The External Kink Mode and the Role of q 95 in Diverted Tokamaks

The disruptive instability in diverted tokamaks when
EUR
q
at the 95% poloidal flux surface,
EUR
q
95
, is driven below 2.0 is
shown to be
a resistive kink
. The mode is a resistive
counterpart to
the current driven ideal mode that traditionally explained the disru
ption in limited cross
sections
when
q
a
, the safety factor at the outermost closed flux surface, lies just below a
rati
onal value
. D
iverted plasmas, in which
q
a
is formally infinite in the MHD model, have
presented a longstanding difficulty since the theory would predict stability, yet, the disruptive
limit occurs in practice when
q
95
, reaches 2.
N
umerical calculations
show
the
resistive kink
mode is linearly destabilized by the rapidly increasing resistivity at the plasma edge when
q
95
<
2, but
q
a
>>
2. The resistive kink also occurs for limiter plasmas but quickly transforms to
the ideal mode when the rational surface
exits the plasma; this also explains
an
observed
small discrepancy in onset conditions.