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Plasma Seminar Series: Additional Information

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Additional information regarding the spring 2006 plasma seminar series is available here.

Monday, February 27

Dr. Raffi Nazikian, Princeton Plasma Physics Laboratory
Kinetic Alfvén Eigenmodes in DIII-D

Abstract

Evidence is presented for a multitude of discrete frequency Alfvén waves in the core of magnetically confined high temperature fusion plasmas excited by the thermal ion distribution. Multiple diagnostic instruments confirm wave excitation over a wide spatial range from the device size at the longest wavelengths down to the thermal ion Larmor radius. At the shortest scales the poloidal wavelengths are comparable to the scale length of electrostatic drift wave turbulence. Theoretical analysis confirms a dominant interaction of the modes with particles in the thermal ion distribution traveling well below the Alfvén velocity. This is counter to the conventional wisdom that thermal ions produce net ion Landau damping and indicates the need for a reexamination of theoretical predictions of high-n Alfvén mode stability in ITER, particularly in enhanced confinement regimes where high ion temperature gradients are expected in regions of weak magnetic shear.

Monday, February 20

Dr. Ilya Y. Dodin, Department of Astrophysical Sciences, Princeton University
Nonadiabatic Ponderomotive Barriers

Abstract

A ponderomotive potential is an effective potential seen by a particle in ac field in average over the fast oscillations. It is not a true potential though, and, if the ac field is in resonance with particle natural oscillations, the particle can exhibit irreversible drift motion [1-3]. A new ponderomotive potential is found for this case that can capture nonadiabatic dynamics [4]. The particle drift in this new potential resembles the motion of a quantum object in a conservative field [5]. Among other applications, these nonadiabatic potentials can perform selective separation and cooling of plasma species or drive electric current by asymmetrically transmitting thermal particles in a preferential direction [1, 2, 6].

[1] N.J. Fisch, J.M. Rax, and I.Y. Dodin, Phys. Rev. Lett. 91, 205004 (2003).
[2] I.Y. Dodin, N.J. Fisch, and J.M. Rax, Phys. Plasmas 11, 5046 (2004).
[3] I.Y. Dodin and N.J. Fisch, J. Plasma Phys. 71, 289 (2005).
[4] I.Y. Dodin and N.J. Fisch, Phys. Lett. A 349, 356 (2006).
[5] I.Y. Dodin and N.J. Fisch, Phys. Rev. Lett. 95, 115001 (2005).
[6] I.Y. Dodin and N.J. Fisch, Phys. Rev. E 72, 046602 (2005).

Monday, March 27

Prof. George Tynan, University of California, San Diego
Observation of Turbulent-Driven Shear Flow in a Cylindrical Laboratory Plasma Device

Abstract

A turbulent-generated azimuthally symmetric radially sheared plasma fluid flow is observed in a cylindrical magnetized helicon plasma device with no external sources of momentum input. A turbulent momentum conservation analysis shows that this shear flow is sustained against dissipation by the turbulent Reynolds stress generated by collisional drift fluctuations in the device. In the wavenumber domain this process is manifested via a nonlinear transfer of energy from small scales to larger scales. Simulations of collisional drift turbulence in this device have also been carried out, and clearly show the formation of a shear flow quantitatively similar to that observed experimentally. The results integrate experiment and first-principle simulations and validate the basic theoretical picture of drift-wave/shear flow interactions.

Monday, April 24

Dr. Stefan Gerhardt, Princeton Plasma Physics Laboratory
Studies of Free Boundary Field Reversed Configurations with Improved Stability in the Magnetic Reconnection Experiment Device

Abstract

A recent campaign of field reversed configuration (FRC) studies in the Magnetic Reconnection Experiment (MRX) has produced MHD-regime oblate FRCs with improved stability to low-n MHD modes, with an associated extension of the plasma performance. These FRCs are produced by the merging of two spheromaks with anti-parallel toroidal fields. The plasma shape in controlled through three independently energized pairs of equilibrium field (EF) coils; we parameterize the shape of the equilibrium field by its mirror ratio. A conducting center column has been used in some cases to provide important stabilization of the spheromaks during formation and translation before merging, and provides some stabilization of the final FRC state. The FRC without a conducting center column is observed to be unstable to n=1 tilt/shift motions. The dangerous n=1 tilt is suppressed by the center column except at very low mirror ratio, but n=2 & 3 modes often remain. These modes can be reduced by forming plasmas with very large EF mirror ratio. These very oblate plasmas have a minimal amplitude of low-n perturbations, and the longest lifetime. Note that both shape control and passive stabilization are required: plasmas whose n=1 activity is reduced by the central conductor may still be quickly terminated by n=2 activity if the mirror ratio is not sufficiently large. Free-boundary equilibria for these FRC plasmas are computed with a new Grad-Shafranov solver, which constrains the equilibrium solution to the available data. The calculations indicate that as the EF mirror ratio varies from 2 to 4.5, the elongation (kappa) varies from 1 to 0.6 and the triangularity (delta) varies from 0.5 to -0.4. The equilibria are used in a model for rigid-body tilt/shift motions, which indicates that these oblate FRCs are in the MHD tilt-stable regime. Initial 3D MHD calculations with the HYM code indicate improved stability to both radially and axially polarized low-n instabilities.

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