Coherent magnetic field fluctuations and locked modes in a reversed-field pinch

Abstract

Paper IAEA-CN-56/C-4-3]. The dominant modes are m= l/n=7-9, resonant near the axis. The fluctuations are coherent, corresponding to nearly continuous rigid body rotation with poloidal and toroidal frequencies of f a= 130-170 kHz and f4= 11-14 kHz, respectively. Discrete dynamo events (in which toroidal magnetic flux is generated) of two different types are observed at high values of pinch parameter (e > 1.6). The first type is triggered by a single, wall-locked m= l/n=7 mode. The slowing down of the mode rotation is in rough agreement with a simulation which assumes the effect of induced eddy current in the vacuum liner. In the subsequent relaxation event, the locked mode decays and the m= 1 spectrum cascades to higher-n modes. However, in some cases the saturated m = l/n=7 mode exists for periods up to t=0.4 msec, resembling single helicity Ohmic states. The second type of event is characterized by phase locking of several m= l/n=7-9 modes and the appearance of large m =0/n = l-5 modes. Mode rotation continues through the event, similar to the normal machine operating condition.

Observation of tearing mode deceleration and locking due to eddy currents induced in a conducting shell

“…In both configurations, mode deceleration and locking can occur with the growth to large amplitude of a single, internally resonant tearing mode. [1][2][3] In the late 1980s, a theory was proposed for the tokamak and RFP that linked the deceleration and locking of a single mode directly to the amplitude of the mode. 4 -7 According to this theory, eddy currents induced in the conducting shell surrounding the plasma exert a braking torque on the plasma in the immediate vicinity of the mode's resonant surface.…”

What is a Reversed Field Pinch?

“…Transient states where the magnetic field fluctuations spatial spectrum is dominated by one individual (m = 1, n ∼ 2R/a) MHD mode, were first detected in all large RFP's [69][70][71][72][73][74], and found to last for several energy confinement times. Later on, the single helicity (SH) states found in MHD simulations [29], triggered the discovery in the data base of RFX of similar states lasting over the whole flat top at about 1 MA [76].…”

Local measurements of tearing mode flows and the magnetohydrodynamic dynamo in the Madison Symmetric Torus reversed-field pinch

“…Magnetic fluctuations play a substantial role in the global dynamics of the RFP and much work has been carried out previously to characterize magnetic tearing mode behavior in the RFP. 5,[27][28][29][30][31][32][33] Direct measurements of the tearing mode spectrum in the core of MST are not available for any quantity, including magnetic fields. Nevertheless, magnetic mode spectral mea- surements are routinely made at the plasma wall by arrays of magnetic coils.…”