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AAC'04 Agenda
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High Energy Density Physics and Exotic Acceleration Schemes Working Group
kunioki mima: PIC simulation and experimental researches on high energy ion generation
  High energy ion generation with ultra-intense lasers has been interested because of possibilities of applications to high energy density plasma production, nuclear science opened up by short lifetime particle acceleration, plasma diagnostics, medical applications like positron emission tomography and cancer therapy, and so on. Multi-10MeV ion generation with ultra-intense laser has been explored at the Institute of Laser Engineering of Osaka University by using 50 TW and 500TW /0.5ps lasers and with large scale PIC simulations. One of the specific issues investigated by PIC simulations is a multi stage acceleration of ion by using multi foil target. It was found that relatively monochromatic high energy ions can be generated by synchronizing ion beams with successively generating sheath fields on rear surfaces of multi foils. In the simulation, we found that protons are accelerated to 16MeV with 1019 W/cm2 laser intensity, where the 1% of incident laser energy is converted to the ion energy within 5% band width. The ion energy produced by this scheme can be higher than 100MeV with 1020 W/cm2 laser intensity. As for experiments, high current and high energy ions beams generated by peta watt laser at Osaka University with hemisphere target are focused on a solid target to heat dense plasma instantaneously. This may contribute to high energy density physics and the fast ignition core plasma heating. The other geometry like cone target for ion acceleration has been explored by experiments and simulations. In this talk, we present the status of the above researches at ILE, Osaka University on high energy ion acceleration and related plasma phenomena.
Levi Schachter: Active Media Accelerators
  In either circular or linear accelerators, electrons are accelerated by electromagnetic energy stored in macroscopic cavities or coupled cavities (disk-loaded structure). In the case of active medium acceleration electrons may be accelerated by electromagnetic energy (photons) stored in microscopic cavities. At the level of a single particle, the effect can be conceived as the inverse Frank-Hertz effect in the sense that an electron moving in the vicinity of an excited atom may gain energy from the latter while the bounded electron jumps to a lower energy state. Active medium may accelerate a pulse of electrons pre-bunched at the resonance of the medium or may actively enhance the quality factor of a macroscopic cavity that in turn accelerates electrons. It is the purpose of this talk to present a variety of phenomena occurring when electrons move inside or in the vicinity of a resonant medium.
Benjamin Bowes: Ultrafast 2-D radiative transport in a micron-scale aluminum plasma excited at relativistic intensity
  Intense, high contrast femtosecond (fs) laser pulses deposit energy into the electrons of a solid faster than it escapes from the initially-excited volume and much faster than the target surface expands hydrodynamically. When initial electron temperature kT_e exceeds several hundred eV radiative heat transport begins to dominate over collisional transport. The physics of this regime underlies energy transport in stars, as well as ultrashort pulse x-ray generation. Past experiments in this regime used loosely focused, ~1J, ~1ps pump pulses and probed the target transversely in transmission, and thus were restricted to observing late stages of 1D radiative transport in an optically transparent material on a time scale of tens of picoseconds. We present new measurements using 1 mJ, 24 fs pump pulses focused to a diffraction-limited lambda^2-size spot (1.5µm diameter) to excite a metal target surface at relativistic intensity (up to 1.8*10^18W/cm^2). We probe the target in reflection through microscope optics. This geometry enables us to observe the earliest stages of radiative transport in 2D on any target material on a sub-picosecond time scale. New features of radiative transport are expected on this space-time scale. For example, simple transport models in the diffusive limit (i.e. Rosseland radiation mean free path lambda_Ross << heated spot size lambda) predict very different temporal evolution of the thermal/ionization front in 2D vs. 1D. Moreover, since our focal spot size is on the order of lambda_Ross, our experiment may open up fs studies of nonlocal radiative transport. As such, we experimentally observe a thermal/ionization front expand radially at ~10^8cm/s from a lambda^2-size spot of an aluminum target excited at >10^18W/cm^2.
Kazuhisa Nakajima: Laser Ponderomotive e+e- Collider and Proton Acceleration up to the range of Ultra-High Energy Cosmic Rays
  Relativistic ultrahigh laser fields can produce plasmas through quantum mechanical tunneling ionization mechanism, and accelerate produced electrons and ions to generate a relativistic electron beam and energetic ions in plasmas. This process will be followed by creation of electron-positron pairs through interaction of relativistic electrons with a Coulomb field of a nucleus in plasma ions or a strong laser field.

In a relativistic strong laser field, the longitudinal accelerating force exerted on an electron is proportional to the square of the electric field, whereas the transverse quivering force is just linearly proportional to it. This is essence of the relativistic ponderomotive acceleration that dominantly produces energetic particles in interaction of ultraintese laser fields with particle beams and plasma. Therefore a tightly focused laser field can accelerate an electron bunch longitudinally up to a remarkable energy and at the same time confines it transversely in the superposed ponderomotive potential of an intense ultrashort laser pulse. Here we propose acceleration and focusing of the pair beam by the ponderomotive acceleration scheme to compose a high energy electron-positron collider with very high luminosity.

In a plasma where a group velocity of laser pulse is less than the vacuum speed of light, ponderomotive potential scatters particles on the front of the laser pulse above a threshold intensity. In this regime of ponderomotive acceleration the laser pulse energy is effectively transferred to particle energy in a point-like interaction. It is feasible to conjecture that the over-threshold ponderomotive acceleration can accelerate protons in a cosmic relativistic plasma up to extreme high energies of the order of 10^20eV, known from observations of ultra-high energy cosmic rays.

Paul Bolton: Ion Production via Optical Field Ionization of Atoms and Ions: Results from Early Work
  Optical field ionization (OFI) of atoms and molecules is ultrafast and distinguished with high laser intensities of ultrashort duration. For an initially neutral atom or molecule, field driven ionization rapidly progresses sequentially through successively higher charge states. Consequently, the time-dependent field of a single high intensity laser pulse can multiply ionize a multielectron atom during its risetime such that the peak intensity predictably ‘sees’ ion targets with the highest possible charge state (as determined by the peak intensity level). Within a short time interval (during which ions are relatively immobile) the spatial distribution of charge states is then determined by the laser field and target distributions. Closed form expressions for threshold and saturation laser intensities (in terms of relevant ionization potentials) bracket a useful range for ion production. They can also facilitate the design of ion sources and laser diagnostics. Results from the early OFI studies conducted at the Lawrence Livermore National Laboratory will be presented.
Eric Esarey: Beam Manipulation with Laser-Plasma Wakefields: Beam Conditioning, Emittance Selection, and Beam Chopping
  The strong electric fields associated with laser-plasma wakefields are discussed and how, also, tailoring the transverse profiles of the laser and plasma density allows one to control the wakefields. This ability enables a number of applications to electron beam manipulation. Three possible applications are considered. The first is beam conditioning,which enhances free electron laser (FEL) gain by introducing a correlation between the amplitude of transverse oscillation and theparticle energy. The second is transverse emittance selection, again advantageous for FELs, in which particles with large amplitudeoscillations are removed. The third is the generation of short (100 fs) pulses, accomplished by chopping a beam longitudinally.

This work supported by DoE, DE-AC03-76SF0098.

Alex Kanareykin: New Low-Loss Ferroelectric Materials for Accelerator Application
  Ferroelectric ceramics have an electric field-dependent dielectric permittivity that can be altered by applying a bias voltage. Ferroelectric ceramics have been widely used recently in rf communication technologies, radar applications, etc. Ferroelectrics have unique intrinsic properties that makes them attractive for high-energy accelerator applications: very small response time of ~10-11 sec, considerably high breakdown limit of more than 100 kV/cm, good vacuum properties. Because of these features, bulk ferroelectrics may be used as active elements of tunable accelerator structures [1], or in fast, electrically - controlled switches and phase shifters in pulse compressors or power distribution circuits of future linear colliders [2]. One of the most critical requirements for ferroelectric ceramic in these applications is the dielectric loss factor. In this paper, the new bulk ferroelectric ceramic is presented. The new composition shows a loss tangent of 4-5*10-3 at 11 GHz. The ceramics have high tunability factor: the bias voltage of 50 kV/cm was enough to reduce the permittivity from 500 to 400. The material chemical compound, features of the technology process, and mechanical and electrical properties are discussed. It is shown that there is no fundamental physical limitation in reducing the loss tangent down to 1*10-3. [1] A. Kanareykin, W. Gai, J. Power, E. Sheinman, and A. Altmark, AIP Conf. Proc. 647, Melville, N.Y., 2002, p. 565.[2] V.P. Yakovlev, O.A. Nezhevenko, J.L. Hirshfield, and A.D. Kanareykin, AIP Conf. Proc. 691, Melville, N.Y., 2003, p.187.
Teh Lin: Characteristics of High-Intensity Laser Produced Proton Beams
  We report on the dependence of high-intensity laser accelerated proton beams on material properties of various thin-film targets. Evidence of star-like filaments and beam hollowing (predicted from the electrothermal instability theory) is observed on Radiochromic Film (RCF) and CR-39 nuclear track detectors. The proton beam profile also varies with initial target conductivity and target thickness. For resistive target materials, these structured profiles are explained by the inhibition of current due to the lack of a return current. The conductors, however, can support large propagating currents due to the substantial cold return current which is composed of free charge carriers in the conduction band. A plot that shows the relation between the maximum proton energy and the target thickness also supports the return current and target normal sheath acceleration (TNSA) theory. We have also observed filamentary structures in the proton beam like those expected from the Weibel instability in the electron beam. The maximum proton energy and the spectrum of protons energy are obtained by the time-of-flight method. The electron distributions produced from the interaction between the high-intensity laser and solid targets are also investigated in this report.This work is supported by National Science Fundation.
Peter Messmer: Ion acceleration and terahertz wave generation by ultrashort laser-foil interaction
  The interaction of a strong fs laser pulse with a thin over-dense foil target is investigated via particle-in-cell (PIC) simulations. The interaction leads to the formation of an electrostatic shock wave propagating through the foil and accelerating bulk ions to keV - MeV energies. These ions leave the foil as a second population of high-energy ions, in addition to the sheath accelerated ions from the foil backside. Terahertz perturbations are seen in the post-foil region. This will be discussed in more detail.
Atsushi Ogata: Energy Enhancement of Produced Protons by Prepulses and Back Focusing in Interaction between T-cube Lasers and Thin Foils
  The maximum proton energy is enhanced by factor of five in the interaction between thin organic foils and TW laser pulses with prepulses, when the waist is focused behind the target foil. The maximum energy obtained by our 50mJ 50fs 800nm laser is 900keV. This finding gives possibility for small TW-class lasers to obtain MeV protons. A model is proposed in which prepulses first ablates the target to produce neutral gases. The Kerr effect of the gas then guides the main pulse and tighten its focusing on the target.
Tomas Plettner: New experimental approaches for the LEAP experiment
  The availability of commercial high peak power compact near infrared lasers in the past decade has drawn interest to explore laser driven particle acceleration in vacuum as a feasible scheme for achieving high acceleration gradients. The LEAP experiment is a proof of principle experiment for laser driven particle acceleration in vacuum caused by the longitudinal electric field produced by two crossed Gaussian laser beams.

  The LEAP experiment is located at the SCA-FEL facility at Stanford University. During its first 6 years the instrumentation, software and diagnostics for the experiment were developed. Also, a fused silica accelerator cell having a 1mm long laser-electron interaction space for the acceleration and with 10 microns wide entrance and exit apertures was implemented.

  Single electron bunches were successfully transported through the entire system and characterized, however the very narrow entrance and exit apertures of the accelerator cell caused significant loss of beam and made the electron beam tuning a very difficult task. Furthermore the damage threshold of the cell limited the laser power and thus the acceleration from the cell to 20 keV, which was impossible to detect with the existing noise characteristics at the SCA-FEL facility. Finally, our old timing resolution between the laser and the electron beam was about 100 psec, forcing us to very long time scans of the laser.

  Due to these shortcomings key elements of the experiment have been drastically modified. To circumvent the laser damage threshold problem a “disposable” cell consisting of a very thin reflective boundary in the form of a tape has been designed. The reflective boundary stops only the laser while allowing the electrons to traverse. The need for slits and the transmission problem through the cell has disappeared. Furthermore we have added an IFEL upstream of the tape as a precise timing diagnostic between the laser and the electron beam to within 1 psec.

Carl Schroeder: Electron Beam Conditioning by Thomson Scattering
  A novel method is proposed for conditioning electron beams via Thomson scattering. The conditioning provides a quadratic correlation between the electron energy deviation and the betatron amplitude of the electrons, thereby circumventing emittance limitations, resulting in enhanced gain in free-electron lasers. Quantum effects imply conditioning must occur at high laser fluence and moderate electron energy. Conditioning of x-ray free-electron lasers (e.g., LCLS) should be achievable with present laser technology, leading to significant size and cost reductions of these large-scale facilities.

This work supported by DoE, DE-AC03-76SF0098.

Jonathan Wurtele: Beam Conditioning for FELs: Consequences and Methods
  The consequences of beam conditioning in four example cases of soft x-ray to hard x-ray Free electron lasers (FELs) are examined. In parameter regimes of interest for x-ray FELs the emittance is a major limitation to FEL performance. It is shown that in these cases proper conditioning allows operation with larger emittance, stronger focusing in the undulator, and decreasing gain lengths by up to a factor of two, while increasing the saturated power output. General analytic work is presented showing it is possible to design beam lines that both produce conditioning and, simultaneously, provide exact matching into an FEL. Various conditioners, employing conventional components, are considered, and expressions derived for the amount of conditioning provided. The possibility of using laser and plasma systems for conditioning is explored in a companion paper [Esarey, Sessler and Wurtele, AAC04].

Donald Umstadter: Acceleration of an Electron Beam in Vacuum by a High-Intensity Laser Pulse
  The interaction of a laser produced electron beam with an ultra-intense laser pulse in free space is studied. We show that a transversely propagating optical pulse imparts longitudinal momentum to the electron beam, causing it to deflect. The results are found to be in good agreement with the theoretical model that takes into account the longitudinal fields present when a Gaussian pulse is tightly focused. It is also demonstrated that this technique can be used to temporally characterize a sub-picosecond electron bunch.