N." "Ohyabu

National Institute for Fusion Science (NIFS-Repository)

§47. Density Profile in the Ergodic (SHC) Boundary Ohyabu, N. A new boundary control schen1e (SHC boundary) has been proposed, which could allow sin1ultaneous achievement of the H-n1ode type confinement improvement and radiative cooling with wide heat flux distribution. In our proposed configuration, a low n1 island layer sharply separates a plasma confining region fron1 an open "ergodic" boundary. The degree of openness in the ergodic boundary must be high enough to make the plasma pressure constant along the field line, which in tum separates low density plasn1a just outside the plasma confining region (the key external condition for achieving a good H-n1ode discharge) from very high density, cold radiative plasma near the wall (required for effective edge radiative cooling). An example of such proposed SHC boundaries for Heliotron typed devices is shown in Fig.l. We examine the constant pressure assun1ption in the ergodic boundary ( one of the key assun1ption of the SHC concept). In TEXT and Tore Supra experiments, however, inversion of the density profile has not observed in the ergodic boundary, which the observed normal temperature gradient there is expected to accompany through the effect of the constant pressure. This may be due to high anomalous diffusion and viscosity or ion-neutral collision, as discussed below. In the ergodic boundary, the ten1perature profile is determined mainly by the electron parallel thern1al conductivity and the power dissipation. For simplicity, we assun1e the temperature profile as illustrated in Fig.2(a). The radial particle flux (1 r (r)) in a steady state is determined by div r = S (particle source) and is high near the wall and becotnes low beyond the ionization zone. The 1r consists of the anmnalous diffusion (- D V l.Il) and the parallel flow (null), 1r (r) =- D V l.Il + <bb I B>null - -- (1) where <Db I B> is the average angle of the field line toward the radial direction. This parallel tlow 52 may cause parallel n1on1entun1 loss due to the anomalous viscosity and the ion -neutral collision, described as * 2 - V 11P - Mn D V ..L u11 - Mnvu11 = 0 - - - (2) where M is the mass of the bulk ion and o* and v are the viscosity and the ion-neutral collision frequency, respectively. Replacing V ..L 2 and V 11 by L\ -2 and <Db I B> V ..L respectively, Eq.(4) becomes null = -<Db I B> V ..LP I M(D*L\-2 +V) -- (3) Eliminating null from Eqs.( 1) and (3 ), we obtain - V _LP/P=(i r+D V ..Ln)M(D* i\-2 +V)/2nT <Db/ B>2 --- - (4). Here P = 2nT is assUined. Eq.(4) is rewritten as V _Ln In = [- V ..LT !f -111r I nD] I (1+11) -(5) where 11 = D(D*i\-2 +V) I Vthi2<Db I B>2 . The density scale length ( V _Ln I nf 1 is plotted as a function of 11 in the Fig.2(b). When 11 << l, V mIn ~ - V _1_ T (f , i.e., the constant pressure assumption is valid. For the opposite extren1e case ( 11 >> 1 ), the conventional relation 1r ~ -D V _Ln is recovered. This constant pressure condition ( 11 << 1 ) provides the key information as to how to generate a SHC boundary in real devices. 3.5 4 I' T(r) (a) V.LD/n Lr ~-R (m) 0 Fig.l Ergodic Boundary r VuP = 0 r"' -DV.LR -r/nD -- - - - -------- -· -- -- --- - i Fig.2 (b) 

Density Profile in the Ergodic (SHC) Boundary

Abstract

Similar works

Full text

Available Versions

National Institute for Fusion Science (NIFS-Repository)