Experimental study of the reduction of field emission by gas injection in vacuum for accelerator applications

LGEP 2014 ID = 1590International audienceField emission current from surfaces under vacuum and at high field strengths can be reduced by the injection of gas into the evacuated volume. In this paper, the effects of H 2 , He, N 2 , and Ar on this "dark" current emitted from a tungsten carbide point cathode for 2 cm gap distance is studied. Exposure to any of these gases at pressures on the order of 10 −3 –10 −2 Pa was found to reduce the emission current by up to 90% with a time constant on the order of ∼1 minute as compared to the current at 10 −6 Pa. The effect was strongly dependent on the gas nature, with Ar and N 2 having larger effects at lower pressures than He and H 2 . The reduction was reversible, with the current increasing to near its original value with a time constant on the order of ∼1–10 minutes after pumping down. The effect of the gas remained in the absence of electric field, whatever the gas pressure. Mechanisms for these and related phenomena are discussed

Measurement of the amount of liquid and vapour created by an electric arc on an electrode – case of Ag and AgSnO

In this paper an original experimental device is presented. It allowed to obtain, for the first time, the total amount of liquid and vapour of metal created on the electrode surface by a non stationary electric arc (600 A/20 ms) burning in air at atmospheric pressure. The results are presented for two different materials Ag and AgSnO2 and for electrode gap values in the range 1–10 mm. The amount of liquid and vapour created under the arc action is compared with usual erosion rate in the same experimental conditions. In the case of Ag electrodes the amount of liquid metal created on the anode may be 3 to 5 times higher than on the cathode although the usual erosion rates are more important at the cathode. For the anode, the usual erosion may represent a very low percentage (<10%) of the total amount of metal liquid created showing then that a very small part of the liquid created during the arc is ejected. In the case of AgSnO2 electrodes the amount of liquid metal is smaller. The usual erosion rates at the cathode are higher than for the anode and the usual erosion represent 10 to 50% of the amount of liquid and vapour created

Surface temperature measurement of a copper anode submitted to a non-stationary electric arc in air

This work concerns the assessment of the surface temperature of copper anodes submitted to an electric arc in a non-stationary regime in air at atmospheric pressure. An infrared camera is used to measure the decrease of the surface temperature just after a very fast controlled arc extinction. Results are presented for different mean values of the arc current intensity (30, 70 and 130 A) with an electric arc duration in the range of 2–5 ms. The temperature decrease after the arc extinction allows an assessment of the surface temperature just at the moment of the arc switching off. In the present experimental conditions the mean temperatures reached for copper anodes are in the range of 750–1200 °C according to the arc current intensity values. Comparison between experimental results and a numerical modeling of the electrode heating allowed one to assess the surface power balance. The values for the volt equivalent are found about 12 V and the values for the surface power density are found to be near 2 × 109 W/m2

Study of the properties of high intensity switching under 36 VDC. Influence of the contact material

This paper concerns the study of contact opening under DC voltage in the case of high intensity currents (a few hundred amperes). The application fields are mainly automotive and aeronautical systems. An experimental device has been designed in order to study the opening of contacts under various conditions. The influence of several parameters on contact opening is studied. These are the following: the opening velocity, the current intensity through the closed contact, the nature of the contact materials (Ag, AgSnO$_{2...})$, the value of the inductance in the electrical circuit... The measured parameters characterizing the opening phase are: the arc duration, the value of the electrode gap when the arc extinguishes, the arc energy

Dynamics of microparticles in vacuum breakdown: Cranberg’s scenario updated by numerical modeling

International audienceMicroparticles (MP) and thermofield emission in vacuum are mainly caused by the roughness present at the surface of electrodes holding a high voltage. They can act as a trigger for breakdown, especially under high vacuum. This theoretical study discusses the interactions between one MP and the thermofield emission electron current as well as the consequences on the MP’s transit. Starting from Cranberg’s assumptions, new phenomena have been taken into account such as MP charge variation due to the secondary electron emission induced by energetic electron bombardment. Hence, the present model can be solved only numerically. Four scenarios have been identified based on the results, depending on the electron emission current from the cathode roughness (tip) and the size of the MP released at the anode, namely (i) one way; (ii) back and forth; (iii) oscillation; and (iv) vaporization. A crash study of the MP on the cathode shows that the electron emission can decrease if the MP covers the thermoemissive tip, i.e., if the MP is larger than the tip size—a phenomenon often called “conditioning”—and helping to increase the voltage holding in vacuum without breakdown

Study of the electrode gap influence on electrode erosion under the action of an electric arc

In this paper we present an experimental study concerning electrode erosion under the sole action of an electric arc. An experimental device allowing to generate electric arc current pulses of constant intensity has been realized. It allows also to ignite the arc for a given electrode gap up to 10 mm by means of a HV discharge. The influence of several parameters on the electrode erosion has then been studied. These parameters are: the nature of the contact material (Ag, AgSnO2, AgCdO, Cu, CuSnO2), the value of the electrode gap, the nature of the cover gas... Films realized with the help of a high speed framing camera have brought interesting elements of interpretation concerning the test erosion results