Session BO1: Basic Plasma Physics I

Positron--Electron, Pair-Plasma Production on OMEGA EP

It is shown that an e$^{+}$e$^{-}$ pair-plasma can be created on OMEGA EP, a feat yet to be achieved in the laboratory. We calculate that a yield of between 10$^{11}$ and 10$^{12}$ positrons can be produced on OMEGA EP by a combination of the Bethe--Heitler conversion of hard x-ray bremsstrahlung\footnote{ J. D. Bjorken and S. D. Drell, \textit{Relativistic Quantum Mechanics}, International Series in Pure and Applied Physics (McGraw-Hill, New York, 1964); D. A. Gryaznykh, Ya. Z. Kandiev, and V. A. Lykov, JETP Lett. \textbf{67}, 257 (1998); K. Nakashima and H. Takabe, Phys. Plasmas \textbf{9}, 1505 (2002).} and the trident process,\footnote{ E. P. Liang, S. C. Wilks, and M. Tabak, Phys. Rev. Lett. \textbf{81}, 4887 (1998).} assuming a total laser energy of 5~kJ. For this expanding e$^{+}$e$^{-}$ cloud to be a plasma, there must be many particles in a Debye sphere, and the cloud must be many Debye lengths in size. A magnetic field produced by a second OMEGA EP beam will provide the necessary confinement. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460.   

Quasi-Linear and Nonlinear High-Harmonic Generation from Ultraintense Ultrafast Laser Plasmas

High-harmonic generation from ultra-intense laser-solid interaction has the potential for efficient generation of attosecond pulses, from a plasma so thin that phase-matching issues need not arise. However, extremely difficult constraints on laser-pulse contrast for solid interaction have routinely meant that new effects of nonlinear optical physics cannot be teased out from the hydrodynamic effects. New beamline-engineered systems of double plasma-mirrors have changed this completely, and we have discovered that the process of high-harmonic generation in laser-solid interactions is actually comprised of two disparate processes, a quasi-linear process of Coherent Wakefield Emission (efficient for ultrafast laser pulses down to I = 10$^{15}$ W/cm$^{2})$, as well as the nonlinear Relativistic Oscillating Mirror mechanism. Both depend crucially on aspects of optical and plasma-wave phase-control during the interaction.   

Spectroscopic Analysis of Electron Beam Diodes for Flash X-ray Radiography at Sandia National Laboratories*

Experiments were performed on RITS-3, a Marx driven, three stage IVA (5.5MV, 120kA), and are continuing on RITS-6, a six stage IVA (10MV, 120kA), to study the role of plasma formation and propagation on electron beam focusing for flash x-ray radiography. It is believed that plasmas formed on electrodes or by interactions with background gases limit e-beam focusing and stability and cause pulse shortening of the diode. These are concerns as higher doses (1000 Rad at 1m) from smaller sources (2mm dia.) are required for future radiographic applications. Diagnostics include time and space resolved visible and uv emission spectra using 1 meter Czerny-Turner monochromators equipped with framing and streak cameras. Line and continuum analysis are conducted using a time-dependent CR model. Self-consistent line shape calculations measure Stark, Doppler, and opacity broadening. Electron density and temperature determinations as well as neutral and ion species parameters are obtained. Such data is crucial to the fundamental understanding of electron beam diode behavior and aids in the continued development of these sources. *Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract DE-AC04-94-AL85000.   

Coherent X-Ray Emission from a Plasma Discharge-Driven Hydride Target

A. B. Karabut originally reported a soft x-ray laser ($\sim $1.5 keV) using metal targets (Ti ,Pd) as the cathode in a high-current pulsed deuterium plasma discharge [1, 2]. We have undertaken experimental and theoretical studies of the emission process at the UIUC. Coherent x-ray emission is observed during discharge operation at a 0.1-0.5 T and cathode and anode spacing of 4 mm, voltage of 1-2 kV, pulsed current $<$ 2 A. Square-shaped 0.2-2.0 ms current pulses with 0.1 us rise time give X-ray output of 13.4 mW/cm$^{2}$, dose = 3.3 $\mu $J/cm$^{2}$, efficiency $\sim $ 10 $^{-4}$. The proposed mechanism assumes a thermal shock-induced D-diffusion process near the cathode's surface (generating high order harmonics) due to the high current deuteron bombardment. This results in penetration of recoil deuterons into the inner (LII) electron shell of the cathode material (Pd). The excitation energy is then lost as the electron returns back to its native orbital. [3, 4] Work is underway to extend studies to higher currents and voltages. [1] A. Karabut, ``Solid X-ray Laser {\ldots},'' Proc., 11th Intern'l. Conf. on Emerging Nuclear Energy Systems, Albuquerque, NM. 2002, pp. 374-382. [2] G . H. Miley, A. Lipson, and A. Karabut, ``A New Type of Phonon-Driven X-ray Laser'', Novel Accel. and Laser-Beam Interactions, ICFA-6, Oxford, UK, July, 2003. [3] P. B. Corkum, Phys. Rev. Lett, Vol. 71, 1994. [4] M. Drescher, et al., Nature, 419, 803 (2002).   

Structure of an Ionizing Shock Wave in a Guide Tube

This contribution describes new measurements of an ionizing shock wave, having a shock velocity of $\sim 6$ km/s, routinely produced in a guide tube filled with gas at atmospheric pressure. Shock waves of this velocity generate temperatures and pressures that are high enough to ionize the gas in which the shock wave is traveling. The plasma generated by this ionizing shock wave is characterized using Rogowski coils, Langmuir probes and visible spectroscopy. Charge separation, or the difference in the electron and ion densities is measured by Rogowski coils. Electron temperatures and densities are measured using Langmuir probes at the edge of the guide tube. Ions are identified by using fiber coupled spectrometers and streak cameras that view the shock wave along the guide tube and radially across it. The measurements show charge separation, an electron temperature of 2 eV and an electron density of 10$^{24}$ m$^{-3}$. These measurements are the first to show an axial charge separation as well as to characterize the ionizing shock wave on such a fast time scale, ($< 1 \mu$s).   

Stationary electrostatic structures revisited

With recent advances in computer simulations and measurements of naturally-occurring nonlinear electrostatic structures in space, it is worth revisiting the relationship between solitons, phase-space holes, and double-layers. Pseudopotential methods are used to explain several interesting new features of 1D nonlinear electrostatic structures. Analytic and numerical solutions explain why the scaling of bipolar field width-to-height ratios are \textit{opposite} for electron phase-space holes and for ion-acoustic solitons. Preliminary results suggest that weak electrostatic field structures in the solar wind may correspond to ion-acoustic solitons. In other studies we show how double layers and solitons can morph into each other. Slight alterations in the pseudopotential for a double layer (potential ramp) can transform the double layer into a soliton (bipolar field) and vice-versa. Double layers, ion solitons and the relationships between them are studied for the case of one ion species drifting with respect to electrons and for the case of two ion species drifting with respect to each other and with respect to electrons. Vlasov simulations relevant to the upward current region of the auroral ionosphere where oxygen and hydrogen are accelerated to different velocities [D. Main, this meeting] demonstrate these properties.   

Observation of a devil's staircase behavior and experimental control of chaos in wave-particle interaction

A specially designed Traveling Wave Tube allows to carefully study wave-particle interaction; in the case when the beam electrons can be considered as test particles a devil's staircase has been observed by recording the beam velocity distribution function vs the amplitude of two induced waves. Its existence was related to the transition to large scale chaos in such a system, paradigm of two degrees of freedom systems in Hamiltonian dynamics$^{1}$. Chaos often represents an obstacle for the control of experiments; we showed the robustness of a control method for Hamiltonian chaos by varying methodically the pertinent parameters of system$^{2}$. Increasing the beam intensity self-consistent phenomena gradually enter; commonly leading to wave growth and further nonlinear saturation. A generator able to produce subnanosecond electron pulses at high repetition rate was installed allowing to explore the influence of the relative phase of electron packets with respect to waves$^{3}$. \textbf{1} A. Macor, F.Doveil and Y.Elskens, Phys. Rev. Lett. \textbf{95}, 264102 (2005) and Chaos \textbf{16}, 1 (2006) \textbf{2} C.Chandre et al., Phys. Rev. Lett. \textbf{94}, 074101 (2005) \textbf{3} A.Macor, F.Doveil and E.Garabedian, J-NPCS (in press)   

Dominance of second growth rate peak in Alfv\'enic behavior and instability transition of relativistic electromagnetic ion cyclotron instabilities near threshold

The second peak of the growth rate spectrum has a weaker driving term than the first peak. This driving term is extra due to relativistic effect. Surprisingly, numerical results show that, for higher fast ion energy such as 14.7 MeV, the second peak has managed to overcome the Alfv\'{e}nic condition of a cubic instability near the threshold while the first peak remains stable. From the analytical theory, the threshold slow ion temperature increases with the fast ion energy. The effect of finite slow ion temperature is more important at higher wave number regime for the slow ion cyclotron resonance, but not for the non-resonant slow ion inertia. As a result, their balance on determining the coefficient $P$ in the dispersion relation is changed. Thus, the $P$ peak moves to higher wave number regime near the second peak from where the first peak locates. The Alfv\'{e}nic condition is first satisfied near the $P$ peak. Thus, the second peak appears first near the threshold; the first peak overcomes the condition later. When the slow ion temperature or density is about twice than that of the threshold, the second peak has transited from the cubic to a coupled quadratic instability while the first peak remains as the cubic instability.   

Accelerate the transition of radioisotopes or unwanted weapons-grade $^{239}$Pu into stable nuclei with a system of high frequency modulation for a net energy gain

A process of high frequency stimulation of nucleons can be utilized for the accelerated fission, decay or controlled transition of unstable isotopes. For example $^{238}$U could be persuaded to transition promptly into $^{206}$Pb, where portions of the total mass difference of 29873.802 MeV per nucleus becomes available energy. The proposals of this paper describe an effective system for nuclei stimulation configured to accelerate such a series of 14 transitions over several milliseconds, instead of 4.47 x 10$^{9 }$years. Positive ions or ionized capsules of fuel suspended by magnetic fields and subjected to the system of correlated frequency modulation of multiple beam lines, tailored to the specific target, will emit sufficient energy to stimulate subsequent targets. The system can be applied to all radioisotopes, including $^{232}$Th, nuclear waste product isotopes such as $^{239}$Pu, and a variety of other suitable unstable or stable nuclei. Through the proposed confinement system and application of high frequency stimulation in the 10$^{22}$ to 10$^{24}$ Hz regime, the change in rest mass can be applied to both the fragmentation of subsequent, periodically injected targets, and the production of heat, making a continuous supply of energy possible. The system allows the particle fragmentation process to be brought into the laboratory and provides potential solutions to the safe disposal of fissile material.   

A Phase Space Study of an Electron Beam Interacting with Lasers and Plasma Waves

The phase-space dynamics of the electrons in a beam that is intended to be used as a diagnostic of plasma waves has been studied theoretically, numerically, and experimentally. Previous numerical models predicted that the emittance of the electron beam passing through the plasma wave should be less than about 1 mm-mrad in order for the scattered electrons to retain information on the amplitude of the plasma wave after they exit the wave. In updates to the model, the interactions of the electrons with the CO2 laser beam that excites the plasma wave and the YAG laser beam that diagnoses the plasma wave, have been added. Also discussed will be the results of attempting to control the beam emittance by varying the power to the thermionic emitter in the electron gun. The techniques for measuring the emittance will be discussed.   

Micro- and nano- second time scale, high power electrical wire explosions in water.

Experimental and magneto-hydro-dynamic simulation results of micro- and nanosecond time scale underwater electrical Al, Cu and W wires explosions are presented. A capacitor bank with stored energy up to 6 kJ (discharge current up to 80 kA with 2.5 $\mu$s quarter period) was used in microsecond time scale experiments and water forming line generator with current amplitude up to 100 kA and pulse duration of 100 ns were used in nanosecond time scale experiments. Extremely high energy deposition of up to 60 times the atomization enthalpy was registered in nanosecond time scale explosions. A discharge channel evolution and surface temperature were analyzed by streak shadow imaging and using fast photo-diode with a set of interference filters, respectively. Microsecond time scale electrical explosion of cylindrical wire array showed extremely high pressure of converging shock waves at the axis, up to 0.2 MBar. A 1D and 2D magneto-hydro-dynamic simulation demonstrated good agreement with such experimental parameters as discharge channel current, voltage, radius, and temperature.   

Dipolar radiation from spinning dust grains coupled to an electromagnetic wave

We present a model for the emission of radiation by elongated polarized dust grains under the influence of both an external electromagnetic wave and a constant background magnetic field. The dust, exhibiting rotational motion at the external electromagnetic field frequency wo as well as quivering motion at frequency 1$^{-}$2o, proportional to the em field amplitude, will radiate with frequencies that will depend on the external field wavelength and amplitude. Two different types of radiated spectra can be observed, one, pertatining to small field amplitudes, with frequencies wo, and sidebands wo $_{f }$flo and wo f 212$_{0}$ and the other, corresponding to large field amplitudes, exhibiting frequencies at wo, and also at the quivering frequency S$^{-}$2o flo t wo and at twice the quivering frequency 21Zo 212o f w. Since the amplitude and the frequency of the background electromagnetic field are independent parameters, this model establishes a correlation between different spectral components of galactic dipolar emission, which may explain the correlation between a component of the Galactic microwave emission and the 100$\mu $m thermal emission from interstellar dust that has been recently measured.