Less than 1250 mrem in one year to an accelerator facility staff member.

3. Engineered Safety Systems Requiring Calibration, Testing, Maintenance, and Inspection

·        An NSLS Interlock Safety Policy and Requirement Manual (PRM) procedure shall be in place to manage facility interlocks and shall be in compliance with SBMS Standard 1.5.3 Interlock Safety for Protection of Personnel. 

·A program shall be in place to manage radiation monitors that annunciate locally.  These monitors shall be placed in locations to ensure operator and staff awareness of transient radiological conditions associated with ATF operations. Responses to radiation conditions identified by this system shall be established in formal procedures in the ATF Fault Response Procedure. These monitors will be calibrated annually and tracked through the NSLS Controlled Measurement and Test Equipment Database.

4. Administrative Controls

·        Two persons must be present at the facility during accelerator operation, at least one of whom must be a fully qualified operator or duty operator.  The operator shall follow guidelines and procedures as set forth in the ATF Handbook.  The second person does not require operator training but does require NSLS facility specific and GERT training as well as ATF Beam Line Operations and Safety Awareness (BLOSA) training.

·        An NSLS Safety System Work Authorization PRM procedure shall be in place to manage radiation shielding configuration.

·        An NSLS Experiment Safety Review PRM procedure shall be in place to manage experiments and shall be in compliance with SBMS Standard 1.3.5 Planning and Control of Experiments. Any proposed experiment that would require operation outside of the approved ATF SAD and ASE requires additional evaluation and/or revision of the SAD and ASE prior to its operation.

·        An NSLS Work Planning and Control System Procedure shall be in place to manage routine work and shall be in compliance with SBMS Standard 1.3.6 Work Planning and Control for Operations.

·A Radiological Control Division Radiological Posting Requirements program shall be in place for the deployment and management of radiological postings by Radiological Control Division personnel.

·Personnel and area radiation TLD dosimeters shall be deployed and managed by Radiological Control Division personnel.

5. Operating Envelope

The Operating Envelope denotes facility operating parameters or systems with a significant link to safety.  Operation within the conditions identified in the Operating Envelope provides a buffer against exceeding the ASE Limits in Sections 2, 3 and 4 above.  Operation of the facility in excess of a parameter identified in the Operating Envelope shall be curtailed as soon as possible but does not constitute a violation of the ASE, as long as other provisions of the ASE are not exceeded.  Excursions beyond the levels of the Operating Envelope shall be recorded in the ATF Operations Log and will be investigated using the NSLS Nonconformance Reporting system.

The ATF shall not exceed the Maximum Electron Beam Energy of 120 MeV.  This Maximum Electron Beam Energy will be limited by the installed capability of power systems.  No active monitoring of the beam energy will be provided.  Any increase in RF power that may potentially result in exceeding the Maximum Electron Beam Energy of 120 MeV will require review and approval of the ASE prior to such change.

·        The maximum average electron current during ATF operation shall be limited to 600 nA.

·        The radiation dose Administrative Control Level (ACL) for personnel working at the ATF is 100 mrem whole body dose for a calendar year.  Approval to exceed this ACL must be obtained from the NSLS Chairman, the ALARA Committee Chairperson and the ESH Facility Representative from the Radiological Control Division.

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

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