Filamentary discharges in a krypton long-arc lamp

The application of a high voltage at low current across the terminals of flash tubes results in the formation of a filamentary discharge which wanders inside the lamp volume. This discharge is characterized by a very thin plasma channel caused primarily by a fill pressure that is greater than a tenth of an atmosphere. If the lamp is driven with an alternating current, then the discharge filament is more likely to quickly jump position and path upon polarity reversal, which to the naked eye gives the impression that multiple discharges exist at the same time.

Here the plasma is generated inside a krypton-filled pulsed lamp driven with a 15 kV HF power supply operating at 30 kHz. There are several interesting phenomena that are visible in the picture above, such a discharge light emission which is particularly rich in red, a characteristic of krypton (xenon has a much cooler light color). That noble gas has two strong atomic lines at 758.7 and 760.2 nm in the deep red end of the visible spectrum. The overall blue color of the tube is from the fluorescence of the trivalent cerium ion which is used as a quartz dopant to absorb UVs. Finally, it is interesting to note that the filamentary discharge often sticks to the wall. There are several reasons for that, such as the enhancement of the local electric field at the glass-gas interface due to the abrupt change in dielectric permittivity there, the emission of photoelectrons by the glass material, and the trapping and release of surface charges. Also, strong radial displacement currents, caused especially by the high operating frequency (i.e., high dV/dt) and by quartz's high relative dielectric constant (3.7-3.8), results in an effective capacitive coupling between the filamentary discharge and the grounded surroundings, which pulls the plasma towards the wall.

It is possible to get multiple filamentary discharges to exist at the same time in such lamp, but this requires some very specific operating conditions, such as a very low drive current in order to get a very high plasma impedance. An other way is to drive the lamp at high frequency in the so-called "single-electrode" mode where only one electrode is connected to the power supply while the opposite electrode is the grounded surrounding. In that case the dynamic positive (capacitive) impedance of the glass wall of the lamp counters the negative (ohmic) one of the plasma, enabling the discharge current distribution to be spread out over multiple locations. In such regime the quartz wall effectively works as a distributed ballasting impedance. The lamp shown here was not driven this way as both electrodes were connected to the HV power supply.