Ionization and charge transport phenomena in liquid helium induced by corona discharge

Abstract

Corona discharge around a point electrode has been used to study ionization and charge transport phenomena in liquid helium (LHe) in a wide range of hydrostatic pressure (0.1–10 MPa). All experiments have been done by using 0.45 and 2.5 μm point electrode radii for negative and positive polarities of the electrode. Corona threshold voltages have been found to be pressure independent which indicates that corona initiation takes place in the liquid phase of He. Extinction of the corona discharge occurred for a voltage lower than that for corona initiation. The space-charge limited corona current depended on applied pressure both negative and positive point electrode polarities. This involves considering the charge carriers in LHe as “micro-bubbles” for negative carriers and “snowballs” for positive ones. The parameters of the corona plasma generated close to the negative point electrode have been analyzed using a quasi-one-dimensional numerical model for the simulation of the ionization and the drift regions. An evaluation of the Townsend ionization coefficients is proposed for liquid He. The model provides the space and time variations of the electric field, the electron and positive ion densities in the vicinity of the point cathode. The formation of a steady ionization zone occurs in some nanoseconds due to the multiplication of the cathode-emitted electrons. The electric field strength in the ionization zone of the burned corona is less than the space charge free field before the corona onset. This explains the lower observed corona extinction voltages compared with the initiation ones.

Introduction

Field emission of electrons into liquids is of interest for investigation of the different processes occurring in high-electric field, e.g. electron avalanches, space-charge limited (SCL) current, bulk motion of the liquid, local heating of the liquid, vapor bubble formation, etc. [1], [2], [3]. Fine tungsten points with radii ∼0.1 μm have proved to be useful for an injection of charge carriers in liquefied helium [1], [4], [5], [6]. Currents of electrons or positive ions as large as several 10⁻⁷ A have been injected into the liquid by means of field emission or ionization near a metal tip at the potential less than 2.5 kV [4]. The electron emission current has been observed with the pronounced maximum at 1.7 K [4] and the pressure dependence in the superfluid HeII [5]. The measured currents for positive and negative tips followed the current–voltage dependence of a self-sustained corona [6].

The features of the excited species in liquid helium (LHe) (both superfluid and in normal states) have been under investigations during last years [7], [8], [9], [10]. Excitation of the liquid has been realized by electronic beam [4], [5] and by synchrotron radiation [6], [7]. Corona discharge is a simple way for the excitation of atoms in the quantum liquid. The excited species are created by electronic impact as it occurs in low-density gases.

The electrical breakdown of LHe has been investigated for small inter-electrode gaps (30–200 μm) [11]. The breakdown voltage (2–5 kV) increased with an inter-electrode distance. This result is in agreement with the first observation of a corona in LHe using a 2 μm point electrode and voltages in the range of 1–3 kV [12]. Electrical breakdown of LHe in the rod–rod electrode configuration requiring higher voltage (e.g. 22 kV for 1.5 mm gap) has also been studied [13].

Corona discharge is a complex phenomenon which assumes different forms under various conditions and involves numerous microscopic mechanisms [14]. A divergent electrode geometry (e.g. point-plane) is required to avoid spark breakdown of the inter-electrode space when a high-electric voltage is applied. On the other hand, such an electrode geometry allows to initiate and to sustain a discharge in a very dense medium, such as LHe. Close to the point, ionization processes determine the corona onset voltage V_s. When a sufficiently high voltage V>V_s is applied to tip-plane electrode configuration, a discharge current appears.

Near a point electrode, the strong electric field initiates an electron avalanche due to ionization of the medium by electron impact. The positive ions density increases exponentially with time in this region and it distorts the electric field which becomes a non-Laplacian field. The distorted field creates a thin layer (“cathode sheath”) with intense ionization inside it [15], [16], [17]. In the layer, the electric field and the ionization provide a strong enough electronic current for the maintenance of a steady state electric current inside the inter-electrode gap. Such mechanism is well known for corona discharge in gases [18] and it has been used for the description of the corona in a high-density medium such as liquid Argon [19], [20]. Calculations have shown that the properties of a “cathode sheath” in the liquid differ from that in the gas. The latter corresponds to a layer of a quasi-neutral plasma with equal electron and positive ion densities and constant electric field strength. The properties of such “gaseous cathode sheath” are similar to the positive column of glow discharge in low-density gases [18]. As shown by calculations, this neutral structure is absent in the liquid as a result of a stronger impact ionization in the liquid compared to that in the gas at the same density, of a lower mean free path of the electrons and of the compression in space of ionization process. This is the first feature of the corona in LHe which has to be taken into account.

The second feature of the negative corona specific to liquids such as liquid hydrocarbons, liquid argon, and nitrogen was the appearance of short duration (∼ns) current pulses. After each impulse of current, a shock wave is detected following by several growths and collapses of a vapor bubble [2], [3]. This phenomenon of current impulse correlated to bubble formation is also expected in LHe.

The third feature concerns the transport properties of charged particles in LHe. In a typical gaseous corona, for a point cathode, the electrons produced at the point move towards the anode and come into a low-field region (drift zone) where their drift determines the mean electric current. There is no ionization outside the ionization zone and the electric current is carried by electrons (or negative ions in electronegative molecular gases) in the drift zone. Due to the large difference in spatial scales (small size of the point electrode radius and the ionization zone compared with the large inter-electrode distance and the radius of the current spot on the plane electrode) the problem of unipolar charge carriers motion through the drift zone can be solved separately [18] with the boundary conditions for electric field strength and carrier density on the border of the ionization zone. The solution gives a quadratic dependence of the electric current on the voltage applied (SCL current regime). In this case the charge carriers mobility can be deduced from the current–voltage characteristics [1], [21], [22].

For the negative corona, i.e. when a negative high voltage is applied to the point electrode, the negative carriers are electrons or negative ions if an electronegative impurity (e.g. oxygen) is present in the liquid. In LHe, thermal electrons are trapped in empty cavities (“micro-bubbles”) and their drift is governed by the hydrodynamic flow of the viscous liquid [23]. Their mobility is very low compared with the electron mobility in a dilute gaseous He and it shows a pressure dependence. This occurs because both the micro-bubble radius and the liquid viscosity are depended on the hydrostatic pressure applied [23].

For the positive corona, the electric current through the drift zone is carried by positive ions which are “clothed” in a snowball of He atoms. The radius of the snowball is pressure independent [23], [24] and this fact can be established by the measurement of the positive corona mean current as a function of the pressure applied.

So, the properties of the corona discharge can be divided into two parts. The first part determines the mean corona current due to the transport of charge carriers. The second part determined by ionization processes relates to the corona onset voltage and to the intensity of the light emission from the ionization zone.

In this work, emphasis was put on the phenomena of generation and transport of charge carriers occurring in LHe using a point-plane electrode configuration and varying the applied hydrostatic pressure P over a wide range. For both positive and negative corona, the mobility and the nature of the charge carriers have been determined from the analysis of the mean current–voltage characteristics and from theoretical models of charge transport. The onset corona voltage V_init and the current–voltage characteristics have been measured for 0.45 and 2.5 μm point electrode radii as a function of P. The results have then been analyzed and compared to a theoretical model of ionization processes in LHe.