Plasma start-up design and first plasma experiment in VEST

doi:10.1016/j.fusengdes.2015.05.015

Fusion Engineering and Design

Volumes 96–97, October 2015, Pages 274-280

https://doi.org/10.1016/j.fusengdes.2015.05.015 Get rights and content

Highlights

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First plasma start-up experiment in VEST has been successfully carried out.
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A start-up design code is developed to generate start-up scenarios in predictive manner.
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Start-up scenarios are developed with the code and confirmed experimentally in VEST.
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Full tokamak equilibria (I_p = 70 kA, κ = 1.6 and q_edge = 3.7) are achieved from the scenario.

Abstract

First plasma start-up experiment in Versatile Experiment Spherical Torus (VEST) has been successfully carried out with conventional start-up scheme using a central solenoid. A start-up design code has been developed for the development of start-up scenarios in VEST, which can predict evolutions of vacuum field structure and plasma current with given operation parameters from the PF coil power supply circuit considering the eddy currents. Start-up scenarios are successfully developed with the code, generating field null region in the inboard side at the onset of loop voltage in order to maximize the connection length, and then providing the required poloidal magnetic field for stable equilibrium as the plasma current evolves. Hydrogen is used as a working gas and the toroidal field of 0.1 T with 0.2 s flat-top is applied on magnetic axis during the discharge. With the assist of ECH (Electron Cyclotron Heating) pre-ionization (6 kW, 2.45 GHz), a plasma current of ∼70 kA has been generated with the pulse duration of ∼10 ms. The elongation and edge safety factor are estimated to be 1.6 and 3.7 respectively.

Introduction

The VEST (Versatile Experiment Spherical Torus) is the first Korean spherical torus, located at Seoul National University. The main device parameters are major radius of 0.4 m, aspect ratio of >1.3 and toroidal magnetic field of 0.1 T on axis. The main objective of VEST is to conduct basic research on a compact, high-β spherical torus with an elongated chamber and novel partial ohmic solenoids. Partial solenoids will be utilized for the double plasma merging start-up, which was pioneered by UTST [1], [2], [3]. UTST has utilized outer PF coils to provide the loop voltage for double plasma merging start-up but partial solenoids of VEST are expected to provide higher loop voltage with less stray field inside the vacuum vessel. Details of the device from key design concepts to commissioning results can be found in elsewhere [4], [5], [6], [7], [8].

After successful construction and commissioning of VEST, first plasma experiments have been conducted. ECH (Electron Cyclotron Heating) pre-ionization has been utilized for the reliable start-up with limited volt-second under severe eddy currents, and start-up scenarios have been developed systematically by developing a start-up design code, which can predict the evolution of vacuum field structure with given operation parameters of the PF coil power supply circuits by considering eddy currents at the thick vacuum vessel wall. With the start-up code the start-up scenario of VEST has been improved so that plasma current, I_p of ∼70 kA with pulse duration of ∼10 ms has been achieved so far. Overall schematic drawing of VEST as well as the PF coil configuration is shown in Fig. 1.

In this paper, detailed descriptions on the first plasma experiments of VEST are presented. In Section 2, start-up scenario development by using the start-up design code is described. In Section 3, results of the first plasma experiments are presented. Conclusions and future plans are followed in Section 4.

Section snippets

Required conditions for the reliable start-up

The plasma breakdown in tokamaks including spherical tori is generally described by Townsend avalanche theory [9], [10], [11]. Following empirical formula is widely used to evaluate the condition for the reliable start-up with pre-ionization in practical use [9], [10]; $E_{t} \frac{B_{t}}{B_{p}} > 100 [V/m]$ where E_t, B_t and B_p are the toroidal electric field, toroidal magnetic field and poloidal magnetic field, respectively.

After successful current initiation with sufficient plasma breakdown, the equilibrium field

Typical discharge procedure and conditions

Typical discharge procedure for the first plasma experiment in VEST is depicted in Fig. 5. At the onset of a discharge, the TF power supply is triggered and it takes about 200 ms for the TF coil current to reach the flat-top value of 8.3 kA (0.1 T at R₀ = 0.4 m). Relatively long TF flat-top with the duration of 400 ms is generated with battery-powered TF coil. The TF power supply is turned off at 550 ms by sequentially turning off each battery module [4], [7]. The PF#1 coil current is ramped up at 387

Conclusions and future plans

The start-up design code has been developed for the development of start-up scenarios in VEST, which can predict the vacuum field structure and the I_p evolution with given operation parameters considering the eddy currents at the wall and their influence on the PF coil current self-consistently. First plasma start-up experiments in VEST have been successfully carried out with the conventional start-up scheme using a central solenoid based on start-up scenarios developed by the start-up design

Acknowledgments

This work was supported by National R&D Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (No. 2014M1A7A1A03045367 and 2008-0061900) and also supported by the NRF (No. NRF-2012K2A2A6000505).

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View full text

Plasma start-up design and first plasma experiment in VEST

Highlights

Abstract

Introduction

Section snippets

Required conditions for the reliable start-up

Typical discharge procedure and conditions

Conclusions and future plans

Acknowledgments

Fusion Eng. Des.

Fusion Eng. Des.

Fusion Eng. Des.

Fusion Eng. Des.

Vacuum

Plasma Fusion Res.

Electric. Eng. Jpn.

Nucl. Fusion

Plasma Sci. Technol.

Nucl. Fusion