The Advanced Accelerator Concepts workshop is the only acknowledged and fully sponsored forum that provides a platform for inter- and cross-disciplinary discussions on various aspects of advanced accelerator and beam physics/technology concepts covering a wide range of applications.

The workshop will comprise plenary sessions and working groups. Details of the working groups work plan can already be found on the workshop’s web site. The Invited Talk program will be published there soon.

Our web site is at
http://www.bnl.gov/atf/AAC04.htm

Participation in the Advanced Accelerator Concepts Workshop series is by invitation only. If you are interested in participation in this meeting, please use the form on the web site to request an invitation. Filling out the form does not commit you to participation.

Students working in advanced accelerator R&D are encouraged to participate; there will be significant financial aid to help students to participate.

I hope you will take part in this exciting bi-annual event.

The main past event was the 12th ATF Users’ Meeting and ATF Program Advisory Committee meeting, which took place on January 8-9, 2004. The meeting was a great success in a number of ways. Participation was extremely strong, drawing 57 participants from all over the country. The facility presentations have shown the great progress that the ATF made towards better serving its users by improved performance, better support and improved amenities. The reports from the users were rich in various subjects related to advanced accelerator physics and generation of coherent radiation. Finally, an indication that the ATF science will continue to be vibrant and innovative also in the future was made by the six new proposals that were presented to the ATF Program Advisory Committee and the audience.
The meeting web site,
http://www.bnl.gov/atf/Meetings/ATF2004/Default.htm
provides the agenda, presentation slides and posters from the ATF tour.

In this newsletter you can find beautiful results in hardware, measurements and new ideas from the VISA experiment presented by Gerard Andonian and Alex Murokh. A report from Wayne Kimura announces the successful termination of the STELLA II with the demonstration of the first laser accelerator producing mono-energetic acceleration. He also shares the ideas towards the next step in this successful series, the STELLA-LW experiment, which was proposed to the committee in the users meeting. Also you can read Igor Pogorelsky’s report on the progress of the ATF picosecond terawatt laser, which reached power in the terawatt class and some new trends in advanced diagnostic and microbunch manipulations by Takahiro Watanabe. As always, we conclude with Karl Kusche’s report on ESH&Q.
Besides the material in the meeting’s presentations and the contents of this newsletter, I wanted to highlight two items that were presented at the meeting by Vitaly Yakimenko.

First item is Operations, Performance and Upgrades. The presentation emphasizes that:

· New experiments demand more from ATF beam and diagnostic.
· Ease of achieving top beam parameters were considerably improved with high energy line rebuild.
· New laser based survey proved to be very successful for an accurate and very efficient quadruples and BPMs alignment.
· Chicane magnets for electron bunch compression were installed.
· Newly installed sextuples helps to deliver beam with large energy spread/chirp to the experimental hall.
· MathCAD based automated beam measuring/tuning procedures are used in every day operations.
· New ESH person at ATF helps user to conduct experiments safely and avoid unfortunate canceled runs due to unresolved ESH issues.

Many new power supplies, improved BPM, standardized experimental chamber, fail safe vacuum valve arrangement were discussed among other improvements to improve efficiency of experimental runs at ATF.

The other item is a report on an experiment, the Plasma Experiment, with results that were recently published in PRL. We reported the generation of plasma wakefields by a relativistic electron bunch and study relative phasing between the longitudinal and transverse fields in the wake. The leading edge of the electron bunch excites a high-amplitude plasma wake inside the overdense plasma column and the acceleration and focusing wakefields are probed by the bunch tail. By monitoring the dependence of the acceleration and focusing forces upon the plasma density we approached the beam matching condition and achieved an energy gain of 0.6 MeV over the 17 mm plasma length, corresponding to an average acceleration gradient of 35 MeV/m. Wake-induced energy and angle modulation of the electron bunch are mapped within a wide range of plasma density. We confirm the theoretical predictions for the phase offset between focusing and accelerating components of the wake.

I can think of no better way to close my comments than by quoting some the words of the advisory committee:
“Our overall view is that the ATF is one of the premier facilities for doing beam physics in the United States. It is run by a group of dedicated scientists and engineers who are excited about the science that is being done there.”

Ilan Ben-Zvi.

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VISA-COMPRESSOR Experiment Progress Report, AE24 (REPORTED BY GERARD ANDONIAN AND ALEX MUROKH)

 

Hardware

Here is a short summary of the installations and improvements that have occurred at F-line and ATF tunnel since summer:

1) Chicane centerpiece diagnostic was installed and tested. Beam image was obtained in the middle of chicane. Due to phosphor layer poorly attached to the screen, the beam profile information is not very informative. Yet the position readout is rather accurate, allowing the chicane commissioning in the nearest future.
2) Installation of additional Beam Profile Monitors along the F-line. Two (2) additional BPMs were added to perform the e-beam dispersion studies.
3) Some old trims were replaced by the new magnets to improve beam control in the dispersive section. The new trim magnets are also more compact, freeing up valuable space along the beamline.
4) Installation of sextupole magnets. The cornerstone of the VISA II experiment is operating under running conditions where non-linear dispersive effects are mitigated to preserve initial e-beam chirp through the transport and at undulator injection. The installation of two (2) sextupoles along the F-line will achieve these goals. A third sextupole will be installed during the next ATF shutdown.

Figure 1: Sextupole installed along F-Line
5) Improved undulator diagnostics. Some additional diagnostics were added to the VISA undulator, including a diagnostic for far-field distribution, and a diagnostic for the differential angular spectrum of the FEL radiation.

Experimental Results of VISA I-B (June 2003)

A first experimental step was to run under VISA I conditions except with a higher chirp and to test out newly installed diagnostics. Once stable lasing at high gain was achieved, various aspects of the FEL output were measured. The resulting spectrum displayed uncharacteristic broadness: ~11% total bandwidth (see figure). The beam admittance through the High Energy Slits is roughly 2% for VISA running conditions.

Figure 2: Measured spectrum of FEL radiation
Start-to-End simulations to study this regime show some interesting results. The simulated spectrum reproduces the unique features observed during the run. The large bandwidth is explained by the angles involved in the lasing process.

Figure 3: GENESIS Simulation of FEL spectrum under VISA I-B conditions
Analysis of the phase space shows that there is strong compression along the F-line with a very high peak current (~300 A) and an emittance growth due to the nonlinear dispersion mismatch.

Figure 4: ELEGANT Simulation of phase space at undulator injection

Experimental Results (December 2003)

The goals of subsequent VISA runs have been to reproduce the VISA I-B conditions and further analyze the properties of that FEL output. In December, we achieved stable lasing and examined the differential angular spectrum as well as the far-field radiation.
The far field radiation exhibits an atypical spiral structure.

Figure 5: Far-Field FEL radiation

The differential angular spectrum measurements allow us to unfold the correlations between angle and frequency. The usual Doppler pattern is observed but an anomalous pattern is observed with higher bandwidth. The rich spectral structures are currently being analyzed for publication.

Figure 6: Experimental Setup of Differential Angular Spectrum Mesurement.

Figure 7: Differential angular spectrum of FEL radiation (x-axis is angle and y-axis is SASE spectrum)

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Progress Report, STELLA Experiment -AE20 (REPORTED BY WAYNE KIMURA)

STELLA and STELLA-LW Update

The STELLA experiment formally ended on Dec. 31, 2003 and a presentation summarizing the experimental results was given at the ATF Users' Meeting on Jan. 8, 2004. The results will be published in Physical Review Letters later this year. To summarize, during the STELLA program we successfully demonstrated the ability to stage two laser-driven accelerators, create and efficiently trap ~3-fsec microbunches in the laser field ponderomotive potential well, and accelerate these microbunches by over 20% while maintaining a narrow energy spread. Agreement between the data and model is very good with the model being particularly useful in helping to interpret the experimental results.

This experiment validated the basic STELLA approach of using the laser acceleration mechanism to initially modulate the e-beam energy thereby creating microbunches whose lengths are a small fraction of the accelerating wavelength. These microbunches can then be rephased with an acceleration wave in a subsequent laser acceleration device. Under the proper conditions it is possible to trap a large portion of the electrons in the e-beam and monoenergetically accelerate them.

The next experiment to be performed at the ATF builds upon the STELLA success by applying the STELLA basic approach to laser wakefield acceleration (LWFA). This new experiment is called STELLA-LW, where "LW" stands for "Laser Wakefield." In LWFA an intense laser pulse creates a wakefield in a plasma. Electrons can then be accelerated by this wakefield. Very high acceleration gradients have been demonstrated in LWFA experiments around the world; however, typically only a relatively small amount of electrons have been accelerated and their energy spreads tend to be large. By applying the STELLA approach to LWFA we hope to eventually accelerate many electrons with narrow energy spread.

STELLA-LW will also be the first to use a CO2 laser beam to drive the wakefield process. This is made possible by the recent upgrade of the ATF CO2 laser to terawatt levels. We will also be investigating a new pseudo-resonant operating regime for LWFA [see N. E. Andreev, et al., Phys. Rev. ST Accel. Beams 6, 041301 (2003)]. In pseudo-resonant LWFA, pulse steepening of the laser pulse occurs within the plasma, which makes the laser pulse effectively shorter thereby allowing it to create a good wakefield. (Normally the laser pulse length needs to be of order the plasma wavelength divided by 2c, c is the speed of light. Pulse steepening in pseudo-resonant LWFA essentially creates the Fourier components that exist in a shorter laser pulse, which are needed to resonantly drive the wakefield formation.)

Over the next three years we will be concentrating on Phase I of the STELLA-LW program. During Phase I we will be constructing a capillary discharge similar to the one already used in the recent ATF Laser Channeling and Plasma Wakefield Acceleration (PWFA) experiments. This capillary discharge will provide the plasma medium. The ATF CO2 laser beam will be focused into the end of the capillary tube to create the wakefields, and then the electrons will be sent through the capillary tube for acceleration. We are also considering an alternative hydrogen-gas-filled capillary discharge developed by S. M. Hooker, et al., at the University of Oxford.

Constructing the needed diagnostics will also be a key component of the Phase I effort. We plan to use coherent Thomson scattering (CTS) as our primary means for detecting the formation of the wakefield. Although CTS is a well-established technique, STELLA-LW represents the first time CTS will be used to probe a capillary discharge.

At the end of the Phase I effort our goal is to modulate the energy of the ATF e-beam via LWFA driven by the ATF CO2 laser beam. Assuming a 5-J, 2-ps laser pulse delivered to a 1.2-cm long discharge, we expect to observed ~7-8 MeV acceleration.

After completion of Phase I, STELLA-LW will be ready to pursue the next phases of the program where we would investigate creating microbunches from an LWFA device and demonstrating greater energy gains by scaling to longer capillary discharge lengths.

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Review of theATF CO2 Laser Status and Recent Progress (REPORTED BY IGOR POGORELSKY)

Our CO2 laser, being synchronized to the ATF linac, enables laser/e-beam interaction experiments. A number of such user’s experiments have been successfully completed over the past years. These include:
•Inverse Cherenkov Accelerator
•IFEL Accelerator
•Staged Electron Laser Accelerator (STELLA I and II)
•High Gain Harmonic Generation (FEL)

Currently active experiments:
•Compton Scattering
•Laser Driven Cyclotron Autoresonance Accelerator (LACARA)
•Structure-based Laser Driven Acceleration in Vacuum
•Plasma channeling
•Microbunching of electron beams

And there are more proposals are presently under consideration by the ATF Steering Committee

Until recently, the ATF CO2 laser operated at the 180 ps 50 GW level. Relatively long pulse duration was due to a narrow bandwidth of a preamplifier. These parameters are
not sufficient. In order to meet requirements for advanced applications including high-gradient particle acceleration and nonlinear Compton scattering, a number of practical steps has been recently made at the ATF in order to attain ~1 TW laser power primarily by shortening pulse duration. They include:
Generation of ~3 ps second harmonic (SH) of YAG laser in a KD*P crystal
Gating ~3 ps CO2 pulse with a fast semiconductor or Kerr switch controlled by YAG SH
Acquiring 10-atm preamplifier
Presently, all these hardware upgrades are completed and integrated into the system that operates at 30 ps 0.5 TW (to be confirmed by additional measurements). A principle optical diagram of the ATF CO2 laser is shown on Fig.1. Fig.2 shows high pressure laser amplifiers.

Among other recent upgrades and developments that facilitate laser operation and user’s experiments are:
•Final CO2 amplifier is upgraded with adding extra 2 passes (6 total) and external mirror adjustment
•On-line diagnostics including acousto-optic energy meters and single-shot autocorrelator.
•6-foot motorized optical delay stage for laser/e-beam synchronization
•New universal laser/e-beam interaction cell to be installed in beamline #2.
•Capillary discharge as a plasma source and CO2 laser channeling

Demonstrated and prospective (in parenthesis) CO2 laser performance is summarized in the table:
preamplifier final amplifier
Pulse length 30-180 ps (3ps) 30-180 ps (3ps)
Energy 10 mJ 15 J
Repetition rate 1 Hz 1/20 Hz limited by power supply
Peak power 1 GW 0.5 TW (>1 TW)
Focal spot (s) 32?mm (~10 mm)
Laser strength (a) 0.1 (>2)
Beam size (on transport mirrors) 50 mm

We will proceed with a gradual improvements leading to a shorter pulse and higher peak power by optimizing the YAG and CO2 setup.

Figure 1, Schematic layout of the laser

Figure 2, Photo of the preamplifier and final amplifier

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Development of New Capabilities (TAKAHIRO WATANABE)

BEAM ANGLE MONITOR

A beam angle monitor that is supposed to be utilized for linac daily operation and experiments will be studied. The monitor is based on a wave front, or far-field, measurement of Cherenkov radiation (Figure 1, Beam angle monitor). Since the wave front of the radiation is plane in one-dimensional geometrical optics, it can be focused onto the camera by the far-field focusing optics. The expected resolution was estimated prior to the experiments (Figure 2, Estimation of resolution). It was then found that the resolution of the scheme is mainly decided by a beam scattering inside the radiator and the diffraction of the radiation. ‘Proof-of-principle’ experiments will be preliminary performed in early 2004, followed by a couple of detailed experiments.

MICROBUNCH DIAGNOSTICS AND CONTROL

A pick-up of a single microbunch from microbunch train is now being planned. In order to aid the experiment, developments of diagnostics for microbunches via coherent transition/diffraction radiation (CTR/CDR) techniques were considered. In order to discuss the feasibility of the spectrometer and the interferometer for CTR/CDR, some GUI code was developed (Figure 1, Interferogram and spectrum of CTR). Additionally a simple tracking code for microbunching was developed in order to see the space-charge effect as a function transverse beam envelope (Figure 2, Tracking code for microbunching). We expect from the two codes that both the spectrometer and the interferometer will be useful for the characteristic of microbunches, including picked-up microbunch. The experimental setup will be designed by utilizing the codes and then experiment will done.

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ES&H (REPORTED BY KARL KUSCHE)

Many Environmental, Health, Safety and Quality Assurance (ESH&Q) issues have been dealt with since the last ATF Newsletter:

1) The streamlining of the Experimental Safety Review (ESR) process for certain ATF experiments is underway. Specifically, we hope to develop ways to allow quick approval of basic non-hazardous experiments and the use of lasers in the experimental hall (avoid extensive redundant paperwork);

2) New procedures to rapidly assess & enforce compliance with BNL training and documentation requirements are in place. The ATF ESH officer now checks all training records and each ESR for signatures and completeness prior to the issuing of each ATF running schedule;

3) Revision of ATF Awareness training & migration to convenient web-based recertification is in its final form. All ATF staff and users will be notified when this course is posted on-line, which may be as early as next week. Annual recertification will be required;

4) Long-term remediation of lead brick contamination in the machine shop is nearing completion;

5) The facility-wide program for replacement of bare lead with high-impact encapsulated bricks continues, with the gun clamshell and experimental hall next on the list;

6) Local air conditioning units & computerized monitoring to improve temperature & humidity control in the mezzanine & experimental areas have been installed;

7) ATF successfully navigated safety audits by OSHA, NRC, DOE and the BNL Physics Department;

8) Long-term revision of ATF ESH documentation continues, with the Safety Approval Document (SAD) due for submission to and review by the BNL ESH committee. A formal approval process is expected to start in February;

9) The "Ad-hoc Safety Advisory Committee" recently interviewed all ATF staff, and continues its in-depth assessment of ATF operations & documentation;

10) ATF has been awarded Pollution Prevention (P2) funds to retrofill the klystrons with non-PCB oil;

11) Completion of the new ATF User's Setup Area is less than one month away. The asbestos-based floor tiles have been removed as per OSHA recommendation and new tiles have been installed. The room will get new window blinds and ceiling tiles, along with a fresh coat of paint. We will install a networked PC & printer, nitrogen gas supply, a large 4-tier shelf for storage of delicate experimental apparatus, work bench & table surfaces (and maybe a microwave & refrigerator).

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