|icctalk.pdf||2006-02-21 14:07:12||Alan Hoffman|
Confinement and Current Drive Measurements for Steady State FRCs
Author: Alan L. Hoffman
Submitted: 2005-12-09 12:53:41
Co-authors: R.D. Milroy, H.Y. Guo, K.E. Miller
University of Washington
14700 NE 95 St.
Redmond, WA 98052
FRCs are diamagnetic entities and, at the most basic level, FRC confinement is dependent on the power needed to overcome ohmic voltages and prevent flux loss. Rotating Magnetic Fields (RMF) can overcome electron-ion friction by exerting a force on the electrons, and its application has been studied in the Translation, Confinement, & Sustainment (TCS) FRC device. The RMF force is exerted mostly near the FRC edge due to a self-regulating partial penetration, and is transferred throughout the FRC by various mechanisms. Analysis of the sustainment process thus involves equating the RMF edge drive torque with the total resistive torque. Balancing these torques results in a density scaling of nm(10^19m-3) ~ 50Bw^2(mT)/eta^1/2(uOhm-m)Be(mT), where nm is the peak density, Bw is the RMF magnitude, Be is the external confinement field, and eta is an average resistivity near the FRC field null. Typical experimental conditions produced in TCS with Bw = 5 mT were Be = 18 mT and nm = 2.0x10^19m-3. Measurements of this type can be used to determine the scaling of cross-field resistivity, which is of general interest for both confinement and current drive efficiency. Data over a wide range of densities yield a scaling of eta = etao/nm^1/2(10^19m-3) with etao = etao(Tt, rs) where Tt is the total plasma temperature and rs is the FRC separatrix radius. etao = 50 uOhm-m for TCS conditions with Tt ~ 40 eV and rs ~ 38 cm. The resistivity scaling dependence on ne^-1/2 is identical to that measured in high density theta-pinch formed FRCs. The temperature range was limited in TCS by a high impurity content and resultant radiation barriers, so the scaling with Tt is less certain, but the data that does exist (at early times before impurities were ingested, and for anti-symmetric RMF drive which reduces other losses) strongly support a Tt^-1/2 dependence for etao. Scaling from the smaller STX device suggests a 1/rs dependence. Many measurements point to a much higher resistivity in the narrow edge layer where the RMF drive produces high electron drift velocities vde (relative to the ion sound speed vs) which are necessary for its penetration. When the ratio of Bw/Be is high, this high edge layer resistivity is also attested to by excess RMF power absorption proportional to the axial screening currents. When the ratio of the screening jz to the azimuthal jtheta is reduced, either by running at higher temperatures or lower values of Bw (resulting in Be/Bw ratios up to 6), this excess power dissipation disappears. The results are strongly suggestive of a Chodura type resistivity which is high when the ratio vde/vs exceeds unity. New experiments in TCS/upgrade, which is designed to reduce impurity content and utilize anti-symmetric RMF drive, should shed better light on the resistivity temperature dependence, and significantly advance the knowledge base of the attractive, singly-connected, high beta, steady-state FRC reactor concept.