Študij obrnjenega plazemskega plašča z enodimenzionalnim kinetičnim modelom in simulacijami delec v celici

BAJT, KRIŠTOF (author), Gyergyek, Tomaž (mentor), Kovačič, Jernej (comentor)

Kadar je elektroda potopljena v plinsko plazmo in odklopljena od zunanjega vezja, se praviloma nabije na potencial, ki je negativen glede na potencial plazme. Pravimo mu potencial lebdenja elektrode. Pred elektrodo se formira tanko območje, kjer obstaja presežek pozitivnega naboja. Večji del padca potenciala med plazmo in elektrodo je lokaliziran na to območje, ki mu pravimo plazemski plašč. Če elektroda iz kakršnegakoli razloga emitira elektrone, potencial lebdenja narašča in se približuje plazemskemu potencialu. Ob zares veliki elektronski emisiji, naj bi potencial lebdenja postal celo pozitiven glede na plazemski potencial - prišlo naj bi do pojava obrnjenega plašča. Jasnih eksperimentalnih potrditev tega pojava doslej še ni bilo dosti, razen redkih poročil, da so opazili potencial lebdenja emisijske sonde, ki je bil višji od plazemskega potenciala. Domnevamo pa, da bo v divertorjih velikih tokamakov - na primer ITER - energijski tok na steno tako velik, da bo stena močno emitiralal elektrone in da bo prišlo tudi do pojava obrnjenega plašča, zato je modeliranje in računalniško simuliranje tega pojava zelo pomembno. Ta vrsta reaktorjev bo namreč uporabljena v fuzijskih elektrarnah prihodnosti. V tem delu obravnavamo enodimenzionalni kinetični model obrnjenega plašča v plazemski diodi. Izhajamo iz prepdostavke, da je profil potenciala monoton, porazdelitvene funkcije ionov in elektronov pa so zaradi tega rezane Maxwellove. Z modelom smo naredili spekter izračunov in napovedi modela prvič kvantitativno primerjali z rezultati delčnih simulacij. Za te simulacije smo uporabili Berkeleysko kodo XPDP1. V začetku so na kratko opisani bistveni osnovni pojmi, ki jih uporabljamo pri opisovanju plinske plazme. Nadalje so podrobneje predstavljene delčne simulacije plazme in njihova izvedba s pomočjo programa XPDP1. Sledi izpeljava kinetičnega modela obrnjenega plašča in izračuna njegovih rezultatov. Na koncu so primerjani rezultati obeh pristopov, ki potrjujejo predpostavke izpeljave fizikalnega modela, hkrati pa pokažejo odlično ujemanje rezultatov obeh pristopov pri določanju potenciala in električnega polja v sistemu z obrnjenim plaščem.

• Language: Slovenian
• Keywords: plazma, tokamak, divertor, obrnjeni plašč, kinetični model, delčne simulacije
• Work type: Master's thesis/paper
• Organization: FE - Faculty of Electrical Engineering
• Year: 2021
• PID: 20.500.12556/RUL-125552
• Publication date in RUL: 24.03.2021

Secondary language

• Language: English
• Title: Study of inverted plasma sheath by one-dimensional kinetic model and particle in cell simulations

Abstract

An electrode that is immersed in a plasma and disconnected from any external circuit biases itself to a potential, which is usually negative with respect to plasma potential. It is called the floating potential of the electrode. In front of the electrode a thin layer of positive space charge is formed. The largest part of potential drop between the plasma and the electrode is localized in this layer, which is called plasma sheath. If the electrode emits electrons for whatever reason, the floating potential increases and comes close to the plasma potential. If electron emission becomes really large, the floating potential of the electrode might even become larger than the plasma potential. Consequently an inverted sheath should be formed in front of the electrode. So far clear cut experimental confirmation of such a phenomenon has not yet been reported in the literature, except of some observations of positive floating potential of emissive probes. But it can be expected that in the divertors of the future large tokamaks (e.g. ITER) energy fluxes to the walls will be so large that the walls will become so emissive that formation of inverted sheath can be expected. This type of reactors will be used in future fusion power plants. Because of this many activities related to modelling and simulation of such phenomena are under way. In this work a one dimensional kinetic model of an inverted sheath in a plasma diode is presented. The basis for the model is the assumption that the potential profile is monotonic and consequently velocity distributions of ions and electrons are cutoff Maxwellians. Using the model calculations are made and for the first time compared the predictions of the model with the results of particle in cell simulations. For the simulations the Berkeley XPDP1 code was used. The thesis starts with the short description of key plasma parameters defining the sheath formation. Secondly particle simulations of plasma systems are described, manly the principles behind XPDP1 simulation program. Next kinetic model of inverted sheath is derived and results calculated. Through the comparison of the results initial assumptions of the model are confirmed to be valid and excellent coherence of the results presented.

• Keywords: plasma, tokamak, divertor, inverted sheath, kinetic model, particle simulations