Adoption of the ASDEX Upgrade pumping to hydrogen released by an in-vessel cryopump

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Abstract

Detached divertor scenarios at ASDEX Upgrade (AUG) need high gas fluxes, which are mainly pumped with the gas binding in-vessel cryopump (CP). To cope with the large quantities of hydrogen released from the CP during regeneration, a hydrogen compatible pumping system (HCPS), which is built to comply with safety regulations for explosion protection, has been built up in parallel to the already existing gas transfer pumping system (TPS). The amount of gas injected in the vacuum vessel (VV) is limited by an additional volume added to the gas inlet system. A fully automated procedure has been implemented for comprise with safety regulations. This includes daily tests of the components and communications with other relevant systems.

Introduction

For high power plasma scenarios, it is crucial to operate the divertor in a detached regime. This requires a high gas flux and therefore a high pumping speed in the divertor region. The ASDEX Upgrade (AUG) liquid helium cooled in-vessel cryopump (CP) [1] specifically designed for this purpose provides a pumping speed ≈10 times higher compared to the gas transfer pumping system (TPS) installed in parallel [2]. About 90% of the gases puffed for plasma operation is condensed on the cryogenic surfaces and built up an inventory inside the vacuum vessel (VV). Under the previous operating licence, single experiments were not allowed to exceed the inventory of 400 kPa*l. This consequently requires very frequent regenerations and limitations in feasible plasma scenarios. This paper describes the technical concept of extending the inventory from 400 kPa*l to 20 MPa*l, in order to enable high density plasma scenarios, taking into account safety regulations for explosion protection.

Section snippets

Safety concept

During AUG operation, mainly the hydrogen isotopes deuterium (D2) and protium (H2) are used. The explosion range is between 6.7 and 79.6 vol-%. (D2) respectively 4.0 and 77.0 vol-% (H2) in air. Technical devices used in the presence of explosive atmosphere of these conditions have to fulfil the ATEX directive (ATmosphère EXplosible) 2014/34/EU for the explosion group IIC and temperature class T1 [3]. A resume of the safety regulations concerning explosion protection for hydrogen and the

Hydrogen compatible pumping system

The HCPS can be divided into two main subsystems: the vacuum system and the inerting unit. The HCPS operates fully automated and is controlled by the TPS PLC (programmable logic control).

For vacuum generation, a dry screw pump, which fulfils the requirements for category 2 G according to ATEX directive 2014/34/EU [3] and for gases of the explosion group IIC and temperature class T2, was selected. The pumping speed of this pump is up to 650 m3/h and it reaches a final pressure of 0.5 Pa.

Given

Operational procedure

Before starting AUG operation, the entire vacuum system, the inerting unit and the liquid nitrogen supply are inspected by a fully automated functional test. If the proper condition is ensured, the TPS PLC generates a safety token, which is passed on to the SLS via the H1-bus system (Fig. 3). This token is the basic prerequisite for permission of experiment operation. The torus pressure is measured and the technical tightness of the recipient is verified, if the average torus pressure is 1 × 10

The intermediate volume at the gas inlet system

A simple method for verifying the regeneration process of the CP is to calculate the expected pressure in the VV after regeneration from the amount of gas injected. The GES has a fully automated inventory balancing system, which allows recording the amount of H2 and D2 injected into the torus. This balancing is based on the following correlation for the inventory I.I=(p0(H2)p1(H2))*10+(p0(D2)p1(D2))*10

The pressures p represent the pressures DS01L and DS01R (Fig. 5) at the beginning (p0) and

Summary

Most of the gas injected in the AUG VV is trapped on the liquid helium cooled in-vessel CP. For regeneration this gas inventory is released in the VV. To handle this high amount of flammable gas, the TPS was enhanced by a pumping system, which fulfills ATEX regulations. As many devices at AUG did not fulfil the explosion protection criteria the total amount of gas was limited to ensure that in case of vacuum loss the lower explosion level is underrun match the ATEX criteria. Therefore GES has

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014–2018 and 2019–2020 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission.

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