Linear plasma device GyM for plasma-material interaction studies

Abstract

GyM is a linear plasma device operating at Istituto per la Scienza e Tecnologia dei Plasmi, Consiglio Nazionale delle Ricerche, Milan, with the original aim of studying basic plasma physics, such as turbulent processes. Since 2014, GyM experimental program has been mainly focused on the issue of plasma-material interaction (PMI) for magnetic confinement nuclear fusion applications. GyM consists of a stainless steel vacuum chamber (radius and length of 0.125 m and 2.11 m), a pumping system, a gas injection system, 10 magnetic field coils and two magnetron sources at 2.45 GHz, capable of delivering a total microwave power up to 4.5 kW. Highly reproducible steady-state plasmas of different gas species, at a maximum working pressure of ∼10−1 Pa, can be obtained by electron cyclotron resonance heating in the resonance layer at 87.5 mT. Plasmas of GyM have electron and ion temperature ≤15 eV and ∼0.1 eV, respectively. The electron density is in the range of 1015–1017 m−3 and the ion flux is ≤5 × 1020 ions⋅m−2s−1. Main plasma diagnostics of GyM comprise Langmuir probes, an optical emission spectrometer, a mass spectrometer and a fast camera system equipped with an image intensifier unit. For the purpose of investigating the topic of PMI, GyM is provided with two sample exposure systems. Both are biasable at a negative bias voltage down to −400 V, to tune the energy of the impinging ions. One of them is also equipped with a heating lamp and can reach and sustain a temperature of 990 K for several hours, thus allowing to study the role of sample temperature during the plasma-material interaction. This contribution presents the layout of GyM, the diagnostics, the sample exposure systems and the typical plasma parameters. A brief overview of the main PMI activities carried out so far and a description of future machine upgrades are also given.

Study the erosion of Eurofer-97 steel with the linear plasma device GyM

“…Eight mirror finished 10 × 20 × 1 mm 3 Eurofer-97 samples were exposed to the deuterium plasma of the linear device GyM [14][15][16][17]. The magnetic field configuration of the experiments is shown in Fig.…”

“…The plasma parameters remained approximately constant during all the experiments: the electron density and temperature were n 𝑒 = 5.0 × 10 16 m −3 and T 𝑒 = 7 eV, the plasma potential was V 𝑝𝑙 = 25 eV and the deuterium ion flux was 𝛤 = 4.0 × 10 20 m −2 s −1 . A heatable and biasable stainless steel manipulator with a molybdenum (Mo) sample cover (1 mm thick) was used [7,16,17] (see Fig. 1b).…”

“…Neutral impurities should be ionised to sputter the Eurofer-97 samples. However, due to the fairly low GyM plasma density, the values of the mean free path (MFP) for the different ionisation reactions are of the same order of magnitude of or greater than the characteristic dimensions of the device (which is 2.11 m long and 0.25 m in diameter [17]). In particular, the MFPs for electron impact ionisation of sputtered Fe and Mo atoms (for n 𝑒 = 5.0 × 10 16 m −3 and T 𝑒 = 7 eV) are >1 m and ∼0.4 m. Since D + energy was in the range of hundreds of eV, the MFPs have been estimated considering the Thompson-Sigmund energy distribution function with 𝛼 = 5 for the sputtered particles to take account of the energy distribution shrinkage and peak shift to lower energies in comparison to the classical Thompson distribution, more valid for higher incidence ion energies [23].…”

“…This work reports on the investigation of Eurofer-97 erosion behaviour when exposed to the deuterium (D) plasma of the linear device GyM [14][15][16][17]. The ion flux of GyM of 𝛤 = 10 20 -10 21 m −2 s −1 is suitable to simulate the CXNs flux impinging on the recessed components of the DEMO first-wall [18].…”

Analysing the effects of heating and gas puffing in Proto-MPEX helicon and auxiliary heated plasmas *

“…Linear plasma devices can reproduce the divertor conditions of a tokamak reactor, which allows for material development studies and validation of relevant predictive and interpretive simulations. Linear plasma devices also contribute to the understanding of critical fusion reactor issues, such as divertor detachment, plasma fuelling, confinement, heating, and PMI [3][4][5][6][7][8]. The plasma discharge time is relatively small for most current toroidal devices.…”