Impurity sputtering from the guard limiter of the lower hybrid wave antenna in a tokamak

Ou, Jing; Xiang, Nong; Men, Zongzheng

doi:10.1063/1.5006842

Cited by 4 publications

(3 citation statements)

References 23 publications

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“…Due to ion acceleration and electron deceleration of the sheath field, the ion heat flux to the wall is comparable to the electron's for a Debye sheath. For a plasma containing energetic electron components, it was found that the floating potential increased significantly for the sheath containing twotemperature Maxwellian electrons with a drift velocity or with a highenergy electron beam [22][23][24][25][26][27]. In [24,25], the effects of fast electrons on the heat flux to tokamak divertor plates were investigated by considering different fast electron fractions.…”

Section: Interactions Of Plasma and Guard Limiter In Front Of Lower H...mentioning

confidence: 99%

Interactions of plasma and guard limiter in front of lower hybrid wave antenna on EAST tokamak

Xiang

Men

et al. 2019

Nucl. Fusion

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The hotspots on the guard limiters of the lower hybrid wave (LHW) antenna on EAST tokamak not only cause serious damage to the guard limiters, but also strongly degrade the plasma performance due to enhanced impurity productions. In published studies, the heat flux to the limiters is assumed to be carried to limiter walls by electrons which can absorb a small amount of the launched wave energy via interactions with the lower hybrid modes of high parallel refractive index , and the effects of the sheaths formed in front of the limiter surface are ignored. In this work, the heat fluxes to the limiter surfaces are obtained consistently by conducting one-dimension particle-in-cell (PIC) simulations. For the plasma between two guard limiters, while the ions are assumed to be Maxwellian, the electrons are described by using the Fisch model, whose velocity distribution function (VDF) has a resonant plateau in the superthermal region formed by the electron and LHW interaction. Secondary electron emission (SEE) from the limiter surfaces is taken into account by considering two kinds of wall materials, carbon and tungsten. It is found that the sheath potential drop is significantly raised due to the presence of fast electrons. As a result, both ion and electron heat fluxes to the limiters are strongly enhanced compared to those in a Maxwellian plasma, which increases the wall temperature and gives rise to hotspots. The heat flux to the guard limiters increases with the width of the resonant plateau of the electron VDF (i.e. the electron energy). With the enhancement of the sheath potential drop, the physical sputtering yield from the primary and impurity ions on limiter surfaces is found to be significant, which may affect the plasma confinement and discharges.

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Section: Interactions Of Plasma and Guard Limiter In Front Of Lower H...mentioning

confidence: 99%

Interactions of plasma and guard limiter in front of lower hybrid wave antenna on EAST tokamak

Xiang

Men

et al. 2019

Nucl. Fusion

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“…The model is simple but it can capture the main characteristic of plasma-LHW interaction and can explain the fast electron flux in EAST [5]. Under the conditions that the typical electron density n e and temperature T e are n 10 e 17 < < 10 m 18 3…”

Section: Plasma-lhw Interaction Via Electron Landau Dampingmentioning

confidence: 99%

“…A number of theoretical and simulation studies have been shown that the presence of even a small population of energetic electrons can significantly change the potential drop across the sheath and the energy flux to the wall, and then modify the surface temperature of wall material and the resultant plasma-wall interaction processes [13][14][15][16][17][18]. In the case of the plasma in front of the LHW antenna in a tokamak, it is shown from theoretical and simulation results that electron distribution can be described by the background thermal Maxweilian and a small energetic electron energetic component resulting from absorption of a part of the LH spectrum [11,12,19].…”

Section: Introductionmentioning

confidence: 99%

Estimation of carbon impurity from the guard limiter of the lower hybrid wave antenna on EAST tokamak

Ou¹,

Xiang²,

Liu³

et al. 2018

Plasma Phys. Control. Fusion

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Hot spots on the EAST tokamak graphite guard limiters of the lower hybrid wave (LHW) antenna may cause a sudden increase of impurity influx and even ending with disruption. A sheath model is developed by taking into account the energetic electron component from the plasma-LHW interaction via electron Landau damping, and then a self-consistent method is used to study the interaction of the plasma across the sheath at the material surface and material thermal response. It is found that the fast electron fluxes driven by both the 2.45 and 4.6 GHz LHWs modify strongly the sheath potential, which then influence on the bombarding ion energy at the material surface and the surface temperature. By calculating the carbon physical sputtering and chemical erosion from guard limiter of the LHW antenna, the carbon production from the guard limiter in two LHW systems is found to have different behavior for the same edge plasma density and electron temperature. Our results show that carbon impurity is mainly from physical sputtering, except for the 4.6 GHz LHW in a narrow range of the high electron temperature where the chemical erosion has a very sharp increase. Due to modification of the sheath potential induced by the fast electrons, the power density is higher with the 4.6 GHz than the 2.45 GHz LHW system. As a consequence the surface temperature is higher at 4.6 GHz and susceptible to reach several hundred of degrees which is prone to chemical erosion.

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The effect of energetic electrons on the collisionless fusion plasma sheath in ion cyclotron range of frequencies heating

Men

2019

Physics of Plasmas

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A one-dimensional model, consisting of a hydrodynamic radio-frequency (RF) sheath model for the ion cyclotron range of frequencies heating and an equivalent circuit model, is used to study the structure of the collisionless RF sheath of a fusion plasma containing a beam of energetic electrons. For various energetic electron concentrations and velocities at the plasma-sheath edge, a set of equations describing the model are solved numerically to obtain the potential drop across the RF sheath and the sheath thickness, as well as the spatiotemporal variations of the potential, the ion density, and the background electron and energetic electron densities inside the sheath. Under the current EAST ion cyclotron range of heating conditions, it is observed that even at small beam fluxes, the potential drop across the sheath is enhanced at any time in an RF cycle for different bulk plasma densities, ion temperatures, and frequencies and amplitudes of the disturbance current. When the energetic electron component is included, the physical sputtering yields of the RF sheath wall materials such as titanium and iron become significant as a result of the enhancement of the drop in the sheath potential.

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Impurity sputtering from the guard limiter of the lower hybrid wave antenna in a tokamak

Cited by 4 publications

References 23 publications

Interactions of plasma and guard limiter in front of lower hybrid wave antenna on EAST tokamak

Interactions of plasma and guard limiter in front of lower hybrid wave antenna on EAST tokamak

Estimation of carbon impurity from the guard limiter of the lower hybrid wave antenna on EAST tokamak

The effect of energetic electrons on the collisionless fusion plasma sheath in ion cyclotron range of frequencies heating

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