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SUMMARY:"The External Kink Mode in Diverted Tokamaks and the Role of q95"
DTSTART:20150420T103000
DTEND:20150420T113000
DTSTAMP:20260930T194607Z
UID:26bf3301ff21fae3e9e669cd4f1e3d3eb61fbc411f125633626bfd39
CATEGORIES:Conferences - Seminars
DESCRIPTION:Dr. A.D. Turnbull\, General Atomics\, San Diego\, USA\nThe cur
 rent-driven external kink mode is predicted to be unstable in a tokamak wh
 en the edge safety factor at the plasma edge\, qedge\, lies just below a r
 ational value n/m and the current density profile is sufficiently broad. 
  In a cylindrical straight tokamak model\, the corresponding unstable mode
  has toroidal mode number n and poloidal mode number m and is strongly pea
 ked at the plasma edge.  In a torus\, the picture is essentially unchange
 d\, except for additional coupling to neighbouring poloidal harmonics.  T
 he external kink modes have been observed experimentally as the current in
  a tokamak is ramped up and qedge decreases. In particular\, for the case 
 of m = 2 and n = 1 with qedge < 2\, the instability is always encountered.
   However\, for a diverted plasma\, the edge q is infinite and one would 
 naively expect stability to these current driven kink modes.  For many de
 cades\, the limit has been observed instead when q at the 95% flux surface
 \, q95\, reaches 2.0\; the limit q95 = 2\, is still a hard limit and has o
 nly recently been overcome for short times by active magnetic stabilizatio
 n. Ideal stability predictions indicate stability for that case despite th
 e observation of gross instability in the absence of feedback stabilizatio
 n. The conventional view is that the ideal kink mode becomes a tearing mod
 e when the edge rational surface lies inside the finite resistivity plasma
  edge region.  However\, it is shown that instead a resistive kink mode i
 s destabilized by the rapidly increased resistivity at the plasma edge.  
 The resistive kink behaves much like the ideal kink with no sign of a tear
 ing component.
LOCATION:PPB 019
STATUS:CONFIRMED
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