Plasma composition control by mixture pellets at ASDEX Upgrade

P.T Lang,A Drenik,R Dux,M Prechtl,T Jackson,V Rohde,M Schubert,P.A Schneider,B Ploeckl,Asdex Upgrade Team ,R.R Ruess,G Tardini,R.M Mcdermott,O.J.W.F Kardaun

doi:10.1088/1741-4326/aaf294

Abstract

Experiments were performed that aimed to control the plasma isotopic mixture by delivering supplemental components into the plasma via pellet injection. Pellets, mm-sized solid bodies of frozen fuel, are either formed from a mixture of different hydrogen isotopes or from deuterium (D) with added elements. Mixed HD pellets were produced reliably and reproducibly providing the required 1:1 ratio of the protium (H) and D, mimicking the situation in a future fusion reactor with D and tritium (T). By applying pellet fuelling, control of the plasma isotope composition was established while gaining access to the reactor-relevant high-density regime. However, the approach was mainly to demonstrate the controlling capability. Yet, experiments provided information on the impact of the isotopic composition on the plasma performance. There is a strong indication that adding H to a D reference plasma causes a reduction of both the particle and energy confinement. The deleterious impact of H on the particle confinement is, apparently, even stronger than on the energy confinement. Furthermore, small amounts of H can cause a loss of energy confinement that is stronger than anticipated by the H98(y,2) scaling, which is usually employed in reactor design studies. In the course of this experiment, pellets were demonstrated to be an effective and powerful tool for detailed investigations of the isotope effect. A first test demonstrated that producing and launching pellets that contain a mixture of different elements is a feasible technique. The latter was achieved by doping a D host pellet despite technical reasons limiting the permissible amount of processed N.

Highlights

The function of core particle fuelling in the next-step experimental reactor-scale tokamak device ITER and in the planned demonstration fusion power plant EU-DEMO [1] is by no means a trivial one
Pellet core particle fuelling can be considered as an essential technique of nuclear fusion; a couple of physics and technology issues still need further consideration
Our experiments performed at the tokamak ASDEX Upgrade (AUG) employed a mixture of H and D, which mimics the fuel composed of D and T in a reactor

Summary

Introduction

The function of core particle fuelling in the next-step experimental reactor-scale tokamak device ITER and in the planned demonstration fusion power plant EU-DEMO [1] is by no means a trivial one. Cryogenic mm size bodies of solid fuel, taking place at high speed from the magnetic high-field side into the plasma column is expected the most suitable approach for this task. The total inventory of tritium (T) must be as low as reasonably possible, while the effort for processing the exhaust gas has to be kept at a bearable level [7] This exhaust gas will be composed of unburned fuel, T and deuterium (D), as well as the fusion “ash” helium (He). Our experiments performed at the tokamak ASDEX Upgrade (AUG) employed a mixture of H and D, which mimics the fuel composed of D and T in a reactor. N is widely-used in AUG anyway, as it was found to be an efficacious plasma enhancement gas (PEG) improving the plasma energy confinement [9]

Set-up

Findings

Particle confinement