Equilibrium and stability studies of plasmas confined in a dipole magnetic field using magnetic measurements

Abstract

The Levitated Dipole Experiment (LDX) is the first experiment of its kind to use a levitated current ring to confine a plasma in a dipole magnetic field. Unlike most other confinement devices, plasma compressibility stabilizes and allows the plasma to attain a peak beta on the order of unity. Various magnetic sensors have been designed, calibrated, installed, and operated to reconstruct the plasma pressure profile through least-squares fitting to model profiles. Although both isotropic and anisotropic models are used, the latter is critical in deducing the correct beta values for the electron cyclotron resonance heated (ECRH), anisotropic LDX plasmas. The simpler isotropic models give accurate spatially integrated quantities of the pressure profile. The reconstruction of LDX plasmas requires overcoming unique obstacles. Because the floating coil is superconducting, care must be taken when interpreting magnetic signals, which measure the sum of the plasma current and the decrease in the floating coil current. The coupling of these two quantities, in addition to the large physical distances between the sensors and the plasma, makes the magnetic measurements sensitive mostly to the plasma dipole moment. A family of current and pressure profiles with the same dipole moment can therefore fit the measurements equally well. The ambiguity in determining the pressure profile is resolved by supplementing the magnetics with X-ray emissivity data. Internal flux loops close to the plasma will be added in the future to resolve higher order moments.

(cont.) A peak beta of more than 26 % has been measured by the magnetic sensors to date. The high beta plasmas are shown to have supercritical pressure profiles that exceed the MHD limit. The finding confirms that the ECRH produced hot electrons that carry most of the pressure are minimally sensitive to the limit. The MHD gradient limit is slightly increased by incorporating pressure anisotropy, but magnetic data routinely gives a best fit profile that substantially exceeds even the anisotropic limit. It has yet to be seen whether the kinetic analog of the MHD interchange mode, or the hot electron interchange mode (HEI), plays a significant role in limiting the hot electron density gradient. The HEI's have been magnetically measured and shown to correlate with drops in flux measurements. Lastly, it is revealed that LDX plasmas display a linear scaling of stored energy with plasma current (known as the D-P-S relation), much like magnetospheric plasmas. This scaling is used to estimate the energy confinement time of LDX plasmas with different heating frequency compositions.