Tandem Mirror Experiment Research Articles

Electrostatic Plasma-Confinement Experiments in a Tandem Mirror System

Results from the tandem mirror experiment are described. The configuration of axial density and potential profiles are created and sustained by neutral-beam injection and gas fueling. Plasma confinement in the center cell is shown to be improved by the end plugs by as much as a factor of 9. The electron temperature is higher than that achieved in our earlier 2XIIB single-cell mirror experiment.

Drift-mode stability analysis for the Tandem mirror

Drift-mode stability analysis for the Tandem Mirror Experiment is formulated assuming MHD-stability from magnetic field curvature and plasma rotation. The self-consistent field equations describe two potentially unstable drift modes. For k‖ ∼ 2π/Lc, where Lc is the length of the central cell, the electron drift wave occurs with ω ∼ ω*eS(b, ηi) ∼ k‖vA with a radial localization to the region of maximum electron diamagnetic drift frequency ω*e. For flute-like axial modes, the ion drift wave ω ∼ ω*i > k‖vA occurs. The ion drift mode is less radially localized and extends to high frequencies for large k⊥ρi. Stability conditions as a function of ηi, ηe, β and Ti/Te are developed for the drift modes including the guiding-centre drifts and bounce frequency resonances with the electrons.

Analytic approximation to resonant plateau transport coefficients for tandem mirrors

Analytic approximations to resonant plateau diffusion coefficients for ions in a tandem mirror solenoid are developed. These are obtained by assuming that the ∇B azimuthal drift is small compared to E⃗×B⃗ contribution, and that the radial drift per bounce is determined by properties of the vacuum magnetic field. The resulting expressions are used to estimate radial transport life-times for the Lawrence Livermore Laboratory Tandem Mirror Experiment (TMX) and for a proposed future experiment, MFTF-B. These estimates indicate radial loss comparable to axial for TMX, but smaller than axial for most operating modes considered for MFTF-B.

Destruction of cyclotron resonances in weakly collisional, inhomogeneous plasmas

It is shown, both analytically and numerically, that cyclotron resonances can be destroyed in dense (ωp≳Ω, where ωp is the plasma frequency and Ω is the cyclotron frequency), weakly collisional, inhomogeneous plasmas when (ν/Ω) k2r2L≳1, where ν is the collision frequency and rL is the mean Larmor radius. The theory is based upon a model Fokker–Planck equation. It is found that the particles make a transition from magnetized to unmagnetized behavior. This is an important result since it indicates that the ion-and electron-cyclotron-drift instabilities transform into their unmagnetized counterparts, the lower-hybrid-drift instability and the ion-acoustic instability, respectively. The ion-cyclotron-drift (or drift-cyclotron instability) is examined in detail and is found to become the lower-hybrid-drift instability in the region of maximum growth when (me/mi)1/2ω/Ωi≳νii/ Ωi ≳me/mi for Te≈Ti plasmas. The first inequality is required to overcome electron viscuous damping, while the second allows the ions to become ’’unmagnetized.’’ Applications to the equatorial F region of the ionosphere and the tandem mirror experiment (TMX) are discussed.

Superconductivity for mirror fusion

Mirror experiments have led the way in applying superconductivity to fusion research because of unique requirements for high and steady magnetic fields. The first significant applications were Baseball II at LLL and IMP at ORNL, which used multifilamentary niobium-titanium and niobium-tin tape, respectively. Now the USSR at Kurchatov is building a smaller baseball coil with a 6.5 mm square multifilamentary niobium-titanium superconductor similar to the Baseball II conductor. However, the largest advance in fusion magnets will be used in the Mirror Fusion Test Facility (MFTF) now under construction at LLL. Improvements in the technology of the previous LLL experiment, Baseball II, have been made using new conductor joining techniques, a ventilated wrap-around copper stabilizer, and stronger structural welding methods. The MFTF coil winding is proceeding on a separate former to allow parallel construction of the main structure. Not only does this shorten the project schedule to equal that of other conventional constructions, but a second vacuum barrier is created between the magnet helium and the plasma environment for reliable operation. In the future, LLL envisions a superconducting version of the Tandem Mirror Experiment and a possible hybrid reactor leading to economical fusion power.

Tandem‐mirror machines promise better efficiency

A major problem with magnetic‐mirror devices for plasma confinement has always been the leakage of particles out of the ends of the machine. An experiment now under construction at Lawrence Livermore Laboratory, known as the Tandem Mirror Experiment (TMX), proposes to turn this failing neatly to advantage by employing two mirror‐pairs, one at each end of a long straight solenoid. Each pair of mirrors confines positive ions by minimum‐B magnetic‐field wells, and the excess positive charge that develops in these “end cells” confines ions within the solenoid. In other words, the two mirror pairs act as plugs for the ends of the solenoid. Calculations for a reactor based on this concept predict that, under appropriate neutral injection conditions, a plasma of a density and temperature sufficient for controlled fusion will build up in the solenoid. The purpose of the TMX experiment is to test the principles of the concept. Other tandem‐mirror experiments are planned or already under construction at Novosibirsk in the Soviet Union, at the University of Tsukuba in Japan, and at the University of Wisconsin at Madison.