Toroidal Alfven Eigenmodes are shown to be capable of inducing tipple trapping of high energy particles in tokamaks, causing intense localized particle loss. The effect has been observed in TFTR.PACS numbers: 52.35. Py, 52.35.Bj Collective alpha-driven instabilities such as the toroidicity-induced Alfven eigenmodes (TAE) are of concern for future tokamak devices since they can induce anomalous alpha losses. Previously discussed mechanisms of particle loss have consisted of induced transition from passing to direct-loss trapped orbits and radial diffusion produced by stochasticity in particle orbits caused by overlapping resonances 1 . In this work we point out a very effective loss process in devices possessing magnetic ripple wells. It differs from other forms of TAE induced loss in that the mechanism possesses no threshold mode amplitude. The effect has been observed in the Tokamak Fusion Test Reactor (TFTR), where particle fluxes intense enough to damage the vacuum wall were observed.The process is very simply understood using a simple model for the magnetic field.Consider a trapped particle whose banana tip is in the vicinity of a ripple well. In guiding center approximation the particle energy is given byModel the field as a large aspect ratio circular equilibrium modulated by toroidal rippleR with N the number of toroidal field coils, 5 the local ripple strength, and R = Ro+rcos (6) the major radius. To leading orders in gyro radius to system size a trapped particle moves periodically between the bounce points in poloidal angle 9 and slowly precesses from the initial field line with the toroidal angle given by ,
The goal of this paper is to characterize the effects of small non-axisymmetric divertor plate electrodes on the local scrape-off layer plasma. Four small rectangular electrodes were installed into the outer divertor plates of NSTX. When the electrodes were located near the outer divertor strike point and biased positively, there was an increase in the nearby probe currents and probe potentials and an increase in the LiI light emission at the large major radius end of these electrodes. When an electrode located farther outward from the outer divertor strike point was biased positively, there was sometimes a significant decrease in the LiI light emission at the small major radius end of this electrode, but there were no clear effects on the nearby probes. No non-local effects were observed with the biasing of these electrodes.2
The National Spherical Torus Experiment has produced toroidal plasmas at low aspect ratio (A = R/a = 0.86m/0.68m ~ 1.3, where R is the major radius and a is the minor radius of the torus) with plasma currents of 1.4MA. The rapid development of the machine has led to very exciting physics results during the first full year of physics operation. Pulse lengths in excess of 0.5s have been obtained with inductive current drive. Up to 4MW of High Harmonic Fast Wave (HHFW) heating power has been applied with 6MW planned. Using only 2MW of HHFW heating power clear evidence of electron heating is seen with HHFW, as observed by the multi point Thomson scattering diagnostic. A non-inductive current drive concept known as Coaxial Helicity Injection (CHI) has driven 260kA of toroidal current. Neutral beam heating power of 5MW has been injected. Plasmas with β t (=2µ 0 /B 2 = a measure of magnetic confinement efficiency) of 22% have been achieved, as calculated using the EFIT equilibrium reconstruction code. β limiting phenomena have been observed, and the maximum β t scales with I p /aB t . High frequency (>MHz) magnetic fluctuations have been observed. H-mode plasmas are observed with confinement times of > 100ms. Beam heated plasmas show energy confinement times in excess of those predicted by empirical scaling expressions. Ion temperatures in excess of 2.0keV have been measured, and power balance suggests that the power loss from the ions to the electrons may exceed the calculated classical input power to the ions.
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