Predicting ion cyclotron emission from neutral beam heated plasmas in Wendelstein7-X stellarator

Measurements of ion cyclotron emission (ICE) are planned for magnetically confined fusion (MCF) plasmas heated by neutral beam injection (NBI) in the Wendelstein 7-X stellarator (W-7X). Freshly injected NBI ions in the edge region, whose velocity-space distribution function approximates a delta-function, are potentially unstable against the magnetoacoustic cyclotron instability (MCI), which could drive a detectable ICE signal. Prediction of ICE from NBI protons in W-7X hydrogen plasmas is challenging, owing to the low ratio of the ions' perpendicular velocity to the local Alfvén speed, v⊥(NBI)/vA ≃ 0.14. We address this from first principles, using the particle-in-cell (PIC) kinetic code EPOCH. This selfconsistently solves the Lorentz force equation and Maxwell's equations for tens of millions of computational ions (both thermal majority and energetic NBI minority) and electrons, fully resolving gyromotion and hence capturing the cyclotron resonant phenomenology which gives rise to ICE. Our simulations predict an ICE signal which is predominantly electrostatic while incorporating a significant electromagnetic component. Its frequency power spectrum reflects novel MCI physics, reported here for the first time. The NBI ions relaxing under the MCI first drive broadband field energy at frequencies a little below the lower hybrid frequency ωLH, across the wavenumber range kωc/VA= 40 to 60, where ωc and VA denote ion cyclotron frequency and Alfvén velocity. Nonlinear coupling between these waves then excites spectrally structured ICE with narrow peaks, at much lower frequencies, typically the proton cyclotron frequency and its lower harmonics. The relative strength of these peaks depends on the specifics of the NBI ion velocity-space distribution and of the local plasma conditions, implying diagnostic potential for the predicted ICE signal from W7-X

Effective collecting area of a cylindrical Langmuir probe in magnetized plasma

International audienceLangmuir probe diagnostic on magnetic plasma devices often encounters more challenges in data processing than in non-magnetized plasmas, the latest itself being far from simple. In this paper, a theory of particle collection by a probe at the plasma potential in collisionless weakly ionized plasmas is constructed, accounting for velocities distributed according to the Maxwell equation and different mechanisms of particle collection depending on their speed. Experimental validation of the presented theory has been done with 2 cylindrical probes (rpr = 75 mum and Lpr = 1 cm and rpr = 0.5 mm and Lpr = 1 cm) parallel to B --> on a linear plasma device Aline, with magnetic fields of 0.0024-0.1 T and plasma densities of 1015-1017 m-3 in helium. Cylindrical probe measurements are compared to data from a planar probe perpendicular to the magnetic field, and the results for electron density, temperature, and plasma potential are presented. The introduced theory is initially constructed for a cylindrical probe but is applicable to various probe sizes, shapes, and orientations. Alongside the main subject, a number of associated issues are addressed with different details: a probe design issue relative to the magnetized environment, the "intersection" method of plasma potential evaluation, and the robustness of the conventional "1st derivative" method, a current bump near the plasma potential, lower limit for electron temperature estimation, and self-consistent calculation of electron temperature and density

Development of pre-conceptual ITER-type ICRF antenna design for DEMO

ICRF antenna development for DEMO for the pre-conceptual phase is carried out by merging the existing knowledge about multi-strap ITER, JET and ASDEX Upgrade antennas. Many aspects are taken over and adapted to DEMO, including the mechanical design and RF performance optimization strategies. The minimization of ICRF-specific plasma-wall interactions is aimed at by optimizing the feeding power balance, a technique already proven in practice. Technological limits elaborated for the components of ITER ICRF system serve as a guideline in the current design process. Several distinctive aspects, like antenna mounting, integration with the neighbouring components or adaptation for neutron environment, are tackled individually for DEMO

Discharge initiation by ICRF antenna in IShTAR

IShTAR is a linear magnetized plasma test facility dedicated to the investigation of RF wave/plasma interaction. The IShTAR ICRF system consists of a single strap RF antenna. When using the antenna for plasma production without an external plasma source, it is shown that the plasma is either produced in front of the antenna strap or inside the antenna box depending on the antenna parameters. Here, we present experimental and numerical investigation of the plasma initiation parametric dependencies. Detailed pressure and RF power scans were performed in helium at f = 5.22 MHz and f = 42.06 MHz. The experiment shows the parameter ranges for which the plasma is produced in front of the strap, or inside the antenna box. These ranges are validated by simulations with the RFdinity model, and by theoretical predictions

Multi-strap in-port ICRF antenna modeling and development in support of ITER and EU-DEMO

Full-size 3D model of ITER ICRF antenna with 1D plasma electron density (ne) and 3D ne (from EMC3-Eirene) was simulated using the RAPLICASOL (COMSOL-based) code. Impedance matrices and coupled power agree well with TOPICA with 1D ne. Cases with 3D ne show port-to-port differences compared to 1D ne, as well as a lower (about 10%) coupled power. Efficient minimization of ITER antenna near-fields (to reduce RF sheaths by optimizing feeding) calculated by TOPICA and RAPLICASOL is possible with [0;π;π;0] (about balanced strap powers) and is even lower with [0;π;0;π] toroidal phasing (with dominant power from central straps). Lowest near-fields are with [0;π] poloidal phasing, but [0;-π/2] will be used in a load resilience setup with 3dB splitters. Under EUROfusion prospective research and development, in-port ICRF antenna concept for EU-DEMO with 8 quadruplets (4x toroidal by 2x poloidal) is considered to deliver 16.7 MW (3 antennas yielding 50 MW). Areas around the equatorial port and cut-ins in breeding blankets are used, with emphasis on [0;π;π;0] optimization. High-resolution RAPLICASOL calculations with full ne profile (without imposing a minimum ne value) shed light on field distribution with propagative slow wave in detailed antenna geometry

A Test Facility to Investigate Sheath Effects during Ion Cyclotron Resonance Heating

Nuclear fusion is a promising candidate to supply energy for future generations. At the high temperatures needed for the nuclei to fuse, ions and electrons are no longer bound into atoms. Magnetic fields confine the resulting plasma. One of the heating methods is the ion cyclotron resonant absorption of waves emitted by an external Ion Cyclotron Radio Frequency (ICRF) antenna. The efficiency of ICRF heating is strongly affected by rectified RF electric fields at antenna and other in-vessel components (so-called ‘sheath effects’). The chapter presents an overview of ICRF principles. Attention is given to characterising the detrimental sheath effects through experiments on a dedicated test facility (IShTAR: Ion cyclotron Sheath Test ARrangement). IShTAR has a linear magnetic configuration and is equipped with an independent helicon plasma source. The configuration and capabilities of the test-bed and its diagnostics are described, as well as an analysis of the plasmas

Assessment of a field-aligned ICRF antenna

Impurity contamination and localized heat loads associated with ion cyclotron range of frequency (ICRF) antenna operation are among the most challenging issues for ICRF utilization.. Another challenge is maintaining maximum coupled power through plasma variations including edge localized modes (ELMs) and confinement transitions. Here, we report on an experimental assessment of a field aligned (FA) antenna with respect to impurity contamination, impurity sources, RF enhanced heat flux and load tolerance. In addition, we compare the modification of the scrape of layer (SOL) plasma potential of the FA antenna to a conventional, toroidally aligned (TA) antenna, in order to explore the underlying physics governing impurity contamination linked to ICRF heating. The FA antenna is a 4-strap ICRF antenna where the current straps and antenna enclosure sides are perpendicular to and the Faraday screen rods are parallel to the total magnetic field. In principle, alignment with respect to the total magnetic field minimizes integrated E∥ (electric field along a magnetic field line) via symmetry. Consistent with expectations, we observed that the impurity contamination and impurity source at the FA antenna are reduced compared to the TA antenna. In both L and H-mode discharges, the radiated power is 20–30% lower for a FA-antenna heated discharge than a discharge heated with the TA-antennas. Further we observe that the fraction of RF energy deposited upon the antenna is less than 0.4 % of the total injected RF energy in dipole phasing. The total deposited energy increases significantly when the FA antenna is operated in monopole phasing. The FA antenna also exhibits an unexpected load tolerance for ELMs and confinement transitions compared to the TA antennas. However, inconsistent with expectations, we observe RF induced plasma potentials to be nearly identical for FA and TA antennas when operated in dipole phasing. In monopole phasing, the FA antenna has the highest plasma potentials and poor heating efficiency despite calculations indicating low integrated E∥. In mode conversion heating scenario, no core waves were detected in the plasma core indicating poor wave penetration. For monopole phasing, simulations suggest the antenna spectrum is peaked at very short wavelength and full wave simulations show the short wavelength has poor wave penetration to the plasma core.United States. Dept. of Energy (DOE award DE-FC02-99ER54512)United States. Dept. of Energy (Fusion Energy Postdoctoral Research Program administered by ORISE

Third harmonic ICRF heating of Deuterium beam ions on ASDEX Upgrade

We report on recent experiments on the ASDEX Upgrade (AUG) tokamak (major radius R ≈1.65 m, minor radius a ≈ 0.5 m) with third harmonic ICRF heating of deuterium beam ions. Prior to this work, the scheme has been developed and applied on the JET tokamak, the largest currently operating tokamak (R ≈ 3 m, a ≈ 1 m), for fusion product studies and for testing alpha particle diagnostics in preparation of ITER [1]. The experiments reported here demonstrate that this scheme can also be used in medium size tokamaks such as AUG despite their reduced fast ion confinement.This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014-2018 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission.Postprint (published version