Low-frequency fluctuations are observed in a plasma confined by a strong dipole magnet and containing an energetic high-pressure population of trapped electrons. The quasi-coherent fluctuations have frequencies characteristic of drift frequencies of the lower temperature background plasma and have large toroidal and radial extent. They are excited throughout a wide range of plasma conditions determined by the level of neutral gas pressure. However, for a sufficiently high rate of neutral gas fueling, the plasma density profile flattens and the fluctuations disappear.

The Levitated Dipole Experiment (LDX) is an innovative confinement concept that uses an internal superconducting dipole field to confine plasma. Plasma equilibrium is calculated by a least-squares fit of an anisotropic pressure model to magnetic measurements constrained by X-ray images. Reconstructions have been done for different heating schemes using two-frequency electron cyclotron heating at 2.45 and 6.4 GHz. Results show that a maximum local β ∼ 20% has been achieved using two frequency heating at a combined full power of 5 kW. Analysis of the reconstruction results shows that the magnetic sensors are sensitive primarily to changes in the plasma dipole moment. This is partly due to the fact that the dipole current decreases as the plasma current increases (as required by flux conservation through the superconducting dipole) and the magnetic sensors detect the sum of these changes. This paper will present details of the reconstruction procedure and describe how new magnetic sensors will aid in resolving the pressure profile more accurately.

Plasmas in the Levitated Dipole Experiment (LDX) are formed and sustained currently via two electron cyclotron resonance heating (ECRH) sources: 2.5 kW at 2.45 GHz and 2.5 kW at 6.4 GHz. An important topic being investigated is how varying the ECRH affects the confinement and stability of the plasma. We report the results of using different operational combinations of our RF sources, such as varying the power composition, changing the power levels, and sequencing of the onset time.

The levitated dipole experiment (LDX) explores the physics of high-temperature plasmas confined by a dipole magnetic field. Stable high-beta plasma has been created and confined by the magnetic field of a superconducting coil. Discharges containing trapped electrons form when microwaves cause strong perpendicular heating at cyclotron resonance. To eliminate the losses to the supports, the magnetic dipole (a superconducting solenoid) will be magnetically levitated for several hours. The dipole magnetic field is generated by a Nb3Sn floating coil (F-coil), a maximum field of 5.3 T, operating for up to 2 h. A NbTi charging coil (C-coil) surrounds a portion of the vacuum chamber and induces the current in the floating coil. After the F-coil is lifted to the center of the chamber, the levitation coil (L-coil), made from high-temperature superconductor, magnetically supports it. In the first year of operation, the device has been operated in a supported mode of operation while experience has been gained in the cryogenic performance of the F-coil and the integration of the F-coil and C-coil. Current work focuses on the integration of the F and L coils in preparation for first levitation tests. (c) 2006 Published by Elsevier B.V.

The Levitated Dipole Experiment (LDX) [J. Kesner et al., in Fusion Energy 1998, 1165 (1999)] is a new research facility that is exploring the confinement and stability of plasma created within the dipole field produced by a strong superconducting magnet. Unlike other configurations in which stability depends on curvature and magnetic shear, magnetohydrodynamic stability of a dipole derives from plasma compressibility. Theoretically, the dipole magnetic geometry can stabilize a centrally peaked plasma pressure that exceeds the local magnetic pressure (β>1), and the absence of magnetic shear allows particle and energy confinement to decouple. In initial experiments, long-pulse, quasi-steady-state microwave discharges lasting more than 10s have been produced that are consistent with equilibria having peak beta values of 20%. Detailed measurements have been made of discharge evolution, plasma dynamics and instability, and the roles of gas fueling, microwave power deposition profiles, and plasma boundary shape. In these initial experiments, the high-field superconducting floating coil was supported by three thin supports. The plasma is created by multifrequency electron cyclotron resonance heating at 2.45 and 6.4GHz, and a population of energetic electrons, with mean energies above 50keV, dominates the plasma pressure. Creation of high-pressure, high-beta plasma is possible only when intense hot electron interchange instabilities are stabilized by sufficiently high background plasma density. A dramatic transition from a low-density, low-beta regime to a more quiescent, high-beta regime is observed when the plasma fueling rate and confinement time become sufficiently large.

High frequency (f > 1 MHz) electrostatic fluctuations have been observed in high-beta plasma created in the Levitated Dipole Experiment (LDX). We have previously identified these fluctuations as the Hot Electron Interchange (HEI) instability(1). New observations have been made in the presence of the magnetic levitation fields. We find the HEI mode is characterized by frequency sweeping at the drift-resonance of trapped energetic electrons. The fluctuations often appear with coherent structures that have been detected on fast high-impedance electrostatic probes and edge Mirnov sensors. We observe phase shifts using multiple probes that will enable us to determine the toroidal mode number (m) and a higher sampling rate reveals frequency sweeping as high as 40 MHz. Measurements that characterize these modes now incorporate fast magnetic measurements in an attempt to put together a coherent picture of plasma behavior during these modes, including the consequences of these instabilities on plasma formation and pressure limits. (1) E.Ortiz to appear in J. Fus. Energy (2006).

The Levitated Dipole Experiment (LDX) has created high-beta, hot-electron plasmas that are confined by a strong dipole electromagnet via multiple-frequency electron cyclotron resonance heating (ECRH). Multiple frequency ECRH is used to investigate how variation of the power deposition profile may be used to adjust the plasma density and pressure profiles. The initial experiments have been performed using up to 3 kW at 2.45 GHz and 3 kW at 6.4 GHz. Variations included switching on and off a single source while injecting constant power with the other source. We have also investigated the role of magnetic shaping, using external coils, on ECRH phenomena and plasma profile control. The preliminary results of these experiments will be presented.

The Levitated Dipole Experiment (LDX) explores confinement and stability of plasma created within the dipole field of a strong superconducting magnet. During initial experiments, long-pulse, quasi-steady state discharges that last more than 10 s and have peak beta of more than 20% are studied. The plasma is created by multi-frequency electron cyclotron resonance heating (ECRH) at 2.45 and 6.4 GHz. A population of energetic electrons, with mean energies above 50 keV, dominates the plasma pressure. Creation of high pressure, high beta plasma is possible only when intense hot electron interchange (HEI) instabilities are stabilized by sufficient neutral gas fueling. The instabilities resonate with the magnetic drift motion of the energetic electrons and can cause rapid radial transport. Measurements of the electrostatic and magnetic fluctuations of the HEI instability are described along with observations of the instability's spectral characteristics. Fluctuations of the outer poloidal field induced by the HEI show a rapid evolution of the perturbed pressure profile.

A dipole-confined plasma is expected to be stable to interchange modes or convective cells wherever the following relation is satisfied: $\frac{P_{edge}}{P_{core}}\leq\left(\frac{{V_{core}}}{V_{e dge}}\right)^{\gamma}$, where P is the pressure, $V\equiv\oint\frac{dl}{B}$, and γ = 5/3. One of the missions of the Levitated Dipole Experiment is to attempt to exceed the marginal stability condition and drive instabilities. We have two “knobs” with which to explore the parameter space: we can can change the pressure by varying the heating profile, by adjusting the total RF input power as well as the relative power in multiple frequencies; and we can change the magnetic geometry. Initial results of scans of the RF power in two frequencies and scans of the magnetic field are presented. • This work is supported by U.S. DOE Grants DE-FG0298ER54458 and DE-FG02-98ER54459.

The levitated dipole experiment (LDX) explores the physics of high-temperature plasmas confined by a dipole magnetic field. Stable high-beta plasma has been created and confined by the magnetic field of a superconducting coil. Discharges containing trapped electrons form when microwaves cause strong perpendicular heating at cyclotron resonance. To eliminate the losses to the supports, the magnetic dipole (a superconducting solenoid) will be magnetically levitated for several hours. The dipole magnetic field is generated by a Nb3Sn floating coil (F-coil), a maximum field of 5.3 T, operating for up to 2 h. A NbTi charging coil (C-coil) surrounds a portion of the vacuum chamber and induces the current in the floating coil. After the F-coil is lifted to the center of the chamber, the levitation coil (L-coil), made from high-temperature superconductor, magnetically supports it. In the first year of operation, the device has been operated in a supported mode of operation while experience has been gained in the cryogenic performance of the F-coil and the integration of the F-coil and C-coil. Current work focuses on the integration of the F and L coils in preparation for first levitation tests. © 2006 Published by Elsevier B.V. PACS: 52.55.−s; 52.50.SW; 52.35.−g