Spatio-temporal dynamics of a nanosecond pulsed microwave plasma ignited by time reversal

Abstract

In the present paper, a detailed investigation of the spatio-temporal dynamics of the recently developed time reversal microwave plasma source is presented. This novel source allows to ignite a plasma at a desired location in a reverberant cavity by focusing the electromagnetic energy in time and space. An important feature is the possibility to control the plasma position only by changing the input microwave waveform. The source is operated in a repetitive pulsed mode with very low duty cycle (typically 5 × 10−2%). Nanosecond pulses have rise time lower than 1 ns. The generated plasmas have typical sizes in the millimeter range and are observed using imaging for dozens of nanoseconds. The plasma behavior is investigated for different pressures and repetition frequencies. A strong dependence is observed between each discharge pulse suggesting the existence of an important memory effect. The latter is probably due to argon metastable atoms and/or residual charges remaining in the post-discharge and allowing the next breakdown to occur at a moderate electric field.

Space-Time Plasma-Steering Source: Control of Microwave Plasmas in Overmoded Cavities

“…Several techniques can be envisioned to construct this signal so that its emission in the cavity, described by equation ( 1), yields the desired space-time evolution of the field, meaning a field meeting the "Plasma Steering Criterion". Experimentally, we have used two techniques to construct this signal sptq: Time Reversal [17,18] and Linear Combination of Configuration Field [19]. The latter one have been shown to enhance spatial control of plasmas compared to the former one, but is more complicated to implement as it requires more complex signal processing developments.…”

“…These two essential challenges come from the same issue: the difficulty of patterning plasma structures with arbitrary shapes in 2D and even more in 3D. To resolve this issue, a promising way is the innovative concept of microwave plasma source we introduced recently [17][18][19][20]. The idea of this source is to dynamically control the plasma location by changing the waveform of the transmitted signal to a large, overmoded cavity.…”

“…Thus, it allows a dynamic control of plasmas, hence the name "Space-Time Plasma Steering Source" 1 (STPSS). Experimentally, it has been shown to efficiently control plasmas on initiators in an overmoded cavity [17][18][19]. As noted in [20], the most essential step that needs to be taken for the development of this innovative plasma source is the "unhooking" of the plasmas from the initiators.…”

Space-Time Plasma Steering Source: Synthesis and Outlooks

“…It consists in imaging using a camera that enables to measure the spatio-temporal evolution of the light intensity emitted by the plasma during the signal period. These investigations have been made as a function of the pressure 𝑝 and the repetition period of the TR pulses 𝑇 𝑟𝑒𝑝 in our paper [7] and three temporal intervals, with distinct mechanisms, have been identified: the discharge, the "early post-discharge" and the "late postdischarge", as represented on Figure 4. The interested reader must refer to [7] to fully grasp these works.…”

“…First we summarize the experimental results that we obtained since our pioneer paper of 2019 [6]. We will show how the STPSS was first developed using Time Reversal (TR) [6,7] and then using a more elaborated technique: the Linear Combination of Configuration Field (LCCF) [8]. Then we will discuss about promising outlooks for this innovative plasma source, the main and essential one being the "unhooking" of the plasma from the initiators.…”