18 research outputs found
Investigation of mode activity in NBI-heated experiments of Wendelstein 7-X
No full text| openaire: EC/H2020/633053/EU//EUROfusionThe 2018 operation phase (OP 1.2b) of the stellarator Wendelstein 7-X (W7-X) included, for the first time, neutral beam injection (NBI) to heat the plasma. Since the injection geometry at W7-X is not parallel, this generates both passing and trapped fast particles. During longer phases of NBI injection, with the primary purpose to study the heating efficiency of this system, Alfven eigenmodes (AEs) were observed by a number of diagnostics, including the phase contrast imaging (PCI) system, the magnetic pick-up coils (Mirnov coils), and the soft x-ray multi-camera tomography system (XMCTS). Alfven eigenmodes are of great interest for future fusion reactors as it has been shown that the resonant interaction of fast ions with self-excited AEs can lead to enhanced transport of fast ions and potentially to energy losses. This is especially true for so-called gap-modes, Alfven eigenmodes with frequencies in gaps of the continuous spectrum, since they lack continuum damping. These modes are commonly known to be excited by fast ions, but other destabilizing mechanisms, e.g. the electron-pressure gradient are also possible. In this article we present a first analysis of the experimentally observed frequencies from the theoretical side. The calculation of shear Alfven wave continua for selected cases and the assignment of observed frequencies to the gaps of the continuous spectra are presented. Using the ideal-MHD code CKA (Konies A. 200710th IAEA TM on Energetic Particles in Magnetic Confinement System), we find gap modes that match the experimental measurements in terms of the observed frequencies. We emphasize the crucial roles played by the coupling of sound and Alfven waves as well as of the Doppler shift arising as a consequence of the radial electric field in W7-X. We employ the perturbative gyrokinetic code CKA-EUTERPE (Feh ' er 2013 Simulation of the interaction between Alfv ' en waves and fast particles), using a slowing-down distribution function for the fast ions as calculated by the Monte-Carlo particle following code ASCOT (Hirvijokiet al2014Comput. Phys. Commun.185 1310-21) to assess the fast-ion drive. We find that the fast-ion drive is insufficient to overcome the background-plasma damping. The fact that unstable modes were observed experimentally may point to problems with the modelling or indicate the existence of other destabilizing mechanisms, e.g. associated with the electron-pressure gradient (Windischet al2017Plasma Phys. Control. Fusion59 105002) that sensitively depend on the profiles of the background plasma.Peer reviewe
Overview of diagnostic enhancements and physics studies of confined fast-ions in ASDEX Upgrade
No full text
The role of SOL plasma in the confinement of NBI fast ions in W7-X
No full textThe impact of the scrape-off layer (SOL) plasma on deposition, confinement and losses of
neutral beam injected fast ions was investigated in W7-X plasma. The effect of SOL width,
density, temperature profiles, radial electric field, and charge–exchange reactions (CX) was
explored. Ionization and slowing down partly counterbalance each other, as slowing down in
cold SOL plasma compensates for ionization effects in radially decaying model profiles.
However, the effect of SOL plasma on more vulnerable steel components is mitigated over a
wide range of different profiles, because for those components the collisionality effect overrules
the effect of SOL on ionization. The effect of the radial electric field is mitigated for steel
components in the experimentally observed direction of the field. The effect of CX reactions is
shown to lead to a widely spread low power load distribution with no clear effect on peak load.
Statistical challenges caused by hugely varying triangle sizes in the discretization of walls are
discussed
Measurements and modeling of Alfven eigenmode induced fast ion transport and loss in DIII-D and ASDEX Upgrade
No full textNeutral beam injection into reversed magnetic shear DIII-D and ASDEX Upgrade plasmas produces a variety of Alfvenic activity including toroidicity-induced Alfven eigenmodes and reversed shear Alfven eigenmodes (RSAEs). These modes are studied during the discharge current ramp phase when incomplete current penetration results in a high central safety factor and increased drive due to multiple higher order resonances. Scans of injected 80 keV neutral beam power on DIII-D showed a transition from classical to AE dominated fast ion transport and, as previously found, discharges with strong AE activity exhibit a deficit in neutron emission relative to classical predictions. By keeping beam power constant and delaying injection during the current ramp, AE activity was reduced or eliminated and a significant improvement in fast ion confinement observed. Similarly, experiments in ASDEX Upgrade using early 60 keV neutral beam injection drove multiple unstable RSAEs. Periods of strong RSAE activity are accompanied by a large (peak delta S-n/S-n approximate to 60%) neutron deficit. Losses of beam ions modulated at AE frequencies were observed using large bandwidth energy and pitch resolving fast ion loss scintillator detectors and clearly identify their role in the process. Modeling of DIII-D loss measurements using guiding center following codes to track particles in the presence of ideal magnetohydrodynamic (MHD) calculated AE structures (validated by comparison to experiment) is able to reproduce the dominant energy, pitch, and temporal evolution of these losses. While loss of both co and counter current fast ions occurs, simulations show that the dominant loss mechanism observed is the mode induced transition of counter-passing fast ions to lost trapped orbits. Modeling also reproduces a coherent signature of AE induced losses and it was found that these coherent losses scale proportionally with the amplitude; an additional incoherent contribution scales quadratically with the mode amplitude. (C) 2011 American Institute of Physics. [doi:10.1063/1.3574663]open115151sciescopu
Overview of first Wendelstein 7-X high-performance operation
No full textThe optimized superconducting stellarator device Wendelstein 7-X (with major radiusR = 5.5 m, minor radius a = 0.5 m, and 30 m3 plasma volume) restarted operation after the assembly of a graphite heat shield and 10 inertially cooled island divertor modules. This paper reports on the results from the first high-performance plasma operation. Glow discharge conditioning and ECRH conditioning discharges in helium turned out to be important for density and edge radiation control. Plasma densities of 1–4.5 × 1019 m−3 with central electron temperatures 5–10 keV were routinely achieved with hydrogen gas fueling, frequently terminated by a radiative collapse. In a first stage, plasma densities up to 1.4 × 1020 m−3 were reached with hydrogen pellet injection and helium gas fueling. Here, the ions are indirectly heated, and at a central density of 8 · 1019 m−3 a temperature of 3.4 keV with Te/Ti = 1 was transiently accomplished, which corresponds to nTi(0)τE = 6.4 × 1019 keV s m−3 with a peak diamagnetic energy of 1.1 MJ and volume-averaged normalized plasma pressure β = 1.2%. The routine access to high plasma densities was opened with boronization of the first wall. After boronization, the oxygen impurity content was reduced by a factor of 10, the carbon impurity content by a factor of 5. The reduced (edge) plasma radiation level gives routinely access to higher densities without radiation collapse, e.g. well above 1 × 1020 m−2 line integrated density and Te = Ti = 2 keV central temperatures at moderate ECRH power. Both X2 and O2 mode ECRH schemes were successfully applied. Core turbulence was measured with a phase contrast imaging diagnostic and suppression of turbulence during pellet injection was observed
4D and 5D phase-space tomography using slowing-down physics regularization
No full textWe compute reconstructions of 4D and 5D fast-ion phase-space distribution functions in fusion plasmas from synthetic projections of these functions. The fast-ion phase-space distribution functions originating from neutral beam injection (NBI) at TCV and Wendelstein 7-X (W7-X) at full, half, and one-third injection energies can be distinguished and particle densities of each component inferred based on 20 synthetic spectra of projected velocities at TCV and 680 at W7-X. Further, we demonstrate that an expansion into a basis of slowing-down distribution functions is equivalent to regularization using slowing-down physics as prior information. Using this technique in a Tikhonov formulation, we infer the particle density fractions for each NBI energy for each NBI beam from synthetic measurements, resulting in six unknowns at TCV and 24 unknowns at W7-X. Additionally, we show that installing 40 LOS in each of 17 ports at W7-X, providing full beam coverage and almost full angle coverage, produces the highest quality reconstructions.SPCSP
