Experiments in a Tandem Mirror Sustained and Heated Solely by rf

Abstract

A tandem-mirror plasma has been sustained and heated by rf alone without the need for neutral beams. End plug density and energy are maintained by ion cyclotron-resonance heating which traps and heats a fraction of the central-cell loss stream. The central-cell plasma is maintained by gas fueling and rf heating. Magnetohydrodynamic stability limits the ratio of the central-cell to plug plasma pressure, and the central-cell electron temperature must be kept high enough for ionization. A quasi steady state is achieved that lasts much longer than the decay times of the plugs and central cell.PACS numbers: 52.55. Mg, 52.50.Gj, 52.55.Ke The tandem-mirror approach to the development of a fusion reactor promises to have many advantages over simple mirrors as well as other magnetic confinement devices. 1 In a tandem mirror, a central-cell plasma in a solenoid is bounded by "plug" plasmas in mirror cells. The TMX experiment 2 demonstrated that electrostatic confinement of central-cell ions by the plug plasmas significantly reduced end losses. Calculations indicate that the overall Q (power gain) of a tandem-mirror fusion reactor can be quite high (~5-10) if the volume of the central cell greatly exceeds that of the plugs. 3 In previous experiments, plug plasmas were fueled and heated by energetic neutral beams. However, technological constraints on neutral-beam heating when scaled to a reactor could be eased or eliminated by supplementing or replacing neutral-beam power with rf power. We show in the Phaedrus experiment that a tandem-mirror plasma can be sustained by rf alone, without the application of neutral beams.The operation of a tandem mirror can be thought of as taking place in two stages, a transient stage followed by a quasi-state stage. In the transient stage, plasma is injected into the machine by stream guns. In fact, earlier experiments with Phaedrus 4 and Gamma 6 5 were restricted to the transient stage. Such plasmas side-stepped many of the issues of tandem-mirror physics because plasma characteristics were dominated by the external plasma between the plugs and the stream guns. The high-density external plasma line-tied the plasma to the stream guns, significantly reducing both magnetohydrodynamic (MHD) instabilities and microinstabilities and fixing the electron temperature. When the stream guns were turned off in Phaedrus, the plasma decayed away in approximately 150 jus in the plugs and 400 JUS in the central cell. Quasi-steady-state operation after stream guns are turned off requires sources of particles and energy for both the plug and central-cell plasmas and also adds constraints that must be satisfied in order to maintain microstability and MHD stability. In TMX, fueling and heating were provided by neutral beams in the plugs and gas puffing in the central cell. In the Phaedrus experiment, fueling is provided only by gas puffing in the central cell while rf provides heating. In both experiments, stability was provided by high plug energy density and the presence of the central-cell loss flow.The ...

Simulation of α-channeling in mirror machines

“…15,35 In simulating fuel injection and heating with the same waves used also for ␣-particle energy extraction, two system designs were considered.…”

rf Stabilization of an Axisymmetric Tandem Mirror

“…Fueling in the central ceil is by ionization of hydrogen gas puffed into a gas box, and the plug plasma density is maintained by the trapping of a fraction of the central-cell ion-loss stream by the plug ICRH. 3 No neutral-beam injection is used* Plug and central-cell parameters are n p^nc^ 2xl0 12 cm" 3 , T ic^1 60eV, T^TOOeV, T ec^3 5eV, T e/>^2 0eV, plug mirror ratio = 2.5, central-cell mirror ratio = 11, The ponderomotive force can be written 10…”

Applying Alpha-Channeling to Mirror Machines

“…However, injecting fuel ions near the loss phase-space region for α particles [20,30,31] results in the heating of fuel ions by the same waves that channel the α particle energy, with the fuel ions diffusing into the mirror rather than diffusing out of the mirror [28]. This effect might be particularly useful in devices with many cold fuel ions near the trapped-passing boundary [32].…”