Ignition delay of microplasmas at atmospheric pressure

Summary form only given. Microwave field breakdown conditions in gases are important whenever using plasma in pulsed mode. The plasma properties during the steady state regime are different from those during the ignition time. Many efforts have been spent on determining the ignition electric field as a function of pressure. This dependence (Paschen curve) is different for dc and the high frequency regime. Besides the field threshold, the second important parameter for ignition is the delay time between the application of the dc or high frequency field and the breakdown of plasma. Such a delay time was studied in pulsed dc regime in connection rather with memory effects, i.e. the dependencies between the time interval between two voltage pulses and the delay of the breakdown. This work concentrate on measuring the delay time in dependence on the applied microwave power. It is shown that the average time follows a simple power-law, namely t_ign ~ P(W)^-1/6. For excitation we employ a quarter-wave-length slot resonator, operated at 2.2 GHz. A Rohde&Schwarz ZVA 8 vector network analyser with time resolution facility, an external amplifier, a circulator, and a directional coupler constitute the microwave power generation and setup for characterization. The microwave power is switched by a PIN- diode driven by a pulse generator. The same generator triggers time recording of the complex S₁₁ parameter by the network analyser. We detect the ignition of plasma as an abrupt change in the microwave reflection coefficient (R = |S₁₁|²). Measurements have been performed for air at atmospheric pressure and are to be continued for different gases and at different pressures. The statistics of the ignition is characterized by two time constants: the formation time, in which no ignition occurs and an exponentially decaying histogram with the initial slope equal to the so called statistical time. The histogram shows the occurren- e probability of an ignition at a time longer than the formation time. The dependence of the two time constants on microwave power was analysed. We propose a model for electron heating during the formation time that explains our measurements.