Dandl, R.A.

Eason, H.O.

England, A.C.

Ikegami, H.

UNT Digital Library

The ELMO Bumpy Torus (EBT) ExperimentsR. A. Dandl, H. 0. Eason, A. C. England, H. IkegamiOak Ridge National LaboratoryOak Ridge, Tennessee 37830The bumpy torus has received very much1less attention than othertoroidal confinement schemes, primarily because of anticipated MHD instabil-ities and superbanana losses. [Several theoretical papers have pointed outits difficulty with the interchange stability criterion.]In an attempt to circumvent these instabilities and permit studies ofplasma confinement, we have constructed the ELMO Bumpy Torus (EBT), a specifictoroidal array closely modeled after stable, steady-state, high-beta, hot-electron plasmas produced earlier in the ELMO open-ended trap by electron-cyclotron heating. The EBT is expected to derive MHD stability from theaverage minimum-B generated by the high-beta, mirror-confined, hot-electronshells typical of the ELMO plasma. Such annular hot-electron plasmas havebeen experimentally generated in each mirror section of EBT with only theoutermost edges of the annuli line-tied against flutes by cold-plasmaconduction to chamber walls.We are encouraged by the observed stability in the present. EBT, aswell as in straight and canted-mirror experiments, as long as the pressureexceeds a critical value. Although this stability is ascribed to some formof line-tying, there is evidence to indicate that conducting end plates maynot be necessary. It is possible that the stabilization stems from the elec-trostatic confinement of a cold-electron component produced by ionization ofthe ambient background gas [1]. >'MASTER*Research sponsored by the U. S. Atomic Energy Commission under contractwith tha Union Carbide Corporation.Bumpy torus confinement of single particles has. been shown by a numberof authors [2] for a wide range of magnetic parameters. The neoclassicaldiffusion coefficient is given approximately by [3,4]D - \ <vT2> -g-^-z .where 0 is the precessional frequency, vT is the toroidal drift velocity, andv is the effective collision frequency. The plasma confinement time of EBTin a collisionless3 neoclassical regime can be approximately given byR 2T " (I) *ei 'where R/a is the aspect ratio of the torus. Because the character of theparticle drift surfaces is independent of mass, no distributed space chargedifficulties seem likely, so that we can discuss the equilibrium from thestandpoint of a charge-neutral guiding-center-fluid. We cannot, however,ignore the plasma currents which are expected to modify the magnetic fieldsignificantly. The effects are studied experimentally since it is difficultto obtain a unique pressure profile theoretically from first principles.Part of our program is therefore to optimize the vacuum field and to developthe analytical and experimental techniques for study and control of theequilibrium.A schematic of EBT is shown in Fig. 1. The 24 coils are arranged in thebumpy torus configuration so as to give a mirror ratio of 2:1 with 1;-° cantangle between the planes of adjacent coils. This requirement is based uponexperiments in the canted-mirror facility in which total storedplasma energy was observed to decrease by only 25% for 15° as comparedto 502 for 22.5° cant angle. The major radius of the torus is 150 cm with anaspect ratio 10:1. The radius of the cavity sections enclosing hot-electronannuli is 30 cm with a maximum nridplane magnetic mirror field of 6500 gauss.The plasma is formed and heated fay two 18-GHz klystron amplifiers with30 kW (cw) divided into 24 equal parts in order to provide separate powerfeed to each individual mirror region of the torus. Resonant power feed toeach mirror region is necessary for balanced excitation, because of the strongplasma absorption of power at the electron-cyclotron frequency. Each klystronfeeds alternate mirror regions in order to maintain over-all power feedbalance. A significant feature of the microwave technology is the degree ofcontrol of plasma parameters, such as density, temperature, amount of hotelectrons, and consequently the location of drift surfaces, by the use ofresonant and off-resonant frequencies. Two traveling-wave amplifiers at 55GHz with 10 kW cw generate the off-resonant power which is almost entirelyabsorbed by the hot-electron annul us.The average plasma density was determined by microwave interferometryand by measuring the perpendicular energy by diamagnetic loops coupled to thehot-electron shells; radial profiles, by skimmer probes; bremsstrahlung anal-ysis allowed estimation of the hot-electron temperature and density; andLangmuir probes are used to determine the cold-plasma temperature and densityprofiles.Magnetic field errors are known to destroy plasma confinement in closed-line devices, such as bumpy tori and multipoles, by causing the magneticfield lines to spiral out and strike the wall. In order to esti-mate the amount of error field in EBT, the shift of a pulsad electron beamtrajectory caused by an artificially imposed error field was measured. Assum-ing the error field to be due to the asymmetry of the toroidal coil assembly to beuniformly distributed over the entire toroidal axis, the error field, relativeto the toroidal field strength was measured to be about 1.5 x 1 0 . A morecomplete analysis of plasma losses due to error fields shows them to be thedominant loss mechanism in EBT for a pwith the electron temperature 100 eV.1 r\ ndominant loss mechanism in EBT for a plasma of density less than 10 cmPreliminary experimental results were obtained with four out of the 24cavities energized with 18 GHz resonant power (full power operation will beunder way in mid-March).The diamagnetic signals, which show the stored energy per cavity to in-crease linearly with the microwave power input, indicate that macrostable annularplasmas have been formed in each of four cavities with betas around 0.5.The steady-state, hot-electron component is estimated to have a tempera-ture of about 300 keV, forming a shell structure with a diameter of about25 cm at the midplane. Average plasma density is measured from the microwave12 -3interferometer to be 1 x 10 cm and, after turn-off of the microwave12 -3heating power, plasma density remains at 10 cm for about 1 msec (Fig. 2)11 _•>then decrease to 3 x 10 cm in the next 1 msec, and about 7 or 8 msec laterit again decays abruptly. At this time a dudden burst of x-rays was observed,as shown in Fig. 2, indicating the destruction of the hot-electron shells.These observations indicate some stabilization effects from the high-betaannulus with only four cavities energi2ed.The cold-electron temperature as measured by Langmuir probes indicatethat the plasma outside the hot-electron shell has a temperature of about 4 eVfilling the region adjacent to the wall. The electron temperature shows asteep change across the hot-electron shell, that is, 10 V/cm, and the tempera-ture of the toroidal plasma just on the inner edge of the hot-electron shellis observed to be more than 20 eV. The election temperature of toroidally-confined plasma cannot be conveniently measured by this probe method. We arepreparing to use soft x-ray detectors to measure the temperature of thiscomponent.The electron density radial profile measured by the ion saturation cur-rent also delineated the confined plasma inside the hot-electron shell fromthe unconfined cold plasma outside the shell. -NOTICE-This report was prepared as on account of worksponsored by the United States Government. Neitherthe United States nor the United States Atomic EnergyCommission, not any i-f (licit employee!,, nor any oftheir contractors, subcontractors, or their employees,makes any warran y, express or implied, or assumes anylegal liability or responsibility for the accuracy, com-pleteness or usefulness of any information, apparatus,product or process disclosed, or represents that its usewould not infringe privately owned rights.Expected Information by JuneWe expect to obtain the following experimental information by June.• By controlling the microwave power and gas pressure, we hope to study thetransport mechanisms of the EBT plasma from the medium collisional regimeinto the co.lisionless regime.• We also expect to control the plasma pressure in the hot-electron shell andto some extent its geometry so that MHD stabilization can be optimized.These results could provide insight into stable, hign-beta toroidalconfinement.REFERENCES[1] GUEST, G. E., HARRIS, E. G., Phys. Rev. Letters 27 22 (1971) 1500.[2] KADOMTSEV, B. B., POGUTSE, 0. P., Nucl. Fusion TJ_ (1971) 67.C3] KOVRIZHNYKH, L. M.f MATT-TRANS-82 (Oct. 1968).[4J KOVRIZHNYKH, L. M., Sov. Phys. JETP 29_ (1969) 475.55 GHi (UORH) MICROWAVE FEEDII OF 8 ) -BUS ASSEMBLY (1 OF 8)(9 ̂ v^a;/? &-MI8GHJ (ECH) MICROWfiVE FEED ( I OF 24)DIAMflGNETIC LOOPS (EOF 48)RADIAL SKIMMER (I OF 48)ION GAUGE (1 OF 24)COIL WATERHEAOER flHYDROGEN LEAK (1 OF 24)FIG. 1. Eimo Bumpy Torus.COImOX-Ray Signal3 4 5 6Time (msec)10Interferometer(70 GHz)FIG. 2. Plasma density decay indicating high-beta annul usstabilization effect. The upper trace is x-ray signal detectedby a small ionization chamber inserted on the inner wall of oneof the four energized chambers (resonant power 18 GHz at 6 kWand off-resonant power 55 GHz at 2 kW). The lower trace is the70 GHz microwave interferometer signal. Steady hydrogen gasfeed at 3 x 10~6 Torr.

ELMO Bumpy Torus (EBT) experiments

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ELMO Bumpy Torus (EBT) experiments

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