High heat flux facility GLADIS:: Operational characteristics and results of W7-X pre-series target tests

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Abstract

The new ion beam facility GLADIS started the operation at IPP Garching. The facility is equipped with two individual 1.1 MW power ion sources for testing actively cooled plasma facing components under high heat fluxes. Each ion source generates heat loads between 3 and 55 MW/m2 with a beam diameter of 70 mm at the target position. These parameters allow effective testing from probes to large components up to 2 m length. The high heat flux allows the target to be installed inclined to the beam and thus increases the heated surface length up to 200 mm for a heat flux of 15 MW/m2 in the standard operating regime. Thus the facility has the potential capability for testing of full scale ITER divertor targets. Heat load tests on the WENDELSTEIN 7-X pre-series divertor targets have been successfully started. These tests will validate the design and manufacturing for the production of 950 elements.

Introduction

The operation of long pulse fusion experiments requires plasma facing components (PFCs) capable of steady-state operation with high thermal loads of up to 10–20 MW/m2 and long mean time between necessary replacement. In order to investigate the thermo-mechanical behaviour of such PFCs, extensive tests with heat loads similar to the operational conditions are mandatory. High heat flux (HHF) tests are essential to assess the performance and to define the acceptance criteria for the quality control of industrially manufactured divertor components for fusion experiments such as W7-X and ITER.

As an addition to the existing test facilities, which mostly use electron beam heating [1], [2], [3], [4], [5], the IPP Garching has commissioned the HHF test facility GLADIS (Garching Large Divertor Sample Test Facility) of the ion beam type shown in Fig. 1, Fig. 2. The water-cooled vacuum test chamber (1.5 m diameter, 3.2 m length) is equipped with two H+ ion sources. Both ion sources are inclined at 8° to the horizontal axis of the facility. The preferred position of the target is 3 m in the z-direction from the ion sources at the intersection of the beams. So far, only one of the two RF ion sources has been used for heat loading tests with an operating regime of 3–55 MW/m2 at the target position. A detailed technical description of the design of the main components, the diagnostics, and the control and data acquisition systems can be found in another publication [6].

Ion beams generate homogeneous heating due to the complete absorption of the beam power on the surface of the tested material. They are especially suited for loading of metallic surfaces, e.g. tungsten. In electron beam facilities, reflection and formation of secondary electrons reduces the deposited beam power by up to ∼50% for these materials [7].

A mixed beam of ions and neutrals is used in GLADIS in contrast to other ion beam facilities [8], [9]. Since a complex magnetic ion removal system with deflection magnets, ion dumps and cryopumps is unnecessary, the cost of the facility was significantly reduced. However, the operation of the ion source causes an increase of the hydrogen pressure in the main vacuum chamber to 1 × 10−3 mbar during the pulse. This increase does not affect source performance. The beam dump, integrated in the movable end-door, is able to absorb the full beam power for continuous operation.

GLADIS allows effective testing of large components up to 2 m length and can provide up to 8.5 l/s of cooling water. The high perpendicular heat flux allows targets to be installed inclined to the beam and thus lengthening the heated surface up to 200 mm for a heat flux of 15 MW/m2 in the standard operating regime. Therefore the facility has the potential for the testing of full scale ITER divertor targets [10].

Section snippets

Ion sources

The GLADIS facility uses the two sources previously used to provide radial heating on the W7-AS Stellarator. The current is extracted from a water-cooled 3-grid system originally developed for long pulse heating on the ASDEX tokamak [11]. Two modifications to the sources have been made. The first is the utilization of a cooled Faraday screen in the plasma chamber to allow the pulse length to be extended to CW operation. The second is a re-cabling of the transformer used to isolate the RF

First results of W7-X pre-series target tests

The envisaged steady-state operation of W7-X requires the installation of a divertor target (area 19 m2) assembled from 14 different types of actively cooled elements (total 890) [13]. A target element consists of flat tiles made of CFC (Carbon Fibre reinforced Composite) NB31 as plasma facing material bonded onto water-cooled heat sinks made of CuCrZr copper alloy. It is designed to withstand a steady-state heat flux of 10 MW/m2 and a power load up to 100 kW. Depending on the length of the

Conclusions

The high heat flux test facility GLADIS routinely operates providing a beam power between 150 kW and 1.1 MW from one ion source. The beam pulses are highly reproducible and so far over six months of operation have been achieved with no interruption due to the sources themselves. Presently, one ion source allows high power operation with pulse lengths up to 15 s. The measured beam profiles are in good agreement with the theoretical calculations.

First tests demonstrated that GLADIS is an useful tool

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