Operation of the Tandem-Mirror Plasma Experiment with Skew Neutral-Beam Injection

Abstract

Tandem-mirror plasma with sloshing-ion end plugs has been produced. Ion-velocity distribution functions are peaked at 47° to the magnetic field lines. Low-level ion-cyclotron fluctuations were detected at 1.8 times the plug-midplane frequency and with properties predicted by theory; they did not degrade plasma confinement in the plugs or central cell.PACS numbers: 52.55.KeThe tandem mirror employs mirror-confined plasmas at each end of a long central solenoid to generate electrostatic potentials for confining central-cell ions. 1 ' 2 Tandem-mirror plasmas have been produced by plasma-gun, 3 neutralbeam, 4 and ion-cyclotron-radio-frequency heating. 5 In our previous tandem-mirror experiment (TMX), the end-plug neutral beams were injected at 90° to the magnetic field lines. In the present experiments (TMX-J7), we produced a tandemmirror plasma with sloshing-ion end cells by injecting neutral beams at a skew angle of 47°. For the first time in a beam-driven mirror machine we did not observe fluctuations with the plug-midplane ion-cyclotron frequency. Lowamplitude fluctuations were observed that have the characteristics expected for skew injection. However, we observed no degradation in confinement because of microinstabilities and no heating of escaping central-cell ions by end-plug rf.When ions are injected at a skew angle 6 0 to the magnetic field lines at the field minimum, they are reflected at a field strength where B-Bj sin 2 # 0 . As they bounce or slosh back and forth, their axial velocity is zero at the point of turnabout, thereby causing the density to peak near the turning points. The lower density near the plug midplane produces an axial potential profile with a dip that electrostatically confines low-energy ions and aids in stabilizing loss-cone modes. 6 " 9 As with 90° injection, central-cell losses provide additional ions for stability. In the present experiments, this latter unconfined component is an important ingredient in achieving microstability, which will be greatly reduced ultimately in the thermal barrier configuration. A significant feature of the present experiments is that beam injection at an angle also reduces the anisotropy that drives the Alfv6n ion-cyclotron mode. 10 Such an angular-velocity distribution function has previously been created in gunproduced plasmas. 11 " 13 To confine ions injected at 47° in a mirror configuration, the TMX-C/end plug has a 4:1 mirror ratio 14 ' 15 [compared to 2:1 in TMX (Ref. 4) and 2XIIB (Ref. 16)]. The plug-midplane magnetic field is 0.5 T, and the central-cell field is 0.3 T. The mirror-to-mirror distance of the end plugs is 3 m and of the central cell is 8 m. To maintain minimum-^ magnetohydrodynamicstability properties without severe elliptical distortion in the plasma cross section, the magnet generates a radial magnetic well that increases by only 0.5% at the plasma edge, which is shallow compared with 4% in previous experiments. 4,16 The present experiments show that such shallow wells provide magnetohydrodynamic stability as predictedcby theo...

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Axisymmetric Magnetic Mirror Fusion-Fission Hybrid

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