2005 ATF Newsletters

Feb | Dec
 

    This newsletter begins with a brief summary of the recent facility improvements. Progress reports from some of the ATF experiments follow, and then we conclude with detailed reports on the facility developments.

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    A new RF photo gun has been in operation for a little over a year. The original 1.6 cell electron gun was replaced with a widely used modification GUN IV. The first l.6 cell gun was installed at the ATF in 1996 and achieved a record beam brightness with a 0.8 micron normalized emittance for a 0.5 nC beam. This design has been adopted in dozens of facilities. The main purpose of the upgrade was to eliminate multipactoring which was due to the migration of Mg from the cathode to other gun surfaces, and caused the operation to be unstable.
    We were unable to manufacture a good cathode with a Mg plug in time for installation, so an all-copper cathode was installed instead. Asymmetric tuning, with the field reaching 160 MV/m in the half cell and operating at 110 MV/m gradient in the full cell, allows us to improve beam brightness. With the installation of the “simple” copper cathode, the ATF can supply a stable beam to users and achieve record beam brightness
due to the high gradient: 0.9 micron for a 0.8 nC beam.
    The Mg plug in the cathode provides about an order-of-magnitude improvement in quantum efficiency up to 0.3% but typically limits the gradient. Experimental requirements for higher quantum efficiency led to the installation of a Mg cathode last fall. The gun is currently operated at an ~90 MV/m gradient with emittances of the order of 1-1.5 micron for 0.5 nC beams. We are working to improve the manufacturing of Mg plug cathodes and hopefully these studies will continue to achieve a brightness corresponding to 1µm @ 1 nC.

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    The ATF continues to pursue an increase of CO2 laser peak power by reducing pulse duration. The most recent laser upgrade to 0.5 TW, ~15 ps prepared the way for the next breakthrough in a collaborative US-Japan Compton scattering experiment that demonstrated multiple x-ray beams from a relativistic electron. This is illustrated by Fig.1 with a fundamental 6.5 keV x-ray beam at the left and the filtered-out second
harmonic to the right. The full report will be published in PRL.

Figure 1. The x-ray beams corresponding to the linear component in Thomson scattering to the left and a filtered second harmonic to the right. Please read full report by Igor Pogorelsky below.

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   We have finally been able to commit significant resources to the development of a replacement for the aging Nd:YAG laser that has served the electron gun and CO2 system for over a decade. Part of this commitment includes the hiring of a postdoctoral research associate, Daniil Stolyarov, whom we welcome to the ATF. Daniil has already demonstrated his abilities in his contributions to several ATF experiments, and his research on a new drive laser will allow this project to proceed much faster. Although the old system continues to provide outstanding performance with unmatched reliability, the system lacks the gain in bandwidth to support longitudinal pulse shaping, and has essentially reached the limits of stability of flashlamp-pumped technology. In addition, it is clear that failure of some major components that are over 20 years old, and for which spares are unavailable, could lead to extended loss of running time in a worstcase
scenario. Please read full report by Marcus Babzien below.

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    The new Linux-based control system, commissioned during summer 2004, has now been in operation for over a year. The performance and reliability of the new system has met or exceeded our in-house expectations with ATF users also expressing satisfaction on the improvement in its speed and responsiveness. By introducing a new operating system and a modern hardware platform, the system is becoming more accessible for user experiments and promises to be a solid base from which to continue further developments which will leverage ATF's capabilities. Other new improvements to the control system include:
- Digital Vector I/Q Modulators
- Serial Communications (RS-232) for User Devices:
- Logging of key facility parameters
Please read full report by Robert Malone below below.

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The multi-bunch plasma wakefield acceleration PWFA experiment relies on the resonance between micro-bunch spacing and plasma wavelength, and thus requires precise monitoring of both beam bunching and plasma density. After drifting from the IFEL pre-buncher, beam bunching is monitored near the plasma capillary using coherent transition radiation (CTR). Two thin (1 µm) titanium foils have been installed: the first one, normal to the beam axis, block the CO2 radiation coming from the pre-buncher, while the second one, at 45°, sends the infrared CTR radiation towards the InSb cold detector, through a ZnSe window. The CO2 radiation level is further reduced by an MgF2 window. The CTR is detected in the 1-8 µm wavelength range. Preliminary results show a clear CTR signal that correlates with the bunch energy spectrum broadening expected from IFEL interaction. Note that thanks to thin foils these two diagnostics can now be run simultaneously. Plasma density is monitored by recoding the width of the H_a (=656 nm) and H_ß (=486 nm) lines. The light emitted by the plasma is collected and transmitted to a spectrograph with a gated intensified camera. Typical line widths are in the 5-12 ˜nm. The evolution of the lines width can therefore be recorded during the plasma discharge, at the 50 ns time scale. These two crucial diagnostics are included in the beam line 1 set up (see Figure), and will be used on line during the next acceleration experiments. The plan for the next scheduled run is to finalize the CTR diagnostic and to make a first attempt at the plasma
acceleration measurement.

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   The recent runs of the ATF/UCLA chicane compressor have produced a number of striking results. Runs in September and November showed compression, through coherent transition radiation (CTR) autocorrelation to below 30 microns (100 fsec) rms bunch length, producing peak currents of over 1.5 kA. Breakup phenomena in the phase spaces of the compressed beam have been also investigated, using momentum spectra and transverse phase space tomography. A major goal of the compression studies is the observation of THz coherent edge radiation (CER) from the beam in the final magnets of the chicane. A matrix of CER measurements has been made, exploring the dependence of CER on: polarization (edge radiation is radially polarized), spectral filtering, and RF phase. Follow on running is scheduled for late January 2006, in which we intend to explore the far-field angular spectrum, and wavelength spectrum in more detail.

Figure 2. First autocorrelation measured for the compressed beam at ATF.

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   Steady progress has been made to prepare for laser wakefield acceleration (LWFA) experiments as part of the STELLA-LW program. Two experiments are being prepared in parallel: seeded self-modulated LWFA (SM-LWFA) and pseudo-resonant LWFA. In the former, a ~100 fs e-beam pulse precedes the laser pulse in the plasma to act as a seed to generate wakefields, which are amplified by the laser pulse. This ~100 fs e-beam pulse is generated by using the UCLA chicane compressor. A second witness e-beam pulse immediately follows to probe the wakefields. In pseudo-resonant LWFA, a ~2.5 TW laser pulse excites wakefields in the plasma, which are probed by a single e-beam pulse. Currently, the ATF TW CO2 laser system has sufficient peak power for the seeded SMLWFA experiment. It is being upgraded to provide the higher peak power needed to drive the pseudo-resonant LWFA experiment.
    Both experiments will be using a new hydrogen-gas-filled capillary as the plasma source, which was designed and constructed at STI Optronics (STI). It has been installed at the ATF and is able to use the same high-voltage pulse power system that drives the polypropylene vacuum capillary. Off-line tests have demonstrated the ability to generate stable discharges with the gas-filled capillary at densities down to 10¹⁶ cm^-3. To protect the ATF linac from the hydrogen gas expelled by the capillary into the beamline vacuum, a 1-micron-thick titanium pellicle foil will be inserted into the beamline pipe upstream of the capillary. A special beam position monitor (BPM) is being fabricated at STI that will ensure the foil makes a vacuum seal when it is inserted. The foil will also be oriented at 45-degrees to the e-beam trajectory so that when the ebeam
passes through the foil it will generate coherent transition radiation (CTR) that can be measured.
    For ~100 fs e-beam pulse lengths, the CTR is at THz frequencies. A series of THz cut-on filters are being calibrated, which will permit determining the onset frequency of the CTR. This onset frequency is directly related to the e-beam pulse length. For the seeded SM-LWFA experiment, dual e-beam generation has been demonstrated with the first (seed) e-beam pulse compressed by the chicane while the second (witness) e-beam pulse follows approximately 10 ps afterwards with no compression. The next step is to test focusing the dual e-beam pulses into the gas-filled capillary. It is critical that the e-beam alignment through the e-beam optics upstream of the capillary is dead center because the dual e-beam pulses have a natural energy difference of ~1 MeV. If the e-beam is off-center, then the optics will deflect and focus the two pulses differently. The pseudo-resonant LWFA experiment only needs a single e-beam pulse; hence, it is ready to begin once the ATF TW CO2 laser upgrade is completed. Our apparatus is flexible such that we can easily switch between the two LWFA experiments, which we are prepared to do depending on which experiment is most ready at the time.

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    PASER is one of the experiments [1-3] to be conducted at the BNL-ATF utilizing the CO2 microbunching
technique. A 70MeV electron beam bunched in a wiggler by a CO2 laser is injected into a discharge cell that contains an excited mixture of gases typically used in CO2 lasers. Diamond windows of about 2 micron thickness separate a pressure vessel from a high-vacuum beam-line.
    PASER stands for Particle Acceleration by Stimulated Emission of Radiation. In order to understand the physical essence behind the PASER acceleration scheme one should first understand the inverse effect. When a particle moves within a lossy medium or in the vicinity of a metallic structure it induces eddy currents. The heat generated by these currents comes at the expense of the kinetic energy of the particle and therefore the latter is decelerated. This deceleration is actually a direct result of the interaction between the moving particle and the secondary field generated by the lossy medium as a reaction to the primary field attached to the moving particle in vacuum. It was shown theoretically, that if the lossy medium is replaced by an active medium (a gas with the population inverted) the secondary field generated by this medium reverses its phase and the particle is accelerated. In other words, energy stored in the medium is transferred to the moving particle via such an interaction. Theoretically, there are three possible schemes for draining the energy stored in the active medium to the moving particle. First, the wakefield generated by a driving bunch moving in an active medium may be amplified by the medium until it reaches saturation and then used for acceleration of another bunch that trails many wavelengths behind [1]. Second, energy stored in a resonant medium may be used to amplify the Cerenkov radiation generated by a small driving bunch and in turn this amplified radiation may accelerate a trailing bunch [2]. Third, contrary to the lossy medium case where the phenomena is typically broadband, when the resistance is negative i.e. active medium the phenomena is narrow band and therefore, electron-beam modulated at the frequency of the resonance of the medium has maximum interaction hence, acceleration is achieved [3]. The latter scheme is the one that will be tested in the framework of the PASER experiment to be conducted at the ATF. According to rough estimates an increase of the order of 10% may be anticipated in the kinetic energy of the injected electron-beam assuming that the medium reaches saturation.
[1] L. Schächter, PRL 83, 1, P. 92 (1999).
[2] L. Schächter, PRE 62,1, P. 1252 (2000).
[3] L. Schächter, “Train of Micro-Bunches in an Active Medium” in 11th Advanced
Accelerator Concepts Workshop-2004, edited by V. Yakimenko, AIP Conference
Proceedings 737, American Institute of Physics, Melville, NY, 2004, pp. 715-721.

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   Over the past few years, an effort has been made to identify the laser technologies that are best matched to the role of a photoinjector system in general, and more specifically to fulfill the requirements of the ATF drive laser. The goal for the new system is to provide enhanced performance while act as a functional replacement for the existing Nd:YAG laser. Therefore the system is designed to provide shaped UV pulses for the electron gun, sub-picosecond IR pulses for CO2 laser slicing, and IR seed pulses for any future strong field experiments requiring 1 micron wavelength, all synchronized to one another. The operating parameters we are seeking to achieve should allow significant improvement in linac operation, and are summarized below:

These ambitious goals will require significant innovation to achieve simultaneously in a single system, and we expect much of the R&D effort to focus on the operational issues associated with maintaining all parameters at the desired levels for extended periods. Indeed, the system components themselves, and the architecture of the entire system is easily capable of meeting each of the individual requirements above, as has already been demonstrated in most cases. Although we cannot go into detail in this brief newsletter, the system is designed from the start as a photoinjector, instead of an adaptation of existing commercial systems. It is a classical master oscillator/power amplifier configuration, utilizing mixed ytterbium-doped gain media, a solid-state oscillator and power amplifier, and high gain fiber preamplifiers. The oscillator has previously been tested, and we are beginning characterization of the first stage fiber preamplifier. We foresee a very exciting period ahead, and look forward to reporting progress in this R& D effort.

   Our next goal is to attain and surpass a 1 TW milestone by operating the ATF CO2 laser at a 5-3 ps pulse duration. Note, that the bandwidth limit of the high-pressure CO2 amplifiers available at the ATF allows for 3 ps amplification and even shorter pulses. A problem was to produce a short seed pulse using the 14 ps YAG, presently operated at the ATF, which controls optical switching devices to produce a seed picosecond CO2 pulse. This will be resolved by upgrading the CO2 laser front end. A modified laser system shown in Fig. 2 is undergoing tests.

The Nd:YAG laser generates a 10-µm picosecond pulse by turning the CO2 beam polarization in a Kerr cell (filled with CS2). In order to improve the contrast and intensity of the seed pulse directed into a regenerative amplifier, we use two intermediate steps. First, we cut a 10 ns pulse from the 200 ns oscillator output with a Pockels cell and amplify this signal in a CO2 preamplifier. Next we use a split portion of the YAG on a semiconductor optical switch to select a 200 ps pulse. Only after this, do we cut a few picosecond pulse in a Kerr switch. This way we achieve a high contrast CO2 pulse that is sent for amplification. A high-pressure regenerative amplifier traps the seed pulse for the controlled number of double passes inside an optical cavity and releases it when the power reaches several GW. Finally, another large-aperture high-pressure amplifier boosts the energy to 10 J and the peak power to the terawatt level. To operate a Kerr switch faster than allowed by the 14-ps YAG pulse, we use a known technique of shortening its second harmonic (SH) several times below the fundamental pulse duration that is depicted by Fig. 3.

According to this method, we split the YAG pulse in two, rotate polarization for one component and recombine two pulses in a second-harmonic KD*P crystal with a certain delay between them. Generation of a 0.53 m SH of the YAG is achieved within a short overlap between the two pulses. This way, we convert a 15 ps IR pulse into a 5-3 ps green pulse of a higher peak power as demonstrated with a multi-shot autocorrelator (see Fig. 4). Fig. 5 demonstrates that, simultaneously with significant SH pulse shortening, we achieve a noticeable increase in peak power for the optimum delay between two interacting pulses of fundamental frequency. High efficiency of 10-m switching in the Kerr cell is now possible. We wish to thank Wayne Kimura (STI Optronics) for the loan of a YAG amplifier that enabled an efficient second harmonic conversion.

After producing a 3-ps CO2 pulse in the Kerr cell, the remainder of the ATF amplifier system operates in the same manner as previously. However, due to a shorter seed pulse we expect a significant gain in the output peak power. This is confirmed by simulations illustrated by Fig. 6. We thank Prof. Victor T. Platonenko from Moscow University for providing computations.

Case (a) in Fig. 6 was confirmed in the Compton experiment. We will schedule demonstrations of the short-pulse regime (b) in upcoming LWFA and Laser Vacuum Electron Acceleration experiments in January 2006.

Digital Vector I/Q Modulators:
   Two new digital vector I/Q modulator systems have been introduced for control of the RF at the photoelectron gun and linac. The new generation of technology used in these subsystems has resulted in improved stability of the ATF beam and reduction of the coupling between phase and amplitude. The control system keeps the actions transparent to the operator by still allowing entry of phase and amplitude on the control displays, but performs the necessary forward and inverse mappings to/from I and Q.

Serial Communications (RS-232) for User Devices:
   Since its inception, some 15 years ago, the ATF computer control system has always supported communications with devices via RS-232 communications links. However, programming the communications protocol for such devices was always done in-house with the detailed knowledge of the device embedded within ATF applications software. While workable and reliable, it required a fair amount of pre-planning and programming to introduce a new RS-232 device. Recently, a new method for adding new RS-232 devices was introduced: Under this scheme, an ATF server program operates continuously using channels within the database which are provided for users to control the RS-232 interfaces directly. This includes sending and receiving messages and monitoring and controlling the communications link. Communication with a new device requires only reading/writing to the appropriate database channels while the server code handles the details of I/O buffering, string packing, etc. This method already has been used successfully for control of the interferometer used in the beam compression experiment (see report by J. Rosenzweig). Support for other devices will be added as the needs of the experimental program arise.

Logging of key facility parameters:
    Due to the increase in CPU power of the new control system, continuous logging of ATF facility data is now reasonable. Vacuum system history is available for a week's worth of data (logged every second) with similar histories available for the gun attenuator, YAG laser energy and other laser system statistics (logged on each beam pulse.)

Last Modified: December 3, 2007
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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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