Influence of Magnetic Field Ripple on the Intrinsic Rotation of Tokamak Plasmas

Abstract

Using the unique capability of JET to monotonically change the amplitude of the magnetic field ripple, without modifying other relevant equilibrium conditions, the effect of the ripple on the angular rotation frequency of the plasma column was investigated under the conditions of no external momentum input. The ripple amplitude was varied from 0.08% to 1.5% in Ohmic and ion-cyclotron radio-frequency (ICRF) heated plasmas. In both cases the ripple causes counterrotation, indicating a strong torque due to nonambipolar transport of thermal ions and in the case of ICRF also fast ions. Plasma rotation is one of the central topics for toroidal plasma confinement and it is considered to play a key role in tokamak plasma performance. Plasma rotation can have a strong impact on plasma stability and confinement of fusion plasmas, through its ability to stabilize MHD modes [1], affect the strength of transport barriers [2], affect turbulence stabilization due to E Â B shear [3], and reduce the profile stiffness in the presence of anomalous transport [4]. In present day experiments, rotation is often driven by the torques of neutral beam injection (NBI), while in an alpha-heated reactor the momentum input is expected to be small [5]. In reactors with high NBI power, the torque is still expected to be small due to the high injection energy needed for the beam to penetrate deep into the plasma, which reduces the torque per MW of injected power. Thus, there has been a growing interest in the intrinsic rotation of the plasma, which is observed to occur in the absence of momentum sources such as NBI. Besides its relevance for fusion reactors, the residual plasma rotation in confinement configurations, without momentum injection, is an interesting, and not yet understood, problem from a basic physics point of view that has attracted great theoretical and experimental interest in recent years. The extrapolation from intrinsic plasma rotation data observed in several machines suggests that a substantial rotation in the cocurrent direction, i.e., in the direction parallel to the plasma current, will occur in ITER [6]. However, an effect that is still to be accounted for when extrapolating intrinsic rotation observations to ITER comes from the toroidal ripple in the magnetic field, which appears in tokamaks due to the finite number of toroidal magnetic field coils. ITER will have 18 toroidal magnetic field coils and, a ripple amplitude ¼ B=B of 0.5% to 1.2% at the edge (depending on the configuration of ferritic inserts) [7] where B is the averaged toroidal magnetic field and B is the Fourier amplitude of the toroidal ripple. Since the ripple breaks the toroidal symmetry, the motion of individual particles may lead to nonambipolar transport that can affect the plasma rotation through the neoclassical toroidal viscosity [8][9][10].The effect of ripple on plasma rotation during NBI has been reported from and JET [12]. Evidence that ripple transport of fast ions induces edge counterrotation in plasmas with no external momentum input wa...

Observations of rotation in JET plasmas with electron heating by ion cyclotron resonance heating

“…0 π 0 π ) phasing, which gives almost toroidal symmetric wave spectra with little net toroidal momentum. The experiments were carried out with the standard low field ripple (δ = 0.08%) to avoid rotational effects caused by the ripple, which has earlier been found to give rise to counter-rotation [8].…”

“…Some effects have been experimentally confirmed, but there still remain other observations which need explaining. The strongest observed counter-rotation with ICRH alone in JET is obtained with a strong magnetic field ripple δ = (B max − B min )/(B max + B min ) ∼ 1.5% [8], where B max and B min are the maximum and minimum field at the plasma boundary, at the outer midplane. Plasma rotation has also been observed during ohmic heating [14][15][16] and in experiments using waves in the electron cyclotron range of frequencies (ECRF) [17][18][19].…”

“…It is possible that the rotation in this region is sensitive to the magnetic field geometry in the scrape off layer. The outer part of the plasma is, in general, co-rotating except for a large magnetic field ripple [8] and some H-mode plasmas [2,24].…”

“…It is therefore of interest to investigate alternative methods to generate rotation. Toroidal rotation of the plasma is often seen during ion cyclotron resonance heating (ICRH) even when the momentum inputs by the waves are negligible [1][2][3][4][5][6][7][8][9][10][11][12][13]. In general, the rotation is rather small and caused by several weak mechanisms that are not fully understood.…”

“…High-energy ions with broad orbits, created by ICRH, can give rise to radial currents driving the rotation. Most of the earlier rotation studies on JET [3,8] with ICRH were done in scenarios with a dominating ion cyclotron resonance absorption, where the power was transferred to ions and electrons by collisions. Few studies were done with direct electron absorption of the fast magnetosonic waves by transit time magnetic pumping and electron Landau damping (TTMP/ELD), which in current experiments is often a weak damping mechanism.…”