Radial Transport Characteristics of Fast Ions Due to Energetic-Particle Modes inside the Last Closed-Flux Surface in the Compact Helical System

Abstract

The internal behavior of fast ions interacting with magnetohydrodynamic bursts excited by energetic ions has been experimentally investigated in the compact helical system. The resonant convective oscillation of fast ions was identified inside the last closed-flux surface during an energetic-particle mode (EPM) burst. The phase difference between the fast-ion oscillation and the EPM, indicating the coupling strength between them, remains a certain value during the EPM burst and drives an anomalous transport of fast ions. Fast-ion-confinement is one of the most important issues for future burning plasma experiments, such as ITER, since a dominant heating process in such plasmas is an alpha particle heating produced by deuterium-tritium nuclear reactions. In particular, interaction between fast ions and magnetohydrodynamics (MHD) modes is a key issue for fast-ion-confinement [1]. Fast ions may excite various MHD modes, and resulting anomalous transport of the fast ions occurs due to the interaction with the MHD activities [2 -7].In many fusion devices, various Alfvén eigenmodes (AEs) have been destabilized by fast ions in frequency ranges absent of shear Alfvén continuum damping [8]. The characteristics of AEs depend on the plasma parameters and agree well with experimental observations. On the other hand, energetic-particle modes (EPMs) are determined by characteristics of the fast-ion motions such as transit, bounce and precession, and can be destabilized with the frequencies inside the shear Alfvén continuum when the energetic-ion drive is strong enough to overcome the continuum damping [9]. Actually, EPMs are experimentally observed on the condition that the fast-ion pressure is fairly high [3]. EPMs often show nonlinear phenomena such as periodic burst and frequency chirping, which are considered to be related to fast-ion behavior, for example, change of fast-ion profile. The investigation of EPM properties and interaction with fast ions is still in progress [3,10,11].Recently, a directional Langmuir probe (DLP) method was applied to plasmas heated by a neutral beam (NB) for fast-ion measurement, and fast-ion behavior inside the last closed-flux surface (LCFS) can be measured with high time and spatial resolutions [12]. In this Letter, we focus on the experimental studies on the fast-ion-behavior interacting with bursting EPMs. The interaction characteristics between fast ions and the EPM and anomalous transport of fast ions due to EPM are discussed.The stage of our experiments is a helical device (stellarator), named compact helical system (CHS) [13]. The target plasma is heated and sustained by tangential NB injection with port-through power of 800 kW, the beam duration of 100 ms and beam energy of 40 keV. The plasma parameters are magnetic field strength B 0:9 T, lineaveraged electron density n e 0:5 ÿ 1:0 10 19 m ÿ3 , central electron temperature T e 0 0:3 keV, normalized plasma pressure p 0:1%. When the normalized beam pressure increased to similar values as the plasma pressure, fast-ion-driven MHD burst...

Energetic particle physics in fusion research in preparation for burning plasma experiments

“…For a basic study of energetic ion transport, a directional Langmuir probe was developed in the compact helical sysytem (CHS) stellarator/helical device and successfully used to detect the resonant losses of energetic beam ions due to EP modes (EPMs). The probe was inserted inside the last closed flux surface (LCFS) of plasmas heated by low power NBI [75].…”

Heat flux and plasma flow in the far scrape-off layer of the inboard poloidal field null configuration in QUEST

“…In the Compact Helical System, the bursting energetic particle mode and anomalous fast ion loss induced by the energetic particle mode have been observed using a directional Langmuir probe. 15,16 To understand the motion and heat transport of energetic electrons, as the Compact Helical System group did in the case of fast ions, it would be useful to make a direct measurement using a probe. For local heat measurements, a thermal probe [17][18][19] can be used as a diagnostic tool.…”

Convective and Diffusive Energetic Particle Losses Induced by Shear Alfvén Waves in the ASDEX Upgrade Tokamak

“…For single modes, stochastic losses caused by the overlapping of sideband resonances are proportional to ðBÞ 2 [15]. Experimentally, a detailed knowledge of the wave-particle interaction can be gained from direct measurements of MHD induced fast-ion losses in fusion plasmas [25,26]. Fast-ion loss detectors (FILD) in fusion devices obtain typically a crucial information on the phase space of the lost ions [27][28][29][30][31].…”