Bulletin of the American Physical Society
50th Annual Meeting of the Division of Plasma Physics
Volume 53, Number 14
Monday–Friday, November 17–21, 2008; Dallas, Texas
Session NI2: Intense Beams, Plasma Filaments, and Nonlinear Waves |
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Chair: Howard Milchberg, University of Maryland Room: Landmark B |
Wednesday, November 19, 2008 9:45AM - 10:15AM |
NI2.00001: Exponential Frequency Spectrum and Lorentzian Pulses in Magnetized Plasmas Invited Speaker: Two completely different experiments involving pressure gradients across the confinement magnetic field in a large plasma column are found to exhibit a broadband turbulence that displays an exponential frequency spectrum for frequencies below the ion-cyclotron frequency. The origin of the exponential feature has been traced to the generation of solitary pulses having a Lorentzian temporal signature. These pulses arise from the nonlinear interaction of drift-Alfv\'en waves driven by the pressure gradients. The temporal width of the pulses is measured to be a fraction of a period of the initially coherent drift-Alfv\'en waves and sets the scaling frequency for the observed exponential spectrum. The experiments are performed in the Large Plasma Device (LAPD-U) operated by the Basic Plasma Science Facility at the University of California, Los Angeles. One experiment involves a controlled, pure electron temperature gradient associated with a microscopic ($3$ mm gradient length) hot-electron temperature filament created by the injection of a small electron beam embedded in the center of a large, cold magnetized plasma. The other experiment is a macroscopic ($2$ cm gradient length) limiter-edge experiment in which a density gradient is established by inserting a metallic plate at the edge of the nominal plasma column of the LAPD-U. The temperature filament experiment permits a detailed study of the transition from coherent to turbulent behavior and the concomitant change from classical to anomalous transport. In the limiter experiment the turbulence has been associated with blob phenomena. The similarity of the results suggest a universal feature of pressure-driven turbulence in magnetized plasmas that results in non-diffusive cross-field transport. The findings may explain previous observations in helical confinement devices, research tokamaks and arc-plasmas. [Preview Abstract] |
Wednesday, November 19, 2008 10:15AM - 10:45AM |
NI2.00002: Collisionless plasma expansion into vacuum: two new twists on an old problem Invited Speaker: Plasma expansion into vacuum is a generic problem with a broad range of applications. Of particular interest are those regimes where the expanding plasma consists of energetic electrons and cold ions. The expansion is then caused by electron pressure and serves as an energy transfer mechanism from electrons to ions. Collisional plasma expansion is similar to the gas-dynamic expansion, with the fluid description applicable, whereas collisionless plasma expansion requires a kinetic treatment, especially for the energetic electrons. The collisionless expansion is often described under the assumption that the electron distribution is Maxwellian [1]. However, this assumption is not universally relevant, since the expansion may lead to a significant distortion of the electron distribution function. Also, non-Maxwellian electrons may force the quasineutrality condition to break down. This talk presents two problems [2,3] which illustrate the above kinetic effects. The first one is the problem of a magnetic nozzle that transforms an incoming subsonic plasma flow into a supersonic jet. The second is the problem of an expanding nanoplasma (cluster) with a two-component electron distribution. In the nozzle problem, a magnetic mirror, together with the expanding plasma boundary, generates a trapped electron population downstream. This population is decoupled from the plasma source and, consequently, it undergoes adiabatic cooling. The resulting distortion of the electron distribution function is a new element not captured by the usually used Boltzmann relation. In the cluster problem, the key feature is the initial two-component electron distribution with a cold majority and a hot minority both occupying the same volume prior to the expansion. The cluster problem exhibits a breakdown of quasineutrality manifested by a double-layer inside the flow. Both problems are illustrated with closed-form analytical solutions [2,3]. This work was supported by the US DOE NNSA under Contract No. DE-FC52-08NA28512 and Ad Astra Rocket Company. [1] A. V. Gurevich, L. V. Pariiskaya, and L. P. Pitaevskii, Sov. Phys. JETP 22, 449 (1966). [2] A. V. Arefiev and B. N. Breizman, Phys. Plasmas 15, 042109 (2008). [3] B. N. Breizman and A. V. Arefiev, Phys. Plasmas 14, 073105 (2007). [Preview Abstract] |
Wednesday, November 19, 2008 10:45AM - 11:15AM |
NI2.00003: Coherent control of intense terahertz radiation in laser-produced plasmas Invited Speaker: Intense, broadband terahertz (THz) pulse generation is of great current interest owing to its potential application in nonlinear THz optics and spectroscopy. Although such intense THz radiation exceeding tens of microjoules can be obtained from large-scale electron accelerator facilities such as free electron lasers and synchrotrons, there is a present and growing need for high-energy, compact THz sources at a tabletop- scale. One potential scheme is using tabletop, femtosecond, terawatt lasers to produce tenuous plasmas for scalable THz generation. Recently, intense THz generation has been observed upon mixing a femtosecond laser's fundamental and second harmonic fields in gases. The underlying mechanism has been examined and now understood in the context of a plasma current model. In this model, a transverse asymmetric electron current arises when the bound electrons undergo rapid tunneling ionization and acceleration in the two-color field. Since this current surge occurs on the timescale of the laser pulse duration, in the case of ultrafast lasers ($<$100 fs), this process can generate electromagnetic radiation at THz frequencies. Experimentally, we have recently demonstrated a high-energy ($>$5 microjoule), super-broadband tabletop source generating ultrafast THz pulses ($>$75 THz) in gases via two- color photoionization [1]. We also observed strongly anti- correlated third harmonic radiation. By controlling the relative phase between two-color fields, we can switch the output energy between THz and third harmonic [1]. Our current model can be applied to explain this phase-sensitive control, as well as to characterize the carrier envelope phase of few- cycle laser pulses undergoing ultrafast tunneling ionization. \\[0pt] [1] K. Y. Kim et al., Nature Photonics, doi:10.1038/nphoton.2008.153 (2008) [Preview Abstract] |
Wednesday, November 19, 2008 11:15AM - 11:45AM |
NI2.00004: Observations of Regular Filamentary Plasma Arrays in High-Pressure Gas Breakdown by 1.5 MW, 110 GHz Gyrotron Pulses Invited Speaker: Formation of regular two-dimensional plasma filamentary arrays has been observed in long open-shuttered images of air breakdown at atmospheric pressure [Y. Hidaka \textit{et al.}, \textit{Phys. Rev. Lett.} \textbf{100}, 035003 (2008)]. The breakdown was generated by a focused linearly-polarized Gaussian beam from a 1.5-MW, 110-GHz gyrotron with a 3-microsecond pulse length. Each plasma filament is elongated in the electric field direction and separated roughly one-quarter wavelength from each other in the H-plane. The development of this array structure can be explained as a result of diffraction of the beam around the highly conductive filaments. The diffraction generates a new electric field profile in which a high intensity region emerges about a quarter wavelength upstream from an existing filament. A new plasma filament is likely to appear at the intensified spot. The same process continues and results in the formation of the observed array. Electromagnetic wave simulations that model plasma filaments as metallic posts agree quite well with the hypothesis above. With a nanoseconds-gated ICCD camera, we directly confirmed that only a few rows of the observed array are bright at any one moment, as well as that the light emitting region propagates towards the microwave source. Further experimental breakdown research has been carried out with nitrogen, helium, and SF6 at different pressures. Although each species exhibits qualitatively different structures, in general, a lumpy plasma at high pressures transforms into a more familiar, diffuse plasma as pressure is decreased. The propagation velocity of the ionization front has been also estimated both from the ICCD images and a photodiode array. The velocity is on the order of 10 km/s, and increases as the pressure decreases and the power density increases. [Preview Abstract] |
Wednesday, November 19, 2008 11:45AM - 12:15PM |
NI2.00005: Simulations and experiments of intense ion beam current density compression in space and time Invited Speaker: The Heavy Ion Fusion Science Virtual National Laboratory has achieved 60-fold longitudinal pulse compression of ion beams on the Neutralized Drift Compression eXperiment (NDCX) at LBNL. To focus a space-charge-dominated charge bunch to sufficiently high intensities for ion beam-heated warm dense matter and inertial fusion energy studies, simultaneous transverse and longitudinal compression to a coincident focal plane is required. Optimizing the compression under the appropriate constraints can deliver higher intensity per unit length of accelerator to the target, thereby facilitating the creation of more compact and cost-effective ion beam drivers. Experiments utilize a drift region filled with high-density plasma in order to neutralize the space-charge and current of a 300 keV K$^{+}$ beam, and separately achieve transverse and longitudinal focusing to a radius $<$ 2 mm and pulse width $<$ 5 ns, respectively. Simulations predict, and experiments are underway, to demonstrate that a strong solenoid (B$_{z} \quad <$ 10 T) placed near the end of the drift region can transversely focus the beam to the longitudinal focal plane. Measurements and simulations of plasma penetration into strong solenoids for ion beam neutralization, as well as progress on simultaneous charge bunch focusing, are presented. The total achievable current density compression is expected to be strongly dependent upon the level of neutralization provided by the plasma, especially near the focal plane. The upcoming improved accelerator NDCX-II will capitalize on the insight gained from NDCX simulations and measurements in order to provide a higher-energy ($>$2 MeV) ion beam user-facility for warm dense matter and inertial fusion energy-relevant target physics experiments. [Preview Abstract] |
Wednesday, November 19, 2008 12:15PM - 12:45PM |
NI2.00006: Trapping and destruction of long range high intensity optical/plasma filaments by molecular quantum wakes in air Invited Speaker: The propagation of few millijoule femtosecond laser pulses through gases routinely drives a large nonlinear response in the constituent atoms and molecules. This response is central to the extremely long range filamentary propagation of ultrashort optical pulses in the atmosphere [1]. Long range filaments are accompanied by plasma generation and co-propagating coherent white light generation. A femtosecond laser pulse can also drive gas molecules into alignment [2], even in thermal samples at high pressure [3]. The beating of quantum rotational wavepackets excited in each molecule causes alignment to recur at regular intervals well after the pulse. The recurrent alignment propagates behind the laser pulse as a refractive index wake. Here, we demonstrate that the alignment quantum wake inside a pump pulse filament dramatically affects an intense probe pulse filament [4]. We find, depending on femtosecond timescale delays, that wake either transversely pulls and focuses the probe filament into the pump filament path, or destroys it. We also confirm that for pulse lengths $>$50 fs, the dominant air nonlinearity in single pulse filamentation is rotational. Probe filament spectral measurements are also consistent with quantum wake trapping. Our results demonstrate that long range filamentary propagation can be controlled by exploiting the coherent temporal and spatial response of air molecules. [1] A. Couairon and A. Mysyrowicz. Physics Reports 441, 47 (2007) and references therein. [2] H. Stapelfeldt and T. Seideman, Rev. Mod. Phys. 75, 543 (2003). [3] Y.-H. Chen, S. Varma, A. York, and H.M. Milchberg, Opt. Express 15, 11341 (2007) [4] S. Varma, Y.-H. Chen, and H.M. Milchberg, submitted for publication. [Preview Abstract] |
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