The existence of a non-axisymmetric magnetic field in the transition regions between the central cell and end-plugs of a tandem mirror device can lead to significant radial transport of central-cell ions. Self-consistent calculation of the consequences of this non-ambipolar process requires the solution of a highly non-linear charge balance equation for the ambipolar potential. In this paper, radial transport in tandem mirrors is studied, with particular emphasis on the charge balance equation and its consequences. A timedependent radial transport code is presented. Simulations of the Tandem Mirror Experiment (TMX) are performed. Generally, good agreement between code and experiment is obtained. The phenomenon of quenching of radial transport is analysed and demonstrated numerically.
Nonambipolar transport has been measured in the tandem mirror TMX-U [Phys. Rev. Lett. 53, 783 (1984)] by applying charge conservation to the measured electron currents to the end walls. The resulting confinement time τ⊥ is found to depend upon the central-cell potential φ approximately as τ⊥(msec) =3φ(kV)−2. The transport rate, deduced from the data, agrees to within a factor of 1–5 with resonant-transport theory applied to the measured plasma parameters. Attempts to include radial effects by modeling the plasma self-consistently using resonant transport are less successful; near the axis the transport coefficients become too small to explain the equilibrium. Modeling using an ad hoc φ−2 law for the transport coefficients is more successful.
A low-power 60 GHz gyrotron collective Thomson scattering diagnostic has been operating on TFTR to test the feasibility of detecting alpha particles when scattering perpendicular to the magnetic field. An enhanced scattered signal is predicted to result from the interaction of the energetic ions with plasma resonances in the lower hybrid frequency range. Millimeter-wave power levels at the plasma were approximately 200 W for typical pulse lengths of 50 ms. Deuterium and possible fusion product ion cyclotron frequencies and their harmonics were observed during neutral beam heating. These spectra are similar to ion cyclotron emission spectra which are detected with radio-frequency probes on TFTR at the plasma edge. Also, ion cyclotron resonance heating fundamental and harmonic fluctuations were observed. However, a signal has not been definitively detected in the lower hybrid frequency range which correlates to alpha particles. Broadband noise was observed during neutral beam heating which is greater than the predicted alpha particle signal.
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