The performance of an electromagnetic flowmeter head is assessed in terms of a weight vector W such that the output voltage ∝ ∫ v. Wdτ, where v is the velocity and τ the flowmeter volume. The condition curl W = 0 with W → 0 at ∞ is shown to be necessary and sufficient for the velocity to depend only on the flow rate and not on the flow pattern. A class of such ‘ideal’ meters is described. It is shown that meters with point electrodes can never be ideal but may, with considerable complication of the magnetic field, be made immune to asymmetric velocity-profile variations if the flow is rectilinear.
Results are presented from thin (resistive) shell experiments on HBTX and compared with theoretical (linear and non-linear) studies of the plasma stability. Current pulses of 3--5 ms are obtained, compared with the shell time constant for vertical field penetration of 0.5 ms. Theoretically predicted thin shell modes, phase locked to the wall, are prominent experimentally.
The operational limits observed in spherical tokamaks, notably the small tight aspect ratio tokamak ͑START͒ device ͓A. Sykes et al., Nucl. Fusion 32, 694 ͑1992͔͒, are consistent with those found in conventional aspect ratio tokamaks. In particular the highest  achieved ͑ϳ40%͒ is consistent with an ideal magneto-hydro-dynamic ͑MHD͒ Troyon type limit, the upper limit on density is well described by the Greenwald density (a 2 n e /I p ϳ1) and the normalized current (I p /aB t ) is limited such that q 95 տ2. Stability calculations indicate scope for increasing both normalized  and normalized current beyond the values so far achieved, although wall stabilization is generally needed for low-n modes. In double null configurations current terminating disruptions occur at each of the operational boundaries, though the current quench tends to be slow at the density limit and disruptions at high  may be due to the low q. In early limiter START discharges, before the divertor coils were installed, disruptions rarely occurred. Instead internal reconnection events which have all the characteristics of a disruption except the current quench occurred. These various disruptive behaviors are explained in terms of a model in which helicity is conserved during the disruption. Due to the low toroidal field beam ions in START, and ␣ particles in a ST power plant, are super-Alfvénic. This gives the possibility for toroidal Alfvén eigenmodes ͑TAEs͒ to occur and such modes are frequently observed in START neutral beam injection ͑NBI͒ discharges, but seem to be benign. The features of these observed TAEs are shown to be in agreement with MHD calculations.
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