Atomic physics effects on tokamak edge drift-tearing modes [report]

T.S. Hahm
1993 unpublished
The effects of ionization and charge exchange on the linear stability of drift-tearing o modes are analytically investigated. In particular, the linear instability threshold ATh , produced by ion sound wave coupling [Phys. Rev. Lett. 40, 1500 (1978 ] is modified. In the strongly collisional regime, the ionization breaks up the near cancellation of the perturbed electric field and the pressure gradient along the magnetic field, and increases the threshold. In the semi-collisional regime, both
more » ... nal regime, both ionization and charge exchange act as drag on the ion parallel velocity [Phys. Fluids B 4, 2567], and consequently decrease the threshold by reducing the effectiveness of ion sound wave propagation. Q PACS numbers: 52.35.Py, 52.30.Jb, 52.25.Gj, 52.55.Fa t Nt oi r-n I I. Introduction The L to H transitions in tokamaks are usually accompanied by a change in magnetic activity near plasma boundary. 1,2While most of the previous theoretical analyses 3-11 of low mode number tearing instability are based upon tokamak parameters well inside the plasma boundary, the extreme edge plasma parameters are drastically different. For instance, in a diverted tokamak, the magnetic shear diverges to infinity as the last closed flux surface is approached from inside. This increased magnetic shear has a significant impact on various resistive MHD instabilities. 12lt has been also speculated that the enhanced magnetic shear at the edge produced by current ramp down can make the L to H transition easier for Limiter configuration plasmas. 13Other edge specific phenomena are the atomic physics processes such l as ionization and charge exchange. Recently, it has been shown that the ionization can influence the stability of edge dissipative drift wave significantly in both slab 14,15 and toroidall6 geometries. In this paper, we investigate the instability threshold condition of low mode number drift-tearing modes in the presence of ionization and charge exchange. Our theoretical model consists of Braginskii equations with the aforementioned atomic physics effects. Sheared slab geometry is considered for simplicity. We focus on the drift-tearing regime with co.e >> TH_R (the classical tearing mode growth rate) > [_ (ionization rate), "fv(charge exchange rate). The principal results of this paper are as follows. The drift-tearing mode dispersion relations are derived analytically for both strongly collisional regime and semi-collisional regime. Sound wave dynamics, ionization and charge exchange efffects are included in the analysis. In the strongly collisional regime, ionization reduces the effective force to drive the " parallel current, and raises the instability threshold ATh ( A' > ATh is required for instability)
doi:10.2172/6607840 fatcat:4utxcxsjifcxplcy3axjy4ikda