FUSION TECHNOLOGY 1996 C. Varandas and F. Serra (editors) © 1997 Elsevier Science B.V. All rights reserved. 713

Sliding contact tests at high R.F. current under vacuum

G.Agarici, B. Beaumont, L. Ladurelle, G. Lombard, P. Mollard, H. Kuus

Association EURATOM-CEA, Département de Recherches sur la Fusion Contrôlée Centre de Cadarache, 13108 Saint Paul lez Durance Cedex, France

On each of the Tore Supra ICRH antennas [1], 4 variable vacuum capacitors supplied by COMET* are used to match the antenna feed point impedance to the generator output impedance. On such a system, the whole transmission line is working near matched conditions, and high RF voltages are limited to the front part of the antenna. The quality, performance and reliability of the COMET capacitors, fixed or variable type, have been proved by thousands of units in operation in high power oscillators and amplifier circuits. The variable type (10 to 150 pF) fitted in Tore Supra antennas has allowed remarkable results. A record radiated power density of 1.6 kW/cm2, has been achieved, and RF peak voltages in excess of 40 kV are commonly used during plasma shots. However, by construction of the capacitor, a water cooled bellows on the RF current path limits the velocity of the capacitance variation. Reliable RF experiments during transients phenomena like pellet injection, monsters sawteeth or plasma displacements require the matching system to follow rapid variations of loads. Therefore, a new concept of variable vacuum capacitor allowing faster capacitance adjustment, is studied in collaboration with COMET. In this concept, the RF current flows through a sliding contact strip from PANTECHNIK+ working in the sealed vacuum. The RF current path is therefore dissociated from the vacuum barrier. Sliding contacts, both in vacuum conditions and handling high RF current are not in common use in the RF field and have to be tested. This paper describes the existing variable capacitor, the new capacitor concept, the RF contact test device as well as the experimental results.

1. VARIABLE VACUUM CAPACITOR

1.1 Existing variable vacuum capacitors They are composed of two sets of thin concentric cylinders made of oxygen free copper : one is the fixed electrode while the other one, mounted on a shaft, moves for the capacitance adjustment (see figure 1). On this view, one can see the vacuum sealed envelope which is formed by a vacuumtight ceramic brazed between the fixed and the variable side of the capacitor. The flexible hard copper alloy bellows allows to change the capacitance when distance between electrodes varies. This bellows is also the electrical connection between the adjustable electrode and the body of the capacitor

As in most applications, these capacitors are working in a radio frequency domain, (between 30 and 80 Mhz on Tore Supra), where the skin effect confines the current flow to the surface layers of the conductors. Therefore, on the capacitor, the RF current flows from the connecting flange to the edge of the ceramic, and passes inside the capacitor to the electrodes. The variable electrode is in the TS antennas on the grounded side of the capacitor, so that the maximum current runs on the outer surface of the bellows. At the maximum operational conditions of the ICRH antennas, capacitors have to handle a current of 1000 Arms and a voltage of 45 kV, both values are near the maximum ratings. The power deposited by skin effect on the outer surface of the

* COMET : Comet Technik A.G., Berne, Switzerland + PANTECHNIK : Pantechnik S.A., Caen, France

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bellows is about 5 kW calculated at 60 MHz frequency. Cooling of the bellows is mandatory for such a power level and is performed by a forced cooling circuit located inside the bellows. The thickness of the bellows is 0.2 mm, so that this critical element supports high velocity water flow on its internal face, and high RF current in high vacuum environment on its outer surface. In addition, this conception is prone to long term corrosion through the bellows

Ball Bearing

Mounting Ring

Variable Electrode

Fixed Electrode

Figure 1. Cut Away View of a COMET Variable Vacuum Capacitor

Flow restrictions are needed in the cooling circuit to obtain high water velocity and therefore good heat exchange. On another hand, fast movements of the electrode leads to pressure peaks that can accelerate the fatigue of the material. These peaks are difficult to be precisely evaluated depending on a lot of parameters as: the static pressure, the water flow, the bellows stroke and the exact geometry. Therefore, to insure long life of the bellows, the maximum acceleration and velocity are presently limited. On Tore Supra antennas, where the capacitors are working near the maximum operating

conditions, we have limited the velocity at 3 cm/s although future experiments [2] would require rapid variations of the capacitance corresponding at velocity between 1.5 and 2 m/s.

1. 2 New variable vacuum capacitor concept This new concept concerns only the modification of the variable electrode assembly on the existing COMET capacitor. In this design (figure 2) the RF current path always flows from the outside of the capacitor electrode to the inside of the concentric cylinders but now it runs through sliding contacts working under vacuum. The movement of the shaft and the vacuum-tightness are insured by a bellows now located outside the RF current path and made in edge-welded stainless-steel. This element does need any cooling. The axial movement under vacuum is guided by a nitrided stainless-steel shaft sliding in a Cupro-Aluminium alloy bearing. Friction tests performed under vacuum at 2.10"4 Pascal and at temperature of 250°C have been carried out at Cadarache on different material couples. The above couple has allowed to make 50 000 cycles without problem. The water cooling circuit is now built within the shaft, and the internal volume is independent of the electrode movements. Hydraulic parameters can then be adjusted for optimal cooling and are not affected by displacement or acceleration The contact strip from PANTECHNIK is constituted of small cylindrical fingers in Silver/Carbon compound clinched on a silver platted Copper-Beryllium foil. In our application the contact band, using 59 fingers, is rolled up and fitted by small screws on the moving shaft. Each finger ensures the electrical contact to the fixed part of the variable electrode, where the fingers are in touch, by applying a force between 2.3 and 2.9 Newton. The contact strip is calculated to allow high density current up to 65 A per cm of length for a frequency range between 30 to 80 MHz. At 60 MHz for a current of 1000A rms., the calculated loss on each finger is of 7.3 W : 2 W for the contact resistance and of 5.3 W for the surface resistance. Total losses in the contact is then around 430 W If we also take into account the power deposition by skin effect on the outer surface of the shaft and on the fixed part of the electrode, both made in oxygen free high conductivity copper, we find a total power

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Ceramic _ „Moving electrode assembly

St-steel bellows

Sealed vacuum/

N l

_Pantechnik contact _fixed electrode assembly

Figure 2. Cut View of the New Variable Vacuum Capacitor Concept

± water f inlet

■« x water ; T outlet

dissipation of 980 W. This total dissipation has to be compare with the 5 kW found above only on the bellows of the existing capacitor. In order to develop this concept, intensive tests on the use of sliding contacts, both in vacuum conditions and conveying high RF current have been carried out at Cadarache.

2. RF SLIDING CONTACT TESTS

2.1 Tests device. It is, by the dimensions and the material of its components fully representative of the variable electrode assembly in the new capacitor concept. It consists (figure 3) of a 30 Q short-circuited vacuum coaxial line. It is connected to a 400 kW

generator, working at 60 MHz, through a 6 inches coaxial line. The matching system consists of 2 variable stubs placed in parallel along the transmission line. A directional coupler incorporated between the stubs and a vacuum window gives the RF current value. The PANTECHNIK RF contact strip is fitted on the front and on the rear part of the sliding shaft in order to keep the electrical length constant during the movement. The water cooled shaft is actuated by an electric motor on 50 mm stroke. The vacuum window is one of those used in the Tore Supra antennas. A stainless steel bellows, outside the RF cavity, allows the movement of the central conductor and a turbomolecular pump*evacuates the vacuum line.

_9 inches coaxial line Pumping Pantechnik contact 60 Amps/cm

Sliding shaft

High vacuum

Movement 50mm stroke

-St-Steel bellows

Figure 3. RF Tests Device Cut View

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Figure 4. Moving Short Circuit assembly of the Tests Device

A thermocouple situated near the surface in friction with the contact fingers gives the temperature information. Figure 4 shows the moving short-circuit assembly of the tests device with the PANTECHNIK contact fitted on the shaft.

2. 2 Tests and results. At the time of writing, the full tests allowing to completely qualify the contact are not finished but the first results are very encouraging:

- RF current of 1000 A rms. at 60 MHz has been passed during 60 seconds on the system with the contact sliding under vacuum at velocity of 3 cm/s without any RF problem (figure 5). - More than 15 minutes of satisfying RF pulses between 30 and 60 seconds with a current ranging from 650 to 1000 A rms. have been done in the test device with the shaft moving at 3 cm/s.

Slight damages have been found on the soft copper sliding surfaces of the fixed part. Tests with different sliding surfaces in hard zirconium chromium copper alloy are in preparation. Before manufacture of a prototype capacitor, RF tests without movement are also planned as well as the long term qualification of the system by a significant number of RF pulses at 1000 A rms. with and without movement. COMET is also studying for the new capacitor the possibility to

water cool the fixed part of the variable electrode assembly for long pulses compatibility. - For possible future applications, RF tests with the contacts sliding at high velocity, up to 2 m/s are also programmed.

Figure 5. Vacuum Sliding RF contacts tests REFERENCES

1. B.Beaumont and al., "Tore Supra ICRH Antennae Array", proc. 15th SOFT,Utrecht, September 1992, VI, p503 2. L. Ladurelle and al., "First Results of Automatic Matching System on Tore Supra ICRH Antennas; Fast Matching Network for ICRH Systems", this conference