Tungsten transport and sources control in JET ITER-like wall H-mode plasmas

doi:10.1016/j.jnucmat.2014.12.044

Journal of Nuclear Materials

Volume 463, August 2015, Pages 85-90

https://doi.org/10.1016/j.jnucmat.2014.12.044 Get rights and content

Abstract

A set of discharges performed with the JET ITER-like wall is investigated with respect to control capabilities on tungsten sources and transport. In attached divertor regimes, increasing fueling by gas puff results in higher divertor recycling ion flux, lower divertor tungsten source, higher ELM frequency and lower core plasma radiation, dominated by tungsten ions. Both pedestal flushing by ELMs and divertor screening (including redeposition) are possibly responsible. For specific scenarios, kicks in plasma vertical position can be employed to increase the ELM frequency, which results in slightly lower core radiation. The application of ion cyclotron radio frequency heating at the very center of the plasma is efficient to increase the core electron temperature gradient and flatten electron density profile, resulting in a significantly lower central tungsten peaking. Beryllium evaporation in the main chamber did not reduce the local divertor tungsten source whereas core radiation was reduced by approximately 50%.

Introduction

Tungsten (W) as divertor material offers both relevant armor life time [1] and minimum fuel retention [2] for a reactor-grade tokamak operation, as tested in JET with the new ITER-like wall (ILW) [3]. The operational drawbacks are possible wall melting [4] under extreme heat load, and pollution of the confined plasma by radiating high-Z ions. As experienced in ASDEX-Upgrade (AUG) [5], [6] and now in JET [7], this pollution must be controlled to avoid radiation levels incompatible with high confinement (H-mode) operation, generally at a cost of reduced confinement performances [7]. In AUG, H-modes plasmas are generally performed at non-zero gas puff in the divertor to ensure high edge localized modes (ELMs) frequency, reduce divertor electron temperature $(T_{e})$ and thereby the W source [5]. These ELMs prevent tungsten from entering the core plasma, but also increase the energy and particle losses. Central electron heating is also generally applied to repulse tungsten ions from plasma center. Finally, boronizations are performed to reduce the tungsten source. In this contribution we focus on techniques tested in JET-ILW to control tungsten sources and transport in H-mode attached divertor plasmas. In Section 2, we expose some physics involved in plasma pollution by tungsten ions, linked to the interpretation of experimental data and discharge behavior. In Section 3 we present the experimental setup and some aspects of data interpretation concerning tungsten density and source estimate. In Section 4 we discuss the experimental observations. First, the influence of divertor gas puff on discharge pollution is investigated in the scope of W source control, ELMs flushing and divertor screening. The beneficial impact of artificially increased ELMs frequency by magnetic perturbation is discussed. Then, the beneficial impact of main chamber beryllium evaporation is investigated. Finally, core pollution control with central ion cyclotron resonance heating (ICRH) is addressed.

Section snippets

Physics of tungsten source and transport

Tungsten source originates from physical sputtering of tungsten surfaces by impacting ions. Thermal deuterium ions have a small contribution and light impurities generally dominate: residual carbon, boron and oxygen in the case of AUG [8], beryllium in the case of JET-ILW [9]. Energetic deuterium ions originating from ELMs or neutral beam injection (NBI) can however significantly contribute to tungsten source [10], or ions accelerated in rectified sheath potential in the surrounding of ICRH

Plasma scenarios

Control of tungsten is investigated in two relevant ITER baseline scenarios ( $q_{95} \approx 3$ ). The first one relies on ELM frequency control with magnetic perturbations, known as vertical kicks. Under specific conditions, an ELM can be triggered by imposing a fast vertical displacement to the plasma (millisecond time scale). This displacement induces a current perturbation in the pedestal (see [24] for more details), impacting MHD stability. By repeating the perturbation at a given rate, ELM frequency is

Impact of fueling on discharge behavior

Increasing the fueling rate is commonly used in AUG to increase ELM frequency and reduce the W source in order to mitigate the confined plasma radiation caused by tungsten [5]. As shown in Fig. 2 and previously reported [7], a finite fueling rate is mandatory in JET-ILW to avoid H-mode collapse by radiation. In the scenario investigated here, a large divertor fueling rate ( $Γ_{e} \approx 15 \times 10^{21}$ s⁻¹) is applied over typically 1 s prior to H-mode formation, before being reduced to the desired value. By this

Conclusion and discussion

A series of experiments have been conducted on JET-ILW do identify the relevant physical mechanisms involved in the control of tungsten sources and transport. Tungsten accumulation at the very center of the confined plasma can be mitigated with sufficient amount of ICRH. Flattening of the central radiation profile is correlated with a peaking of the electron temperature and a flattening of the electron density profile. However the application of ICRH leads to an increase of the global radiation

Acknowledgment

This work was supported by EURATOM and carried out within the framework of the European Fusion Development Agreement. The views and opinions expressed herein do not necessarily reflect those of the European Commission.

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Tungsten transport and sources control in JET ITER-like wall H-mode plasmas

Abstract

Introduction

Section snippets

Physics of tungsten source and transport

Plasma scenarios

Impact of fueling on discharge behavior

Conclusion and discussion

Acknowledgment

J. Nucl. Mater.

J. Nucl. Mater.

J. Nucl. Mater.

J. Nucl. Mater.

J. Nucl. Mater.

J. Nucl. Mater.

J. Nucl. Mater.

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Phys. Scripta

Nucl. Fusion

Nucl. Fusion

Nucl. Fusion

Nucl. Fusion

Plasma Phys. Control. Fusion

Phys. Scripta

Nucl. Fusion

Phys. Plasmas