C. Teodorescu

Steady supersonically rotating plasmas in the Maryland Centrifugal Experiment

The Maryland Centrifugal Experiment MCX [R. F. Ellis, A. B. Hassam, and S. Messer, Phys. Plasmas 8, 2057 (2000)] studies supersonic rotation and enhanced confinement produced by the application of an electric field perpendicular to an axial confining mirror magnetic field; radial shear in the rotation is predicted to stabilize magnetohydrodynamic (MHD) interchange modes. The MCX mirror field is 2.6 m in length, maximum mirror field 1.9 T, maximum midplane field 0.33 T; an inner coaxial core is driven by a 10 KV capacitor bank, producing the radial electric field which drives azimuthal rotation. MCX produces high density (n>1020m−3) fully ionized plasmas and has two operating modes. In the O (ordinary) mode the plasma rotates supersonically with azimuthal velocities in the range of 100 km/s for discharge times exceeding 8 ms. Ion temperatures are ∼30eV and momentum confinement times 100–200 μs. Sonic Mach numbers (uφ∕vti) in the range 1–2 and Alfven Mach numbers (uφ∕vA)∼0.3 have been achieved for O mode di...

Spectroscopic measurements of plasma rotation and ion and neutral atom temperatures in the Maryland Centrifugal Experiment

In initial spectroscopic measurements on the Maryland Centrifugal Experiment, rotation velocities and directions for C+, C2+, C3+, and N+ ions and neutral hydrogen atoms have been obtained from the Doppler shifts of visible spectral lines. Ion and neutral temperatures have also been determined from Doppler broadening. Different rotation velocities and temperatures are observed for different species indicating that the inner plasma is hotter and rotates more rapidly than that at the edge. The direction of rotation is found to be consistent with the predicted Er¯×Bz¯ direction; and it reverses with the direction of the magnetic field. The magnitudes of the rotation velocities were of the same order as estimated from the applied voltage and magnetic field. For the C+, C2+, and C3+ species, rotation velocities of 20–30 km/s, 40–70 km/s, and 60–100 km/s are observed, respectively. The corresponding temperatures are 10–12 eV, 20–40 eV, and 25–40 eV. Neutral hydrogen atoms are observed to rotate with velocities ...

Radially resolved measurements of plasma rotation and flow-velocity shear in the Maryland Centrifugal Experiment

In diagnosing the Maryland Centrifugal Experiment (MCX) [R. F. Ellis et al., Phys. of Plasmas 8, 2057 (2001)], earlier spectroscopic measurements of averaged plasma rotation velocities have been upgraded to include radial distributions, using a five-channel fiber-optic collection system. Detailed information from each view is now possible with an 8-times increase in spectral resolution, by using a 2m spectrograph and a 2400lines∕mm grating. Inversion of the integrated chordal radiation into a radial dependence of local emissions is performed by two methods: (a) an iterative simulation beginning with assumed emissions in five axially concentric cylindrical zones followed by summation along the five viewing chords, and (b) inversion of a combination of dual Abel-type matrices. The radial profiles of the absolute velocities derived cover a range from 20to70km∕s for both C+ and C++ impurity ions. Previous apparent differences in velocities between ions from a single chordal observation are now explained by th...

Experimental study on the velocity limits of magnetized rotating plasmas

An experimental study on the physical limits of the rotation velocity of magnetized plasmas is presented. Experiments are performed in the Maryland Centrifugal Experiment (MCX) [R. F. Ellis et al., Phys. Plasmas 12, 055704 (2005)], a mirror magnetic field plasma rotating azimuthally. The externally applied parameters that control the plasma characteristics—applied voltage, external magnetic field, and fill pressure—are scanned across the entire available range of values. It is found that the plasma rotation velocity does not exceed the Alfven velocity, in agreement with the equilibrium requirements of magnetically confined plasmas. Measured rotation velocities are also lower than the critical ionization velocity in hydrogen, but a strict limit was not observable within MCX parametric capabilities.

Sub-Alfvénic velocity limits in magnetohydrodynamic rotating plasmas

Magnetized plasmas in shaped fields rely on large, supersonic rotation to effect centrifugal confinement of plasma along magnetic field lines. The results of experiments on the Maryland Centrifugal Experiment (MCX) [R. F. Ellis et al., Phys. Plasmas 12, 055704 (2005)] to document velocity limits are reported. Previous results have shown a limit at the Alfven speed, consistent with equilibrium limits from ideal magnetohydrodynamic theory. Another speed limit, previously reported as possibly related to a critical ionization phenomenon and depending only on the ion species and the shape of the confining magnetic field, is investigated here for a broad range of the applied parameters. We show that this speed limit manifests at sub-Alfvenic levels and that, as externally applied torques on the plasma are increased, the extra momentum input shows up as enhanced plasma density or lower momentum confinement time, accompanied by an increase in the neutral radiation level. Several key parameters are scanned, including the mirror ratio, the length between insulators, and the species mass. We show that this velocity limit is consistent with the species-dependent critical ionization velocity postulated by Alfven.

Experimental verification of the dielectric constant of a magnetized rotating plasma

Direct measurements confirm that the magnetized plasma perpendicular dielectric constant has a linear dependence on the plasma density for fixed magnetic field, as predicted by magnetohydrodynamic (MHD) theory. In experiments performed on the Maryland Centrifugal Experiment (MCX) [R. F. Ellis, A. B. Hassam, S. Messer, and B. R. Osborn, Phys. Plasmas, 8, 2057 (2001)], line-averaged hydrogen plasma electron density is measured using a Mach–Zehnder interferometer. For small rotational Alfven Mach numbers, the measured size of the perpendicular plasma dielectric constant is also in agreement with MHD theory. Plasma density in the range of (1–8)×1020m−3 and relative perpendicular plasma dielectric constant of the order of 106 are measured.

Nonlinear mode coupling and sheared flow in a rotating plasma

Shear flow is expected to stabilize the broad spectrum of interchange modes in rotating plasmas. However, residual fluctuations may still persist. To investigate the presence of such fluctuations, sixteen magnetic pickup coils equally spaced on a crown have been mounted inside the vacuum vessel, at the edge of a rotating plasma in mirror configuration. A comprehensive analysis of the magnetic fluctuations shows that very low spatial mode numbers survive under the imposed shear flow. Nevertheless, temporal Fourier analysis reveals a broadband frequency spectrum. Clear evidence of nonlinear mode coupling is obtained using higher-order spectral analysis, namely the bispectrum and the bicoherence. Two-dimensional simulations of magnetohydrodynamic equations with gravity and imposed shear flow quantitatively model the spatio-temporal characteristics of the observed magnetic fluctuations.

Cross-field plasma injection into mirror geometry

The Maryland Centrifugal Experiment (MCX) and HyperV Technologies Corp. are collaborating on a series of experiments to test the use of a plasma gun to inject mass and momentum into a magnetic-confinement device. HyperV has designed, built and installed a prototype coaxial gun to drive rotation in MCX. The gun has been designed to avoid the blow-by instability via a combination of electrode shaping and a tailored plasma armature. Preliminary measurements at HyperV indicate the gun generates plasma jets with a mass of 160 µg, velocities up to 90 km s−1 and plasma density in the high 1014 cm−3. This paper emphasizes characteristics of the plasma gun and penetration of the plasma jet through the MCX magnetic field. Plans for future injection experiments are briefly discussed.

Analysis and modeling of edge fluctuations and transport mechanism in the Maryland Centrifugal Experiment

The Maryland Centrifugal Experiment [R. F. Ellis et al., Phys. Plasmas 12, 055704 (2005)] is a mirror machine designed to have a plasma axially confined by supersonic rotation and dominantly interchange stable by the radial shear in the azimuthal velocity. Nevertheless, residual fluctuations still persist. To investigate the presence of such fluctuations, an azimuthal array of 16 magnetic pickup coils at the edge region of the plasma has been employed. A comprehensive analysis of the magnetic fluctuations reveals that, under the imposed shear flow, only m=0 and m=2 modes are dominant; yet, the observed frequency spectrum is broadband. Using higher order spectral analysis, clear evidence of nonlinear mode coupling is detected. It is also observed that the amplification of magnetic fluctuations leads to enhanced transport consistent with the drop of the plasma density and voltage. As a result, the magnetic fluctuations start to decrease in amplitude as the central plasma pressure drops. In return, the anoma...

Observations and analysis of magnetic fluctuations in the Maryland centrifugal experiment

Initial results from magnetic probes on the Maryland Centrifugal eXperiment (MCX) [R. F. Ellis et al., Phys. Plasmas 8, 2057 (2001)] provide details of the propagation and azimuthal mode structure of magnetic fluctuations in the edge region. Magnetic coils placed azimuthally along the edge measure changes in the axial magnetic field during the time history of the plasma discharge. The eight evenly spaced coils can resolve azimuthal modes up to m=3. The plasma rotates azimuthally in MCX due to an applied radial electric field. Using a variety of different analysis of the data, it is inferred that the magnetic fluctuations are dominantly convected by the plasma rotation for several rotation periods before significant decorrelation. These findings help to identify the modes at the edge and indicate that there are a few low mode numbers that are dominant during the discharge. Also, the speed of rotation of the modes is found to change dramatically from the High Rotation (HR) state to a low rotation ordinary (...