High Voltage In Vacuum – HVIV

The project is performed in a collaboration between IRFM, LCAR, LPGP and SUPELEC. LCAR and LPGP consider the theoretical aspects of the electron emission under electric field, SUPELEC perform small-scale experiments on dark current and elecric field induced adsorption under well controllled conditions whereas IRFM experiment with a large installation in view of developing a new bushing concept.

Since long it has been observed that the electron current between two biased electrodes («dark current«) can be reduced by adding low pressure gas to the vacuum tank («gas effect«). At the start of the HVIV project we thought that electric field induced gas adsorption was the reason for this effect as it is entirely reversible in the MV testbed at IRFM.
The modelling work by LCAR has shown that the electric field in our testbeds is not sufficiently strong for this to play a role. Experiments by SUPELEC have shown that the gas effect can remain, even if the electric field is turned off. Therefore the hypothesis that the «gas effect« is caused by field induced adsorption is rejected. Other possibilities like the migration of emitting sites are being explored.
On the experimental side an interesting difference between the results by SUPELEC and IRFM on the suppression of the dark current showed up. Whereas IRFM always measure an immediate and strong reduction in dark current when low-pressure gas (around 0.01 Pa) is added to the vacuum vessel, SUPELEC reproduce this only in certain conditions and moreover the gas takes minutes to have effect. There are also differences between the gases, certain gases like nitrogen (or dry air) appear to be much more effective than others (like hydrogen and helium) for dark current suppression. The differences correlate with the ionisation cross-sections of the different gases, we think that ion bombardment of the cathode plays an important role.

Now that it has been shown that electric field induced adsorption does not play a role, theoretical efforts by LCAR and LPGP will concentrate on the role of the cathode ion bombardment, micro particles and chemisorption. LCAR will recruit another post doc in September 2014. The work by LCAR and LPGP is expected to lead to insight in the reduction of dark current due to the presence of low pressure gas in the vacuum chamber.
The SUPELEC experiments, performed under well controlled conditions, aim to gain more insight in the behaviour of the dark current and the differences with IRFM. When modelling and theory results come in, SUPELEC will attempt to validate these experimentally.
IRFM is has tested the high-voltage holding of several materials (stainless steel, copper, titanium, molybdenum) and notably their limits when exposed to high stored energy breakdowns. The differences in voltage holding of these materials scatter with +/- 10% and there seems no need to opt for expensive materials.
All the components for the single stage test bushing have been delivered to IRFM. The assembly failed as the insulater broke during the curing of the epoxy glue, probably due to thermal contraction. Assembly will now be attempted using VITON O-rings or, if that fails, using a flexible epoxy glue.

The project was presented on 22 May 2013 at an international meeting (CCNB) in Heraklion, Greece.
A study on dark current reduction was presented 27 May 2014 at an international conference (ICOPS/BEAMS) in Washington DC, USA.

Vacuum insulation is applied in high voltage apparatus such as power circuit breakers and low loss capacitors. The highest possible electrical breakdown strength should be expected in ideal vacuum, since no charge carriers are present in the inter-electrode gap. Vacuum thus appears as an effective alternative to gas insulators such as sulfur hexafluoride (SF6), perfluorocarbons (CF4, C2F6, or the promising C4F8) which present the drawback of being global warming potential gases. But electrons emitted by the cathode directly cross the gap without any collision phenomena, and pre breakdown is frequently encountered, so limiting the use of vacuum for power circuit breakers. The role played by metallic microprotusions in the field emission mechanism is now well understood but an alternative emission mechanism proposed by Latham et al implies non-conductive or semi-conductive materials such as oxide layers or impurities, including adsorbed gases.
Another field of application is in the controlled fusion domain. Fusion reactions in hot plasmas inside future Tokamak reactors (ITER, DEMO) are initiated by injecting high power beams of neutral D° atoms at high energy (1 MeV for ITER) into the plasma. Negative ions are accelerated by an intense electric field between electrodes at high voltage under vacuum and neutralised in a downstream gas target. The connection between the power supplies under SF6 and the electrodes under vacuum is made through an insulating passage called the bushing.
Experiments at IRFM on the MV testbed (1MV, 100mA) have shown that the voltage holding is limited by the appearance of breakdowns if two electrodes are too close. The voltage holding with distance follows a square root law for distances larger than 1 cm. This dependence indicates according to the theory by Cranberg an exchange of micro clumps that cause breakdowns when they evaporate on the opposing electrode.
Another performance limitation is the appearance of a sizeable electron current (100mA at 400kV) resulting from field emission that appears to follow the Fowler-Nordheim law. This unwanted dark current can be reduced, even eliminated, by the presence of gas in the vacuum vessel.
This very beneficial feature is consistent with an increase of the work function of the metal. This would be caused by the adsorption of gas induced by an intense electric field. This physisorption process allows atoms to stick around emitting micro protrusions. Due to this, the emitting surface is reduced and the work function is increased.
This research project aims to study the field-induced adsorption process by joining theoretical and modelling work with small-scale and large-scale experiments in several laboratories. The objective is to find physical conditions that favour the increase in surface work function, thus leading to an increased voltage holding in vacuum (by suppression of the dark current and absence of breakdowns) under high electric fields (50-100 kV/cm) between large electrode surfaces.
This ANR project proposes specific research on the high-voltage holding under vacuum conditions. Four different laboratories work on 5 different themes:
- Fundamental studies and modelling of field induced gas adsorption (LCAR Toulouse).
- Simulation of field emission from a realistic surface and micro ionisation around emitting micro tips (LPGP, Paris).
- Experimental study of the field emission and field-induced adsorption of gas with adjustable parameters (electrodes surface conditioning, electrodes material, electric field intensity, electrodes gap distance, gas nature and pressure); model validation (Supelec, Paris).
- Study of the high voltage holding in vacuum using different materials and surface treatments to eliminate micro particles and micro tips. Large scale application of the previous more fundamental studies (IRFM, CEA Cadarache).
- Construction and test of a prototype compact bushing, using all the knowledge gained in this project (IRFM, CEA-Cadarache).