Toroidal rotation in RF heated JET plasmas

Abstract

Observations of bulk plasma rotation in radio frequency (RF) heated JET discharges are reported. This study is concentrated on RF heated L-mode plasmas. In particular, the toroidal rotation profiles in plasmas heated by ion cyclotron resonance frequency (ICRF) waves and lower hybrid (LH) waves have been analysed. It is the first time that rotation profiles in JET plasmas with LH waves have been measured in dedicated discharges. It is found that the toroidal plasma rotation in the outer region of the plasmas is in the co-current direction irrespective of the heating scenario. An interesting feature is that the toroidal rotation profile appears to be hollow in many discharges at low plasma current, but a low current in itself does not seem to be a sufficient condition for finding such profiles. Fast ion transport and finite orbit width effects are mechanisms that could explain hollow rotation profiles. This possibility has been investigated by numerical simulations of the torque on the bulk plasma due to fast ICRF accelerated ions. The obtained torque is used in a transport equation for the toroidal momentum density to estimate the effect on the thermal bulk plasma rotation profile.

JET intrinsic rotation studies in plasmas with a high normalized beta and varying toroidal field ripple

“…Pedestal parameters however cannot easily predict JET observations where in many cases central and edge regions rotate in the opposite direction [17,32].…”

“…Statistical errors, determined from the error in the least square fit [22], are shown as shaded areas in the plots below. CXRS intrinsic rotation measurements have been routinely compared with the observed frequency of MHD modes [1,12,17] . The L-mode edge is clearly always co-rotating, while the core is either counter-or co-rotating depending on the plasma current [17], and heating details [12,17] The JET intrinsic rotation database in H-mode plasmas with pure ICRF heating has few pulses, which is due in part to the JET experimental priorities and in part to the initial technical difficulties in maintaining a steady ICRF heating power in H-mode plasmas with edge localized modes (ELMs).…”

“…either +90 0 phasing resulting in launched waves propagating predominantly in the co-current direction or -90 0 phasing resulting in launched waves propagating predominantly on the counter-current direction), a small toroidal momentum in either co-or counter-current can be produced and consequently small effects on the core rotation (~3 krad/s) observed [12]. In the case of minority heating it was also shown that the minority resonance layer position can have an effect on the core rotation [1, 12], as has the minority level [17]. Enhanced rotation is also observed when using an ICRF mode-conversion heating scheme as shown in recent experiments in C-Mod [18] and in JET, where counter-rotation in the core of JET L-mode plasmas, up to a modest value of 10 krad/s, was observed [19].…”

Residual parallel Reynolds stress due to turbulence intensity gradient in tokamak plasmas

“…In terms of its direction and scaling trends, L-mode rotation seems to be a complex phenomenon with various competing effects, which may frequently involve sign flips. 7,9,10 In contrast, H mode rotation is usually in the co-current direction, and the on-axis plasma velocity appears to increase with stored energy W p and decrease with total plasma current I p . 11 This empirical scaling, which has come to be called the Rice scaling, particularly manifests itself strongly in Alcator C-Mod and D-IIID, 1,2 and if taken at face value gives rather optimistic predictions for ITER's rotation speed.…”

Nonlinear flow generation by electrostatic turbulence in tokamaks

“…The toroidal momentum transport has been observed to be highly anomalous in various magnetic fusion experiments, not only for its high level compared to the neoclassical value due to Coulomb collisions, 1 but also for its highly pronounced non-diffusive and non-local nature. A striking finding is the observation of automatic toroidal rotation spin up in nearly all tokamaks, the so called intrinsic or spontaneous rotation, [2][3][4] i.e., toroidal plasmas can self-organize and develop rotation without an external momentum input. Nondiffusive phenomena can also exist in other transport channels such as energy and particle; however evidences so far in experiments appear not as intriguing as the intrinsic rotation.…”