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AAC'04 Agenda
AAC'04 Poster
Organizing Committee
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Laser-Plasma Acceleration: Plasma Guiding Sub-Group
J.T. Mendonca: Plasma waveguides for laser accelerators: Simulations and experiments
  We present the latest results of the GoLP/IST group on the production of plasma waveguides using laser-triggered discharges. Pic code simulations show that bunches of nearly 10 pC and mono-energetic electrons can attain 1 GeV in 1 cm., in a self-injected configuration. Our experimental results show that plasma waveguides compatible with the simulation parameters can be achieved with the 10 TW laser and 200 fs laser beam existing in our laboratory.
Igor Pogorelsky: ATF experiments on laser and e-beam transport in a capillary discharge plasma
  A capillary discharge scheme developed at Hebrew University (Jerusalem) has been adopted at the BNL ATF as a multi-task plasma source for proof-of-principle and user’s experiments. Installed into the 60-MeV electron linac beamline the capillary has been utilized for plasma wake production and characterization. This study evolved later into a pursuit for the resonance wave excitation using an e-beam microbunched with a CO2 laser. In a separate experiment, a high-power CO2 laser has been channeled through a plasma waveguide. Future plans include exploration of the laser wakefield electron acceleration with a guided CO2 laser and Thomson scattering on laser and electron beams counter-propagating in the plasma channel. The experiments are supported by hydro-dynamic and PIC simulations that reveal dynamics of the plasma channel, wake field generation, laser channeling and ponderomotive focusing, and monoenergetic electron acceleration.
Shouyuan Chen: Evidence of Ionization Blue Shift Seeding of Forward Raman Scattering
  We report the results of spectroscopic experiments that wereconducted by focusing an intense ultr-short laser pulse into a heliumgas target. The scattered light from the interaction region wasmeasured spectrally and spatially from various directions as afunction of laser intensity and plasma density and correlated withthe accelerated electron beam. The experiment data showed thatforward Stimulated Raman Scattering (SRS) was sensitive to thefocus position of laser relative to the nozzle. Together with theplasma channel imaged by a CCD camera, the measurements indicatethat SRS is seeded by the ionization blue shifted light. The cross-phase modulation between the SRS and laser beam was also observedin the experiment.
Dimitre Dimitrov: Particle-in-cell simulations of intense laser pulses coupling into plasma channels
  The guiding of intense laser pulses in plasma channels is necessary tomaximize the energy of electrons accelerated in a laser wakefield accelerator (LWFA). In general, a significant fraction of theenergy in the laser pulse can be scattered when coupling from vacuum into a channel. We present initial 2D particle-in-cell (PIC)simulations of this problem, using the VORPAL code. We will discuss new power spectral diagnostics, based on simultaneous FFTsin space and time of the PIC data, which enable quantitative measurements of the scattered pulse energy. Future work will be directed toward VORPAL parameter studies designed to optimize the amount of laser energy that couples into a channel.
Daniel Gordon: Detailed Modeling of Channel Guided Laser Wakefield Accelerators based on Capillary Discharges
  Capillary discharges are simple devices that can be used to generate a plasma channel and guide the drive pulse in laser wakefield accelerators. However, concerns have often been raised regarding the effects of magnetic fields produced by the discharge current, photoionization of carbon ions, and nonideal channel geometries. Another difficulty often cited is that a suitable source of injected electrons must be found. Particle-in-cell simulations will be presented that include these processes and show that they do not preclude the successful use of capillary discharges as accelerating structures. The optical injection scheme considered is a high density limit of laser ionization and ponderomotive acceleration (LIPA). The output from the LIPA simulations is used as an input into the model for the acceleration stage, which includes capillary generated magnetic fields, photoionization effects, dispersion and plasma nonlinearities, and the possibility of discrete density tapering. The simulations of the injector will be compared with experimental results, and end-to-end simulations of a joint U.S. Naval Research Laboratory-Icarus Research experiment to generate near GeV electrons will be presented.
Simon Hooker: Application of the gas-filled capillary discharge waveguide to laser-plasma acceleration
  One limit to the acceleration that can be achieved in a laser-plasma accelerator is the length over which the intensity of the driving laser can be maintained close to the focal intensity. In the absence of any method to guide the laser radiation, diffraction restricts this length to the order of the Rayleigh range, which for the laser parameters of interest is typically a few millimetres. If the target plasma is not fully ionized, additional ionization of the plasma leads to refractive defocusing by the induced transverse electron density profile, which further restricts the laser-plasma interaction length achieved.One solution to this problem is the plasma waveguide in which a plasma is formed, prior to the arrival of the driving laser pulse, with an electron density which increases with radial distance from the beam axis. A profile of this form is able to counteract refraction and diffraction and so guide intense laser pulses over long lengths. Several effects have been utilized to form plasma channels of this type including: hydrodynamic expansion of a laser-produced plasma column, the pinch effect in a fast capillary discharge, and thermal effects in discharge-ablated or gas-filled capillaries.In this talk we present the results of our work on one type of plasma waveguide, the gas-filled capillary discharge waveguide. We describe the mechanisms by which the channel is formed, compare the results of magnetohydrodynamic simulations with interferometric measurements, and present the results of experiments to guide high-intensity laser pulses. The potential for using the gas-filled capillary discharge waveguide in laser-plasma accelerators will be discussed.
Min Hur: Magneto-plasma waveguide
  A new method of making a channel to guide an electromagnetic wave in amagnetized plasma is presented.The basic idea is to use the external magnetic field to createa transverse variation in the refractive index. This is possible since therefractive index of the magnetized plasma is dependent on the externalmagnetic field as well as the plasma density. Particle in cellsimulations with XOOPIC show that the idea works as expected.In the simulations, a right-hand-polarized pulse was launched into auniform density plasma with an axial applied magnetic field whoseintensity varied transversly. Because of the limitation in the magneticfield strength which can be made by current technology, the guided wave is mostlyin the microwave regime. We investigate the possiblity of extending thefrequency range to the laser regime.
Vinod Kumarappan: Guiding of intense pulses in efficient end-pumped plasma channels generated by self-focusing in Ar and H_2 clusters
  We demonstrate that self-guiding of intense short pulses in clustered gases can be utilized to generate long plasma channels, which upon expansion form waveguides suitable for propagation of laser pulses at high intensity. This scheme has several advantages over waveguide-generation in non-clustered gases. The absorption of energy by the target depends on the size of the clusters and not on the average density of the gas, which allows greater control of the density encountered by the guided pulse. In particular, electron densities less than 10^18 per cm^3 are feasible. Moreover, since clusters absorb sub-picosecond pulses very efficiently, channel generation by an auxiliary long-pulse laser is no longer necessary and a considerably simpler setup suffices. The problem of taper at the channel entrance, an old bugbear of side-pumped waveguides in gases, is shown to be significantly reduced.Evidence will be presented of waveguide generation in gases of argon and hydrogen clusters, using different cryogenic sources. A slit source is used for argon, and waveguides < 1 cm could be formed, in which laser pulses with intensity > 10^17 W/cm^2 were guided. The results of a propagation code suggest that even longer channels are well within experimental reach. Argon, however, has the disadvantage that a super-intense pulse would likely produce further ionization, and hence suffer ionization induced defocusing. Hydrogen clusters, which can easily be fully ionized, were formed using a more efficient conical nozzle cooled to 90 K, limiting maximum waveguide lengths to < 3 mm. Though these channels are short, there is no obvious reason why a longer target wont allow longer waveguides to be generated, and the experiments demonstrate the utility of this novel scheme.
Arie Zigler: Guiding of Ultrahigh Laser Intensities in Ablative Capillary Discharge Plasma Channel with Laser Ignition
  We are reporting on the guiding of laser intensities above 5 x 10^18 W/cm^2 over distances of 1.5 cm by ablative capillary discharges. Generation of plasma channels with narrow and deep axial minimum in electron density profiles will be reviewed. Channels with temperatures around 2-4 eV and electron densities of 6 x 10^17 cm^-3 – 2.5 x 10^18 cm^-3 were produced by low-current discharge with laser ignition in polyethylene capillaries of 0.3 mm diameter and 15 mm length. The diameter of the resultant hollow plasma channel at half-minimum electron density was approximately 25% of the diameter of the capillary, and the relative profile depth was as much as 70% of the axial electron density. The ignition of the capillary discharge was obtained using an auxiliary 10 mJ, 10 nsec Nd:YAG laser. This triggering is characterized by both low delay and jitter times. The guiding was achieved using both CH2 and BN capillaries. It was demonstrated that the BN capillaries are capable of withstanding over ~1000 shots.
Chieh Sung: Guiding of 10 &#956;m laser pulses by use of hollow waveguides
  Guiding of intense 10 &#956;m laser light over many Rayleigh ranges is important in many advanced accelerator schemes. We have carried out such guiding experiments using both evacuated and gas-filled tubes of different materials. The CO2 laser pulse length was typically 200ps (FWHM) and the CO2 laser peak power was up to 100GW. A maximum transmission of 15% was obtained when a 2 cm long, 1mm diameter, evacuated Stainless-steel waveguide was used. This dropped to about 8% when 180 mTorr of hydrogen was added to the waveguide. At the peak intensity of 5x10^14 W/cm^2 at the waveguide entrance, we expect the formation of a fully ionized plasma throughout the waveguide. The implications of the study to acceleration experiments will be discussed.