When "the fog comes on little cat feet," we want to see what it's hiding. The millimeterwave regime of the electromagnetic spectrum can show us-if we have the necessary vision. bed}. :is opposed 1o a perfect reflector, which has ,:' = 0 The emissivity of an object (v_hich is polauization-dcpendent) is a function of the dielectric properties of its co_lstitUClllS, its surf4cc roughness, and the angle of observation (,% samplcc_l-the mea_,uredemissixitiesol dixers materials at various frequencies is ei,,en in the table on the next page.) The radiation intensity of a 300-Kelvit+ blackbody t+all_ e×ponentialt> by about eight orders of magnitude from a peak \alue in the 1R to the millimeter-_,ave regime at around 94 GHz (Figure 1). This large dccrease m mtensit._, is partially corn pensated for bv the lower photon energy, th:tt occurs at millimeter-,aave fre quencies. H_,aexcr this '-,iluatioll is dramaticall\ reversed in fog and other inclenlent _ve_tther \_hcn one lakes into accotlilt the _,ignal attelltlation bv atlnospheiic constituents. Ilele the ,qrength of the propagated signal peaks in the millimeter _._,:l_.c region, as the figtire sho_,s. Fhu-,,ecnml reas(m, haldv, 41c Itlllitalions, is due tO the low inillilneler-wave pnv, cl Ilu,, hul i,, not the probleln it once ,aas. Se',eral recent technological advances haxe enabled the exploitation of millimeter waves. Receivers with mixcl ll(ml ends ti_,hlg Scholtky-b;_uricr diodes have demonstrated double ",idchand i_oise figure', :_I (_,to l(} dB over the _4 to 300 GHz regime,
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 ...
Fueling and heating of tandem mirror end cells by rf at the ion-cyclotron frequency have been studied experimentally in the Phaedrus Tandem Mirror. The end cell density is found to increase linerly with rf voltage. The total plasma energy is observed to increase with rf power with no evidence of saturation at high power. The plasma axial length decreases with increasing rf power down to a length of approximately the distance between the two resonance locations in the end cell. The highest density and average ion energy are achieved with the resonance closest to the midplane. It is necessary to assume that the rf electric field at the resonance decreases with increasing density and with distance from the antenna in order to model the ion particle and power balance. The particle and power balance model predicts that dense, hot end cells may be maintained with E+∼1 V/cm and a fueling efficiency of a few percent when the resonance is close to the midplane because of better ion confinement and small plasma volume. Monte Carlo simulation of the trapping process shows that E2+ ∝T1.5ic is required to maintain a given rf trapping efficiency.
A limit to the ratio of central-cell to plug p of 1.8 ± 1.0 is measured in the Phaedrus tandem mirror for central-cell jS's of 2% to 5%. This is consistent with the limiting ratio of 2.0 predicted for interchange stability, and is the first experimental test of the effect on the interchange limit from varying the plasma pressure along a magnetic field line in a tandem-mirror configuration.
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