Key Quantities for ITB Formation and Sustainment
An international ITB database has been constructed to address primarily power threshold scaling, confinement scaling and underlying physics in ITB plasmas. For the power threshold scaling, a precise definition of ITB is required. Either the temporal separation of temperatures in two neighboring measurement channels or the discontinuity in gradients in temperature profiles is often used to judge the ITB formation. However, both of these criteria become unclear for low heating power approaching the threshold for the ITB formation. Given this uncertainty in determining ITB existence,parameters are sought which will better define the time and spatial location of ITB. A parameter frequently employed is Rgeo /LT where Rgeo is the plasma major radius and LT is the local temperature gradient scale length. Another parameter is rT* = rS /LT where rSis the local ion gyroradius at the sound speed; rS =(mi Te)1/2/(ZieB t),mi is the ion mass, Zi is the ion charge number and Bt is the toroidal field . This parameter has a strong Bt dependence and no Rgeo dependence i contrast to Rgeo /LT. The existence of ITBs in JET plasmas can be inferred in regions of space-time where rT* > rITB*, where rITB* is some critical value. Interestingly,a constant value of rITB*, 0.014 ,is valid for a large variety of JET discharges heated by NB and IC with Bt = 1.8-4.0 T when LTe is used as LT. As a first step in finding a parameter or a key quantity that can be generally used to judge the ITB existe ce,the validity of rT* is investigated for various devices in this paper.