ITER RF sliding contact prototype design

Sliding electrical contacts have very distinctive characters to guarantee the electrical connection and mechanical sliding. Generally, sliding electrical contacts contain a large number of parallel louvers which can be compressed elastically. Improving the electrical contact performance and wear performance are the critical issues related with sliding electrical development and usually they are contradictory to each other.

Especially for ITER RF sliding contacts, which are expected to be operated under high current load and have long lifetime, more attention should be paid to the mechanical design and material selection.

3.1.1 RF sliding contact configuration selection

Figure 3-1 Commercial sliding electrical contact designs [19]

As introduced, the RF sliding contact should have compressive flexible contact

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louvers to apply contact force. There are two kinds of louver designs which can realize this purpose: individual or separated louvers (Figure 3-1).

For individual louver sliding contact, the contact louvers are both the electrical current flowing media and the flexible mechanical support. The design is more compact compared with the separated louver configuration. In order to combine these two functions, the material used for this kind of contact should have good electrical, thermal and elastic properties. It’s quite hard to find a material that can meet all the above properties. So, individual louver configuration is only suitable for low current sliding contact application which has low requirements on the material’s electrical conductivity or mechanical performance. ITER RF sliding contact is a heavy load electrical contact, thus individual louver configuration is not suitable.

Separated louvers are the other sliding electrical contact configuration on which a functional division is realized between the contact louvers and the spring element.

The main advantage of this configuration is that the louvers and the spring element can be manufactured with different materials. During design, the selection of the louver materials focuses on their electrical and thermal performances while for the spring element materials the focus is made on mechanical properties (e.g., fatigue strength and yield stress). Separated louvers sliding contact configuration (such as LA-CUT, from Stäubli Multi-Contact [19]) has been tested for ITER RF sliding contact development. With the LA-CUT product, there is no mechanical bonding between the contact strip and the conductor: the LA-CUT contact strip is inserted into a groove (Figure 3-2). Therefore, this type of sliding electrical contact can be replaced easily but at the price of a serious reduction of the heat transfer between the strip and the holding conductor.

Figure 3-2 Schematic of LA-CUT sliding contact assembly

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3.1.2 Base material selection for the RF contact

The Figure 3-2 shows an ITER RF sliding contact design where there are two primary materials: the base materials for the conductors and the base material for the contact louvers. For commercial LA-CUT contact, the contact louvers can be made of oxygen-free high conductivity (OFHC) copper which has high electrical conductivity and thermal conductivity. However, for ITER application, the mechanical properties of copper have to be improved to resist ITER 250ºC (850 days cumulative) outgas baking and high temperature during operation, in which creeping issues can lead to failure[109].

Adding hetero-elements in a metal matrix to generate metal alloys can improve the microstructure of the pure metal and increases some of its material properties. For copper, common copper alloys are CuCrZr, CuBe, CuCr and CuNiBe. Their main mechanical and thermal properties are shown in Figure 3-3 and Figure 3-4. OFHC shows poor performance in yield stress under all temperatures. CuBe and CuNiBe have higher mechanical strength, around seven times higher than OFHC and have a higher thermal stability under mechanical strength under temperature up to 300ºC. On the Large Hadron Collider (LHC) in CERN, RF contacts support a nominal beam current of 0.6 A and CuBe is used for the contact louvers’ manufacturing [110, 111].

For ITER RF contacts which are expected to be operated under 2 kA, the relatively high resistivity and relatively low thermal conductivity of CuBe make it improper to use.

Figure 3-3 Yield stresses of copper alloys under different temperature [112]

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Figure 3-4 Yield stresses and thermal conductivities of copper alloys [112]

CuCrZr shows good mechanical performance even under high temperature at 250ºC and its yield strength is about 5 times higher than OFHC. Moreover, the electrical conductivity and thermal conductivity of CuCrZr at room temperature are similar to OFHC, which are 77% IACS (International Annealed Copper Standard) [113, 114]

and 80% IACS respectively. Consequently, they are suitable for high loading parts such as springs, which conduct electricity, contact wheel, etc [115]. CuCrZr alloy is widely used on fusion experimental devices as heat sink material [116-120], due to its excellent thermal conductivity, strength and fatigue resistance [121]. So, CuCrZr becomes an interesting material for ITER to manufacture the RF contact louvers. Like pure copper, Cu₂O and/or CuO will form on the CuCrZr surface due to oxidation when exposed to atmosphere [121-124]. A functional coating to protect from oxidation and also to minimize contact resistance is then required. Gold or silver coating are generally selected to cover the surface of the contact louvers.

For the base material of ITER RF conductors, currently titanium is selected [17].

However, considering the large mass of the ITER ICRH antenna RF conductors, the application of titanium as base material is costly. Common structural base materials such as 316L and CuCrZr are also proposed. In this thesis study, the base materials of the RF conductor are focused on 316 L and CuCrZr.