86 research outputs found
Frequency Sweeping of Reversed Shear Alfven Eigenmodes during Counter Neutral Beam Current Drive
Modification of the Alfvén wave spectrum by pellet injection
International audienceAlfvén eigenmodes driven by energetic particles are routinely observed in tokamak plasmas. These modes consist of poloidal harmonics of shear Alfvén waves coupled by inhomogeneity in the magnetic field. Further coupling is introduced by 3D inhomogeneities in the ion density during the assimilation of injected pellets. This additional coupling modifies the Alfvén continuum and discrete eigenmode spectrum. The frequencies of Alfvén eigenmodes drop dramatically when a pellet is injected in JET. From these observations, information about the changes in the ion density caused by a pellet can be inferred. To use Alfvén eigenmodes for MHD spectroscopy of pellet injected plasmas, the 3D MHD codes Stellgap and AE3D were generalised to incorporate 3D density profiles. A model for the expansion of the ionised pellet plasmoid along a magnetic field line was derived from the fluid equations. Thereby, the time evolution of the Alfvén eigenfrequency is reproduced. By comparing the numerical frequency drop of a toroidal Alfvén eigenmode (TAE) to experimental observations, the initial ion density of a cigar-shaped ablation region of length 4cm is estimated to be n * = 6.8×10 22 m −3 at the TAE location (r/a ≈ 0.75). The frequency sweeping of an Alfvén eigenmode ends when the ion density homogenises poloidally. Modelling suggests that the time for poloidal homogenisation of the ion density at the TAE position is τ h = 18 ± 4 ms for inboard pellet injection, and τ h = 26 ± 2 ms for outboard pellet injection. By reproducing the frequency evolution of the elliptical Alfvén eigenmode (EAE), the initial ion density at the EAE location (r/a ≈ 0.9) can be estimated to be n * = 4.8 × 10 22 m −3. Poloidal homogenisation of the ion density takes 2.7 times longer at the EAE location than at the TAE location for both inboard and outboard pellet injection
Alfven eigenmode stability and fast ion loss in DIII-D and ITER reversed magnetic shear plasmas
Neutral beam injection into reversed-magnetic shear DIII-D plasmas produces a variety of Alfvenic activity including ´
toroidicity-induced Alfven eigenmodes (TAEs) and reversed shear Alfv ´ en eigenmodes (RSAEs). With measured ´
equilibrium profiles as inputs, the ideal MHD code NOVA is used to calculate eigenmodes of these plasmas. The
postprocessor code NOVA-K is then used to perturbatively calculate the actual stability of the modes, including
finite orbit width and finite Larmor radius effects, and reasonable agreement with the spectrum of observed modes
is found. Using experimentally measured mode amplitudes, fast ion orbit following simulations have been carried
out in the presence of the NOVA calculated eigenmodes and are found to reproduce the dominant energy, pitch
and temporal evolution of the losses measured using a large bandwidth scintillator diagnostic. The same analysis
techniques applied to a DT 8 MA ITER steady-state plasma scenario with reversed-magnetic shear and both beam
ion and alpha populations show Alfven eigenmode instability. Both RSAEs and TAEs are found to be unstable ´
with maximum growth rates occurring for toroidal mode number n = 6 and the majority of the drive coming from
fast ions injected by the 1 MeV negative ion beams. AE instability due to beam ion drive is confirmed by the non-perturbative code TAEFL. Initial fast ion orbit following simulations using the unstable modes with a range of amplitudes (δB/B = 10−5–10−3) have been carried out and show negligible fast ion loss. The lack of fast ion loss is a result of loss boundaries being limited to large radii and significantly removed from the actual modes themselves.US Department of Energy DE-FC02-04ER54698, DE-AC02-09CH11466, SC-G903402, DE-AC05-00OR22725, DE-FG03-97ER5441
Observation of Energetic-Ion Losses Fluctuating with the Same Frequency as Toroidicity-Induced Alfven Eigenmodes in LHD
Studies of Magnetic Configuration Effects on TAE-induced Fast Ion Losses in LHD by an Orbit Following Model
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