Session XI3: Rotating Plasmas and Post-Deadline Invited Talks

Wave induced supersonic rotation in mirrors

Wave-particle interactions in \textbf{E}x\textbf{B} supersonically rotating plasmas feature an unusual effect: particles are diffused by waves in both potential energy and kinetic energy [1]. This wave-particle interaction generalizes the alpha channeling effect, in which radio frequency waves are used to remove alpha particles collisionlessly at low energy. In rotating plasmas, the alpha particles may be removed at low energy through the loss cone, and the energy lost may be transferred to the radial electric field. This eliminates the need for electrodes in the mirror throat, which have presented serious technical issues in past rotating plasma devices. A particularly simple way to achieve this effect is to use a high azimuthal mode number perturbation on the magnetic field [2]. In the rotating frame, this perturbation is seen as a wave near the alpha particle cyclotron harmonic, and can break the azimuthal symmetry and magnetic moment conservation without changing the particle's total energy. The particle may exit if it reduces its kinetic energy and becomes more trapped if it gains kinetic energy, leading to a steady state current that maintains the field. Simulations of single particles in rotating mirrors show that a stationary wave can extract enough energy from alpha particles for a reactor to be self-sustaining. Rotation can also be sustained by waves in plasmas without a kinetic energy source. This type of wave has been considered for plasma centrifuges used for isotope separation [3]. \\[4pt] [1] A. J. Fetterman and N. J. Fisch, \textit{Phys Rev Lett} \textbf{101}, 205003 (2008). \\[0pt] [2] A. J. Fetterman and N. J. Fisch, \textit{Phys. Plasmas} \textbf{17}, 042112 (2010). \\[0pt] [3] A. J. Fetterman and N. J. Fisch, \textit{Plasma Sources Sci. Tech.} \textbf{18}, 045003 (2009).   

Breaking field lines during magnetic reconnection: it's turbulent electron viscosity and not anomalous resistivity

The dissipation mechanism that breaks magnetic field lines during reconnection has remained a mystery since the first models of reconnection were proposed in the 1950s. Classical resistivity is too small to explain reconnection observations in tokamak sawteeth, the solar corona and heliosphere. 3-D particle-in-cell simulations of magnetic reconnection reveal that strong currents and associated high electron-ion streaming velocities that develop near the x-line can drive instabilities. The electron scattering caused by this turbulence produces an enhanced drag, ``anomalous resistivity,'' that has been widely invoked as the dissipation mechanism. We have demonstrated with simulations and analytic modeling that during low-$\beta$ reconnection with a guide field that electron current layers become strongly turbulent. The surprise, however, is that the turbulence driven by an electron sheared-flow instability completely dominates traditional streaming instabilities and the associated turbulent driven anomalous viscosity balances the reconnection electric field and therefore breaks field lines. The turbulence modestly enhances the rate of reconnection. This instability was not seen in earlier simulations because of the limited scale size of earlier computational domains. The instability is electromagnetic, is part of the whistler branch and therefore falls below the electron cyclotron frequency. The ions play no significant role. A second surprise is that a guide field is required for the instability to exist so that reconnection with a guide field exhibits stronger turbulence that anti-parallel reconnection. Signatures of this turbulence that could be explored in laboratory reconnection experiments and satellite observations are discussed.   

Transport Bifurcation in a Rotating Tokamak Plasma

A study of turbulent transport in the experimentally interesting regime of zero magnetic shear, using local nonlinear gyrokinetic simulations with equilibrium flow shear, has revealed the existence of a transport bifurcation. For a given ratio of input torque to input heat it is possible, by varying the overall input power, the temperature or the density, to trigger a transition in the flow and temperature gradients from low values to high values. It is also found that the plasma is linearly stable for all non-zero flow shears, and that therefore the turbulence is subcritical. Furthermore it is discovered that flow shear decreases the profile stiffness at low values of the heat flux, but can increase it at higher values.   

Novel zero vector potential mechanism of absorption in strongly relativistic plasmas

In this talk we introduce a new mechanism of fast electron generation at the vacuum-solid boundary of intense laser pulse interaction with overdense plasma. We demonstrate that for a sharp plasma profile laser energy is first stored into the plasma as electrostatic energy (via compression of the electrons) and then released into fast electron energy by the zeroes of the electromagnetic vector potential. In this zero vector potential (ZVP) mechanism the generation of fast electron bunches and coherent x-rays (high harmonics from overdense plasma) are intrinsically connected. The new mechanism leads to scalings for the fast electron energy, which explicitly depend on the plasma density, thus providing a new insight into relativistic laser-matter interaction. It is shown that for sharp plasma density profiles the new mechanism provides the dominant contribution to the laser-plasma interaction by the injection of energy into the overdense plasma delivered by attosecond-duration electron bunches. This new understanding will allow the future generation of single ultra-bright attosecond x-ray pulse by suitable control of the laser pulse polarization. The process will also allow single pulse attosecond fast electron bunches to be generated that can be further accelerated in laser wakefield accelerators.   

Twisting space-time: Relativistic origin of seed magnetic field/ vorticity

It is shown that a purely ideal mechanism, originating in the space-time distortion caused by the demands of special relativity, can break the topological constraint (leading to generalized helicity conservation) that would, otherwise, forbid the emergence of magnetic field (a generalized vorticity) in an ideal non relativistic dynamics. The new mechanism, arising form the interaction between the inhomogeneous flow fields and inhomogeneous entropy, is universal, and can provide a finite seed even for mildly relativistic flows. Simple estimates of the seed fields in cosmic settings, in particular the early hot universe filled with relativistic particle antiparticle pairs (upto the end of the electron-positron era), are provided. Possible applications of the mechanism in (intense) laser produced plasmas is also explored.   

Temporal characterization of ultrashort electron beams, optically injected and accelerated in a laser-wakefield

Laser-plasma accelerators, driven by ultraintense and ultrashort laser pulses, sustain accelerating gradients of several hundred giga-volts-per-metre and can deliver high quality electron beams with low energy spread, low emittance and up to giga-electron-volt peak energy. The use of two colliding pulses in a collinear geometry can produce a stable source of electrons that is easily tunable in energy. Here, we report on results of recent experiments with two laser beams colliding with an angle of 135$^{\circ}$, thus having the advantage of protecting the laser system from any feedback and facilitating immediate access to the electron beam. For temporal characterization of the electron beam, we measure coherent optical transition radiation in a wide spectral band. Measurements of the absolute number of photons in the mid-infrared spectral band indicate that the electron bunches have durations of only a few femtoseconds. The shape and absolute intensity of the measured CTR spectrum agrees with analytical modeling of electron bunches with durations of 3 to 5 fs [full width at half maximum (fwhm)] and peak currents of 3 to 4 kA, depending on the bunch shape. Under certain conditions, we observe strong oscillations in the visible part of the CTR spectrum. A detailed Fourier analysis reveals that these spectral modulations result from interference of radiation produced by multiple electron bunches. The bunch separation is related to the fringe separation and shifts with plasma density but is always an integer number of plasma wavelengths. It is found that electrons are injected in single- and multiple buckets up to at least ten plasma wave periods behind the first electron bunch.