Experimental Observation of rf Driven Plasma Flow in the Phaedrus-T Tokamak

Abstract

We have observed frequency shifts of the 2͞1 tearing mode in a tokamak that are consistent with injecting momentum with low frequency rf waves. The 2͞1 frequency increased for current drive antenna phasing, decreased for anticurrent drive phasing, and was linear with rf power and driven current. The change in the toroidal velocities derived from the frequency shifts and the calculated rf momentum is consistent. The frequency change also increased with magnetic field, independent of antenna phasing. This is the first demonstration of modifying the 2͞1 frequency through rf momentum input. [S0031-9007(96)00568-6] PACS numbers: 52.55. Fa, Both theory and experiment suggest that plasma rotation can significantly modify the stability and transport characteristics of tokamak plasmas. Simultaneous improvement of the plasma confinement and stability has been identified in reactor studies as the most significant means for improving the economic attractiveness of the tokamak concept [1,2].Plasma rotation's impact on plasma stability has been examined recently. In tokamak plasmas surrounded by a resistive shell, toroidal plasma rotation has been found to increase tearing mode stability [3]. Theoretical analysis suggests that external kink modes are also stabilized by toroidal rotation [4]. Experimental evidence of this stabilization has been reported by Strait et al. where the stabilization was achieved with rotation velocities a fraction of the Alfvén speed [5]. External control of the momentum input can be utilized to prevent locked modes which cause degradation of confinement and often are precursors to plasma disruptions [6,7]. In addition, sheared plasma rotation has been predicted to cause linear coupling of ballooning modes. This increases damping of these modes which may allow tokamak plasmas to reach the second stability regime [8,9].Models developed to describe the transport in experimentally observed enhanced confinement modes have identified the importance of plasma rotation and sheared rotation in the formation of transport barriers [10]. These models are consistent with the observation of shear flows in H-mode plasmas [11] and negative central shear mode plasmas in the DIII-D tokamak [12]. Experiments also have shown degradation of VH mode with decreasing local toroidal plasma flow [13]. In addition, the power threshold for transport barrier formation is predicted to decrease with increased shear [14].External control of the momentum input has been demonstrated with neutral beam injection. While the net momentum transfer is efficient, the deposition profile is broad. In addition, calculations suggest that plasma rotation driven by radio-frequency (rf) waves can be more efficient than neutral beams if the parallel index of refraction N k $ 30 [15]. Several experiments have reported changes in the plasma toroidal rotation velocity with the application of ion cyclotron heating [16][17][18]. The rotation is generated by the spatial diffusion or loss of fast, resonant ions which leads to a modification of the radial...

Ion Larmour Radius Effect on rf Ponderomotive Forces and Induced Poloidal Flow in Tokamak Plasmas

“…Balancing the poloidal force (11) with the neoclassical viscous force, we find for the parameters of TCABR (a 18 cm, R 0 61 cm, B t 1 T, q 0 1.01, T i͞e 400͞500 eV, n 0 2 3 10 13 cm 23 , and all profiles are assumed parabolic) the poloidal plasma flow velocity for wave dissipation of 400 kW, U u,max ഠ 6 km͞s, and radial electric field, B 0 U u ͞c ഠ 60 V͞cm, can also be estimated. This strongly sheared profile of the poloidal velocity is similar to the one in the conditions of the internal transport barriers in the Test Fusion Tokamak Reactor experiment [2].…”

“…The idea to use forces driven by Alfvén waves to reduce radial transport has a rather long history [3,9]. Actually, toroidal plasma rotation (and the current drive also) induced by Alfvén waves has already been demonstrated [10,11], showing the possibility to create ITB.…”

“…The idea to use forces driven by Alfvén waves to reduce radial transport has a rather long history [3,9]. Actually, toroidal plasma rotation (and the current drive also) induced by Alfvén waves has already been demonstrated [10,11], showing the possibility to create ITB.The poloidal flow driven by rf fields has been calculated by many authors in various frequency ranges and with different approximations. The results of calculations of power required to drive substantial flows seem to be rather sensitive to the adopted model and, since many effects act simultaneously, it is not straightforward to pin down the most relevant mechanisms in numerical simulations.…”

The Challenges of Ambient Plasma Wave Propulsion

“…They have been observed in both laboratory and astrophysical settings 7 , 8 . On Earth, they are being investigated as a possible means for plasma heating, current drive, and momentum addition in magnetic confinement fusion systems 9 . In addition, Alfven waves have been proposed as a mechanism for acceleration of the solar wind away from the sun 10 As such, they have been shown to propagate energy and momentum, as well as forming standing wave structures in space.…”

Measurements on rotating ion cyclotron range of frequencies induced particle fluxes in axisymmetric mirror plasmas

“…When such rotating-rf fields are absorbed in the plasma, on the other hand, it is expected that the momentum of this traveling wave is transferred to the plasma particles and a driving force acting along the wave propagation direction appears. The effect of this momentum transfer leads to the generations of steady-state current 3 and plasma rotation 4 parallel to the magnetic-field lines, and may induce perpendicular charged-particle fluxes 5 in the plasma. Concerning the latter point, methods of cross-field flux control by lowfrequency electromagnetic fields were proposed to improve plasma confinement, 6 kinetic theories on the rf-induced cross-field fluxes in magnetic-mirror and toroidal plasmas were reported, 7,8 and an orbit theory on the origin of rfinduced particle drifts has been developed.…”