7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Characterisation of plasma properties for wall conditioning studies on theTOMAS device
K. Crombé1,2, J. Buermans1,2, S. Brezinsek3, D. Castaño-Bardawil1, A. Goriaev1,
D. López-Rodríguez1,2, M. Verstraeten1, L. Dittrich4, Yu. Kovtun5, Ch. Linsmeier3, J. Ongena1,P. Petersson4, T. Wauters6
1
Laboratory for Plasma Physics, LPP-ERM/KMS, Brussels, Belgium
2
Department of Applied Physics, Ghent University, Belgium
3
Institute for Energy and Climate Research, Forschungszentrum Jülich GmbH, Germany
4Fusion Plasma Physics, KTH Royal Institute of Technology, Sweden
5
Institute of Plasma Physics, NSC KIPT, Kharkov, Ukraine
6ITER Organization, St. Paul-lez-Durance, Francee-mail (speaker):
Kristel.Crombe@UGent.beWall conditioning, plasma surface interaction, andplasma production studies are of high importance for aneffective
commissioning
andoperationofsuperconducting fusion devices like W7-X, JT-60SA,and ITER. These topics can be investigated in a
specialised experimental setup known as the TOroidal
MAgnetic System (TOMAS), which has undergonesignificant upgrades in recent years [1]. The TOMAS
experiment is situated at the Forschungszentrum Jülich
in Germany and focuses on ion and electron cyclotron(IC/EC) plasmas. The plasma confinement is achieved
through a toroidal magnetic field generated by 16 copper
coils, reaching values up to 125 mT at the centre. Thedevice has a major radius (R) of 0.78 m, a minor radius(a) of 0.26 m, and a total plasma volume of
approximately 1.1 m3. Figure 1 provides a schematic
view of the TOMAS device. Both the IC and EC systemscan deliver up to 6 kW of power, with the EC frequency
set at 2.45 GHz and the IC frequency ranging from 10 to50 MHz. Additionally, a direct current glow dischargecleaning (GDC) system, utilising a graphite anode with amaximum voltage of 1.5 kV and a current of 6 A, hasbeen installed. Material samples can be exposed to theplasma using a load-lock system equipped with a verticalmanipulator. The device is equipped with various plasma
diagnostics, and additional measuring tools are currently
being incorporated [2]. These include a Langmuir probe
system, optical and particle diagnostics.
Recently, a series of experimental campaigns have been
conducted, focusing on sample exposure, plasma
characterization, and diagnostics commissioning. The
first area of investigation involves exposing material
samples to TOMAS plasmas to study the efficiency ofremoving co-deposited boron layers. The samples weresubjected to GDC in helium as well as IC wallconditioning plasmas in both helium and hydrogen.
Systematic changes were made to the plasma parameters
to determine the optimal exposure conditions. The power
level of the IC system was varied from 1 to 4 kW, andthe electrical connection of the sample holder was
modified, including floating potential, grounding to thevessel, and applying a negative voltage bias. Pre- andpost-exposure analyses were conducted using heavy ion
elastic recoil detection analysis and nuclear reaction
analysis [3].
To complement the research on wall conditioning inTOMAS, a characterization of IC, EC, and combined
IC/EC plasmas was carried out using a radiallyadjustable triple Langmuir probe.
Finally, new diagnostic tools have recently been put into
operation on the TOMAS device. An update will be
provided on the RFEA (Retarding Field EnergyAnalyzer), which is used to investigate ion fluxes, aswell as the optical emission spectrometer applied toscenarios for plasma start-up.

Figure 1. Schematic view of the TOMAS device.
References[1] A. Goriaev et al., Rev. Sci. Instrum. 92 023506 (2021)[2] Yu. Kovtun et al., JINST 18 C02034 (2023)[3] P. Petersson et al., “The influence of plasma parametersand electrical connection on the efficiency of wallconditioning plasmas in TOMAS facility”, presented at 19thInternational Conference on Plasma-Facing Materials andComponents for Fusion Applications, Bonn, 22 – 26 May
2023