Decorative neon lamp on HF power

The picture above shows an old Philips 13501 E/16 decorative neon lamps (star of David) driven with a high-voltage source operating at 25 kHz. Because of the high frequency, part of the electrical current flows outwardly via the capacitive coupling with the surrounding space, causing a voluminous discharge being excited in the whole neon atmosphere of the lamp. That discharge is more intense at the wall due to the dielectric permittivity of the glass which increases the local electric field, and the development of a negative glow at the glass-plasma interface due to the diffusional loss of charges (the glass wall acquires a negative charge due to a high mobility of electrons, so it effectively becomes a cathode).