F Freddy Manders

Speed of streamers in argon over a flat surface of a dielectric

A pin–pin electrode geometry was used to study the velocities of streamers propagating over a flat dielectric surface and in gas close to the dielectric. The experiments were done in an argon atmosphere, at pressures from 0.1 to 1 bar, with repetitive voltage pulses. The dielectric surface played a noticeable role in discharge ignition and propagation. The average speed of the discharge decreased with higher pressure and lower voltage pulse rise rate. It was higher when the conductive channel between the electrodes was formed over the dielectric, rather than through the gas. Space resolved measurements revealed an increase in velocity of the discharge as it travelled towards the grounded electrode.

Pulse, dc and ac breakdown in high pressure gas discharge lamps

An optical study of pulse, dc, and ac (50?400?kHz) ignition of metal halide lamps has been performed by investigating intensified CCD camera images of the discharges. The ceramic lamp burners were filled with xenon gas at pressures of 300 and 700?mbar. In comparison with dc and pulse ignition, igniting with an ac voltage decreases the ignition voltage by up to 56% and the breakdown time scales get much longer (~10?3?s compared with ~10?7?s for pulse ignition). Increasing the ac frequency decreases the ignition voltages and changes the ionization channel shapes. External irradiation of UV light can have either an increasing or a decreasing effect on ignition voltages.

Ac breakdown in near-atmospheric pressure noble gases: I. Experiment

Ac-driven breakdown processes have been explored much less than the pulsed or dc breakdown, even though they have possible applications in industry. This paper focuses on the frequency range between 60 kHz and 1 MHz, at a pin–pin electrode geometry and gap lengths of 4 or 7 mm. The breakdown process was examined in argon and xenon at 0.3 and 0.7 bar. We used electrical and optical measurements to characterize the breakdown process, to observe the influence of frequency change and the effect of ignition enhancers—UV irradiation and radioactive material.

Statistical time lags in ac discharges

The paper presents statistical time lags measured for breakdown events in near-atmospheric pressure argon and xenon. Ac voltage at 100, 400 and 800 kHz was used to drive the breakdown processes, and the voltage amplitude slope was varied between 10 and 1280 V ms −1 . The values obtained for the statistical time lags are roughly between 1 and 150 ms. It is shown that the statistical time lags in ac-driven discharges follow the same general trends as the discharges driven by voltage of monotonic slope. In addition, the validity of the Cobine–Easton expression is tested at an alternating voltage form. (Some figures in this article are in colour only in the electronic version)

Facilitating breakdown in noble gases at near-atmospheric pressure using antennas

Electrical breakdown in near-atmospheric pressure noble gases requires voltages that are quite high, which is undesirable for a large number of possible applications. Metallic structures (antennas) were used on the outer side of the lamp burner to enhance the electric field locally while keeping the same potential difference across the electrodes. Optical and electrical measurements were performed in an argon or xenon atmosphere at 0.3 or 0.7bar, with 4 or 7mm between the electrode tips. We used rod-shaped tungsten electrodes of 0.6mm in diameter. We found that both active and passive antennas facilitate breakdown, and we demonstrated the differences between the two types and their effects on the breakdown process. (Some figures in this article are in colour only in the electronic version)

The Role of Metastables in the Formation of an Argon Discharge in a Two-Pin Geometry

The breakdown process in gases is a versatile research topic. Numerous processes play more or less important roles in discharge formation, strongly depending on the gas mixture, the electrode configuration, the applied electric field, the size of the geometry, and even on the structures surrounding the active volume where the breakdown takes place. We focus our research on the breakdown process in argon at 700 mbar, in a pin-pin (point-to-point) electrode geometry, with increasing positive voltage at the active electrode. The voltage rises by 100 V/ns. We use a 2-D fluid model to examine the formation of a charged channel between the electrodes under given conditions. We find that the results describe previous experiments reasonably well. We also explore the role of excited argon atoms at (4s) metastable levels in the breakdown process, and we conclude that the ionization path with an intermediate step containing the metastables does indeed make a notable difference in the breakdown process.

Transition between breakdown regimes in a temperature-dependent mixture of argon and mercury using 100 kHz excitation

The paper examines the breakdown process at 100 kHz in a changing temperature-dependent mixture of Ar and Hg and the associated transitions between breakdown regimes. Each measurement series started at 1400 K, 10 bar of Hg, and 0.05% admixture of Ar and finished by natural cooling at room temperature, 150 mbar of Ar, and 0.01% admixture of Hg. The E/N at breakdown as a function of temperature and gas composition was found to have a particular shape with a peak at 600 K, when Hg makes up for 66% of the gaseous mixture and Ar 34%. This peak was found to be an effect of the mixture itself, not the temperature effects or the possible presence of electronegative species. The analysis has shown that at this frequency both streamer and diffuse breakdown can take place, depending on the temperature and gas composition. Streamer discharges during breakdown are present at high temperatures and high Hg pressure, while at room temperature in 150 mbar of Ar the breakdown has a diffuse nature. In between those two case...

Numerical investigation of symmetry breaking and critical behavior of the acoustic streaming field in high-intensity discharge lamps

For energy efficiency and material cost reduction it is preferred to drive high-intensity discharge lamps at frequencies of approximately 300 kHz. However, operating lamps at these high frequencies bears the risk of stimulating acoustic resonances inside the arc tube, which can result in low frequency light flicker and even lamp destruction. The acoustic streaming effect has been identified as the link between high frequency resonances and low frequency flicker. A highly coupled 3D multiphysics model has been set up to calculate the acoustic streaming velocity field inside the arc tube of high-intensity discharge lamps. It has been found that the velocity field suffers a phase transition to an asymmetrical state at a critical acoustic streaming force. The system behaves similar to a ferromagnet near the Curie point. Furthermore, it is discussed how the model allows to investigate the light flicker phenomenon. Concerning computer resources the procedure is considerably less demanding than a direct approach with a transient model.

Characterization of Discharge Arc Flicker in High-Intensity Discharge Lamps

High-intensity discharge lamps frequently suffer emission disturbances due to the excitation of acoustic resonances. This paper presents experimentally determined light flicker frequencies and arc motion frequencies at different excitation frequencies and modulation depths. Light intensity fluctuation has been detected with a photodiode. Simultaneously, arc motion was recorded with a camera. The frequencies of both methods are in very good agreement. The behavior of the flicker frequency and the arc motion frequency changes significantly near the acoustic eigenfrequency.

The effect of active antennas on the hot-restrike of high intensity discharge lamps

The ignition voltage of high intensity discharge (HID) lamps with mercury as the buffer gas may rise from 3 kV for the cold state up to more than 15 kV for a hot lamp. By coating a lamp burner with an electrically conductive layer, which operates as an active antenna, the ignition voltage of HID lamps can be significantly reduced. An active antenna connected to one of the lamp electrodes transports the potential from this electrode to the vicinity of the opposite electrode and generates an enhanced electric field inside the burner. On applying a symmetrically shaped ignition pulse, a weak pre-discharge within the first half-cycle produces free charge carriers initiating ignition of the lamp within the subsequent second half-cycle. The authors present a set-up for electrical and optical investigations of hot-restrike in HID lamps. The ignition voltage is measured for two different polarities as a function of the cooldown time. An analysis of its reduction is given. Furthermore, the pre-discharge is investigated by means of short-time photography. It is demonstrated that a negative polarity of the active antenna within the first half-cycle and a positive polarity within the second one is the most effective succession.