General characteristics of 222nm gas discharge
The primary type of gas discharge studied in this paper is corona discharge. As corona discharge develops into flashover or arc, the ultraviolet radiation is similarly enhanced, and the development trends of both are consistent. Therefore, corona discharge is taken as the research object here.
1. Corona Discharge
In a non-uniform electric field, if an electron avalanche reaches the Townsend self-sustaining discharge condition or transforms into a streamer without developing into complete gap breakdown, intense ionization will occur at the high-field electrode, resulting in a localized self-sustaining discharge accompanied by luminescence. This phenomenon is known as corona discharge. Corona discharge occurs under atmospheric pressure or higher, when the electrode has a small radius of curvature, the electric field distribution in the discharge space is highly non-uniform, and the field strength near the electrode surface is very strong.
Near the electrode, there exists a luminous corona layer (also called the ionization layer or glow layer). Inside this layer, the electric field is extremely strong, producing intense ionization and excitation, during which de-excitation of excited particles generates far-ultraviolet radiation, including the characteristic wavelength of 222 nm. Outside the corona layer, the electric field is weak, with little or no ionization or excitation occurring. This region is called the outer zone of corona discharge and appears as a dark region with faint glow. Positive and negative charged particles migrate under the weak electric field, determining the electrical conductivity of this space. The transition from the strong field in the corona layer to the weak field in the outer zone is gradual rather than abrupt. Consequently, the intensity of ionization and excitation also decreases gradually from strong to weak.
In corona discharge, the geometry of the electrodes generally plays a crucial role. The non-uniformity of the electric field confines the main ionization process to regions near the electrode where the local field is extremely high, particularly in thin layers near electrodes with small radius of curvature. Luminescence of the gas occurs only in this region, which is referred to as the ionization zone, corona layer, or glow layer. Beyond this region, due to the weak electric field, ionization is minimal or absent, and current conduction relies on the drift of positive ions, negative ions, or electrons. Thus, the region outside the ionization zone is called the drift region or outer region. If only one electrode produces corona, the drift region contains predominantly charged particles of a single polarity, resulting in unipolar current.
The current intensity of corona discharge depends on the applied voltage between the electrodes, electrode shape, inter-electrode distance, gas properties, and density. Corona discharge is a self-sustaining discharge that requires no external ionization source to initiate or maintain it. Moreover, the voltage drop across a corona discharge does not depend on the resistance in the external circuit but is determined by the conductivity of the drift region. When unipolar space charge exists in the drift region, it impedes current flow, and most of the voltage drop in corona discharge falls across this drift region.
Initiation of Corona Discharge and Transition to Other Discharge Forms
As the voltage between electrodes gradually increases from zero, only non-self-sustaining dark discharge occurs initially, with very weak current determined by residual ionization in the gap. When the voltage reaches the "corona inception voltage" (to be discussed in detail later), the non-self-sustaining dark discharge transitions into corona discharge, satisfying the self-sustaining condition:
In corona discharge, the upper integration limit d does not represent the entire inter-electrode distance but only the thickness of the corona layer near the electrode with small radius of curvature. Within this thickness d, a sufficiently strong electric field is established to produce collision ionization and excitation. Spontaneous radiative transitions of excited particles cause the layer to glow, and certain characteristic spectral lines, such as ultraviolet light at 222 nm wavelength, are particularly prominent in air corona. The corona current typically ranges from microamperes to milliamperes. As voltage increases, the corona layer thickens, luminescence intensity increases, and corona current rises. This continues until the expanding corona layers from both electrodes meet each other or reach the opposite electrode-that is, when the sum of the two corona layer thicknesses (d₁ + d₂ ≥ 2B, where 2B is the electrode spacing) or when a single corona layer thickness d ≥ 2B. At this point, the low-field outer zone of the discharge space ceases to exist. With the disappearance of the low-field region, the discharge current can increase dramatically, ultimately limited only by circuit resistance and power supply capacity. If the circuit resistance is low and power supply is limited, corona discharge transitions to spark discharge; if the power supply can deliver high DC current, corona discharge transitions to arc discharge.
2. Discharge Models
For smooth electrodes, the onset value of E/P under actual Pd (pressure × distance product) conditions is nearly equal to the critical (E/P)₀. However, when surface roughness exists on the electrode, breakdown can occur even when the macroscopic field strength throughout the gap is below the critical value. The reduction in breakdown field strength Es can be expressed using a roughness factor ξ (Eq. 2-21).
2.1 Pedersen Model
Pedersen derived the relationship between the roughness factor, protrusion dimensions, and gas pressure. He modeled surface roughness as hemispherical protrusions of radius r on an otherwise smooth surface, as shown in Figure 2-2. In practical detection, such microscopic protrusions causing localized high fields also often lead to enhanced short-wave ultraviolet radiation, including at 222 nm.
2.2 Tedford Model
Tedford et al. modeled idealized protruding electrodes as semi-ellipsoids of revolution and presented calculation results for various height-to-base ratios (h/b), as shown in Figure 2-3. Clearly, for ξ < 1, the actual ξ begins to decrease with decreasing Ph at Ph ≈ 40 (0.1 MPa·μm) for all h/b ratios. Consider a uniform field gap of spacing d pressurized to the critical field strength with a protrusion of height h on one electrode. The breakdown criterion for electron avalanche development starts from the protrusion surface along field lines:
where Va is the voltage applied across the field region 0 to sc. Since the gap field is at the critical level, the avalanche can develop across the entire gap. If the protrusion effect is negligible, Va equals the external voltage V without protrusion: V = (E/P)₀ Pd = (B/k) Pd
Thus, the equation becomes: BP(d − sc) = K (2-27)
Here, sc is the length of the field line from the protruding electrode across the gap. For any protrusion shape and along all possible avalanche development paths, d − sc ≤ h. From Eq. (2-27): BPh < K (2-28)
If this inequality holds, breakdown will not occur below the critical field strength. Under practical conditions with sufficiently large Pd, the uniform-field breakdown path and field strength closely approach the critical values. In experiments, ultraviolet imaging in the 222 nm band is commonly used to detect early corona discharge phenomena initiated by surface protrusions.
