Modification of the electron energy distribution function during lithium experiments on the National Spherical Torus Experiment

Jaworski, M. A.; Bell, M.G.; Gray, Travis; Kaita, R.; Kallman, J.; Kugel, H.; LeBlanc, B.; McLean, A.G.; Sabbagh, S. A.; Soukhanovskii, V.; Stotler, D.P.; Surla, V.

doi:10.1016/j.fusengdes.2011.07.013

Cited by 28 publications

(33 citation statements)

References 28 publications

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“…To explain the bi-Maxwellian EEDF behaviour, a "heuristic model" accounting for the ionization of neutral hydrogen is proposed in [3]. It has to be noted that, in the limiter shadow, the EEDFs established are Maxwellian with a temperatures of about 6-7 eV which are not enough for ionization.…”

Section: Resultsmentioning

confidence: 99%

“…electron energy distribution function. Dedicated investigations aiming at evaluating the EEDF employing this method have revealed a bi-Maxwellian EEDF in the CASTOR edge plasma [2], in the COMPASS tokamak and, more recently in the liquid lithium divertor area of NSTX [3]. This technique was applied to the investigation of ISTTOK tokamak plasma.…”

Section: Introductionmentioning

confidence: 99%

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Determination of the electron energy distribution function in the ISTTOK tokamak

Dimitrova

Silva

Fernandes

et al. 2014

J. Phys.: Conf. Ser.

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Plasma potential and electron temperature evaluated by ball-pen and Langmuir probes in the COMPASS tokamak M Dimitrova, Tsv K Popov, J Adamek et al. Abstract. The first derivative probe technique was applied to study the ISTTOK tokamak plasma. This technique employs the electron part of the Langmuir probe current-voltage (IV) characteristic and yields information on the plasma potential and the electron energy distribution function (EEDF). The IV characteristic was measured with new electrical probes mounted on a horizontal manipulator, one oriented in parallel and the other perpendicularly to the magnetic field lines. Using the first-derivative probe technique, the plasma potential and the EEDF at different radial positions were acquired. We show that, in the vicinity of the last close flux surface (LCFS), the EEDF is non-Maxwellian and can be approximated by a biMaxwellian one with a dominant cold electron population and a minority group of hot electrons. In the limiter shadow, the EEDF obtained is Maxwellian.The comparison of the plasma parameters evaluated using the first-derivative probe technique for both probes shows a satisfactory agreement.

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Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Determination of the electron energy distribution function in the ISTTOK tokamak

Dimitrova

Silva

Fernandes

et al. 2014

J. Phys.: Conf. Ser.

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“…[17,18,44] where bi-modal distribution functions (distribution function of 2 populations of particles at different temperatures) have been used. Other results are published in the literature describing effects of non-Maxwellians on the Langmuir probe measurements [36][37][38][39][40][41][42][43][44][45][46][47]. The bi-modal approximation is the first efficient way to describe NMDFs, but it assumes the superposition of two populations of particles at the Maxwellian equilibrium and the self-consistency is very restricted since in this bi-modal description the plasma is enough collisional to assure a MDF for each species but do not allow collisions between these two populations.…”

Section: B Corrections For the Langmuir Probes Interpretationsmentioning

confidence: 99%

“…(ii) Another example is the discrepancy of the Langmuir probe interpretation [36][37][38][39][40][41][42][43][44][45][46][47] with respect to the Thomson scattering measurements [48,49] of the electron temperature which has been associated with the presence of super-thermal particles [17,18] (i.e., bulk and super-thermal populations, both at different thermodynamic equilibrium). The Langmuir probe is one of the most used diagnostic for low temperature plasmas.…”

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confidence: 99%

Kinetic corrections from analytic non-Maxwellian distribution functions in magnetized plasmas

Izacard

2016

Physics of Plasmas

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In magnetized plasma physics, almost all developed analytic theories assume a Maxwellian distribution function (MDF) and in some cases small deviations are described using the perturbation theory. The deviations with respect to the Maxwellian equilibrium, called kinetic effects, are required to be taken into account specially for fusion reactor plasmas. Generally, because the perturbation theory is not consistent with observed steady-state non-Maxwellians, these kinetic effects are numerically evaluated by very CPU-expensive codes, avoiding the analytic complexity of velocity phase space integrals. We develop here a new method based on analytic non-Maxwellian distribution functions constructed from non-orthogonal basis sets in order to (i) use as few parameters as possible, (ii) increase the efficiency to model numerical and experimental non-Maxwellians, (iii) help to understand unsolved problems such as diagnostics discrepancies from the physical interpretation of the parameters, and (iv) obtain analytic corrections due to kinetic effects given by a small number of terms and removing the numerical error of the evaluation of velocity phase space integrals. This work does not attempt to derive new physical effects even if it could be possible to discover one from the better understandings of some unsolved problems, but here we focus on the analytic prediction of kinetic corrections from analytic non-Maxwellians. As applications, examples of analytic kinetic corrections are shown for the secondary electron emission, the Langmuir probe characteristic curve, and the entropy. This is done by using three analytic representations of the distribution function: the Kappa (KDF), the bi-modal or a new interpreted non-Maxwellian (INMDF) distribution function. The existence of INMDFs is proved by new understandings of the experimental discrepancy of the measured electron temperature between two diagnostics in JET. As main results, it is shown that (i) the empirical formula for the secondary electron emission is not consistent with a MDF due to the presence of super-thermal particles, (ii) the super-thermal particles can replace a diffusion parameter in the Langmuir probe current formula and (iii) the entropy can explicitly decrease in presence of sources only for the introduced INMDF without violating the second law of thermodynamics. Moreover, the first order entropy of an infinite number of super-thermal tails stays the same as the entropy of a MDF. The latter demystifies the Maxwell's demon by statistically describing non-isolated systems.The observation of non-equilibrium distribution functions is possible in a large number of areas other than laboratory plasma physics such as in astrophysics plasmas, hydrodynamic fluids and molecular dynamics, atomic physics, condensed matter, chemical reactions and in statistics (see Refs. [1][2][3][4][5]). We focus here on laboratory plasma physics with charged particles in a magnetic field relevant to tokamaks and spherical tokamaks for the purpose of energy production by fusion ener...

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“…An additional capability of the HDLP array is to monitor the scrape-off-layer currents that are flowing through the PFC and machine linked currents [3][4][5][6]. Figure 2 shows measurements of the machine-linked currents in a 900 kA, 2MW NBI, ELM-free discharge in NSTX.…”

Section: Summary Of Lld Resultsmentioning

confidence: 99%

Liquid Lithium Divertor and Scrape-Off-Layer Interactions on the National Spherical Torus Experiment:2010 ₋2013 Progress Report

Ruzic

Andruczyk

2013

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The implementation of the liquid Lithium Divertor (LLD) in NSTX presented a unique opportunity in plasma-material interactions studies. A high density Langmuir Probe (HDLP) array utilizing a dense pack of triple Langmuir probes was built at PPPL and the electronics designed and built by UIUC. It was shown that the HDLP array could be used to characterize the modification of the EEDF during lithium experiments on NSTX as well as characterize the transient particle loads during lithium experiments as a means to study ELMs.With NSTX being upgraded and a new divertor being installed, the HDLP array will not be used in NSTX-U. However UIUC is currently helping to develop two new systems for depositing lithium into NSTX-U, a Liquid Lithium Pellet Dripper (LLPD) for use with the granular injector for ELM mitigation and control studies as well as an Upward-Facing Lithium Evaporator (U-LITER) based on a flash evaporation system using an electron beam.Currently UIUC has Daniel Andruczyk Stationed at PPPL and is developing these systems as well as being involved in preparing the Materials Analysis Particle Probe (MAPP) for use in LTX and NSTX-U. To date the MAPP preparations have been completed. New sample holders were designed by UIUC's Research Engineer at PPPL and manufactured at PPPL and installed. MAPP is currently being used on LTX to do calibration and initial studies. The LLPD has demonstrated that it can produce pellets. There is still some adjustments needed to control the frequency and particle size. Equipment for the U-LITER has arrived and initial test are being made of the electron beam and design of the U-LITER in progress. It is expected to have these ready for the first run campaign of NSTX-U.

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Modification of the electron energy distribution function during lithium experiments on the National Spherical Torus Experiment

Cited by 28 publications

References 28 publications

Determination of the electron energy distribution function in the ISTTOK tokamak

Determination of the electron energy distribution function in the ISTTOK tokamak

Kinetic corrections from analytic non-Maxwellian distribution functions in magnetized plasmas

Liquid Lithium Divertor and Scrape-Off-Layer Interactions on the National Spherical Torus Experiment:2010 ₋2013 Progress Report

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