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| 2. |
- Buermans, Johan, et al.
(författare)
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X-mode electron cyclotron heating scenarios beyond the cut-off density
- 2023
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Ingår i: 24th Topical Conference on Radio-Frequency Power in Plasmas. - : AIP Publishing.
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Konferensbidrag (refereegranskat)abstract
- Electron Cyclotron Heating (ECH) at the fundamental resonance in X-mode is limited by a low cut-off density. Electromagnetic waves cannot propagate in the region between this cut-off and the Upper Hybrid Resonance (UHR) and cannot reach the Electron Cyclotron Resonance (ECR) position. Higher harmonic heating is hence preferred in nowadays ECH plasma heating scenarios to overcome this problem. However, if this evanescent region is small compared to the wavelength of the waves, additional power deposition mechanisms can occur to increase the plasma density. This includes collisional losses in the evanescent region, tunneling of the X-wave with resonant coupling at the ECR, and conversion to the Electron Bernstein Wave (EBW) with resonant coupling. Several ECH plasma production experiments were performed on the TOMAS device with simple toroidal magnetic field to identify these additional heating regimes and to study the influence of the heating power on the ECH plasma parameters and the power deposition. Density and temperature profiles were measured with Langmuir Probes. Measurements of the forwarded and reflected power allow to estimate the coupling efficiency. The results help to understand ECH plasma production for tokamak plasma breakdown assistance and Electron Cyclotron Wall Conditioning (ECWC).
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| 3. |
- Crombé, K., et al.
(författare)
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Radio frequency plasma production on the TOMAS device
- 2023
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Ingår i: 24th Topical Conference on Radio-Frequency Power in Plasmas. - : AIP Publishing.
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Konferensbidrag (refereegranskat)abstract
- The study of discharge initiation by an Ion Cyclotron Range of Frequency (ICRF) antenna is of importance for operational scenarios at low magnetic field in e.g., ITER and Wendelstein 7-X. At low magnetic fields ohmic breakdown is difficult because of relatively short connection lengths and the electron cyclotron (EC) systems cannot assist because of a low-density cut-off. In that case ion cyclotron (IC) start-up, with waves in the Radio Frequency (RF) wavelength range, can be a good alternative solution. To explore in more detail the processes at play during the initial breakdown phase and to test measurement requirements, the diagnostic's capabilities of the TOMAS experimental device are being enlarged to allow for IC breakdown studies. A first set of IC breakdown experiments has been performed and data was taken by a newly installed spectrometer and a horizontal triple Langmuir probe. Limitations were found in pressure, ICRF power, and hydrogen concentration. Further diagnostic's upgrades and extensions of the experimental sessions will allow for a more complete study in support of modelling and experiments on other machines.
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| 4. |
- Goriaev, A., et al.
(författare)
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First studies of local ion fluxes in radio frequency plasmas for ion cyclotron wall conditioning applications in the TOMAS device
- 2023
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Ingår i: AIP Conference Proceedings. - : American Institute of Physics Inc..
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Konferensbidrag (refereegranskat)abstract
- To measure the local ion energy distribution functions for support of Ion Cyclotron Wall Conditioning (ICWC) application studies in the TOMAS device, a Semion Retarding Field Energy Analyzer (RFEA) by Impedans Ltd. has been installed in the sample load-lock system. The RFEA system has been commissioned in hydrogen Radio-Frequency (RF) plasmas. The first studies have been performed on the influence of the neutral hydrogen pressure, the input power, the magnetic field and the location on the average ion energy and ion flux density.
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| 5. |
- Hayes, Conor F., et al.
(författare)
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A Brief Guide to Multi-Objective Reinforcement Learning and Planning
- 2023
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Ingår i: Proceedings of the 22nd International Conference on Autonomous Agents and Multiagent Systems (AAMAS). - 9781450394321 ; , s. 1988-1990
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Konferensbidrag (refereegranskat)abstract
- Real-world sequential decision-making tasks are usually complex, and require trade-offs between multiple–often conflicting–objectives. However, the majority of research in reinforcement learning (RL) and decision-theoretic planning assumes a single objective, or that multiple objectives can be handled via a predefined weighted sum over the objectives. Such approaches may oversimplify the underlying problem, and produce suboptimal results. This extended abstract outlines the limitations of using a semi-blind iterative process to solve multi-objective decision making problems. Our extended paper [4], serves as a guide for the application of explicitly multi-objective methods to difficult problems.
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| 6. |
- López-Rodríguez, D., et al.
(författare)
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Characterisation of radio frequency plasmas in the upgraded TOMAS device
- 2023
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Konferensbidrag (refereegranskat)abstract
- To complement wall conditioning research in TOMAS, a characterisation of radio-frequency hydrogen plasmas has been performed using a radially movable triple Langmuir probe. Experimental measurements of electron temperature and density radial profiles at different magnetic field on axis strengths and neutral pressures have been performed. First results of simulations of the radial profiles with the code TOMATOR-1D can qualitatively reproduce the measurements of the diagnostic and may be used to understand the behaviour of the waves inside the plasma.
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