Non-equilibrium plasmas at atmospheric pressure
Much of my work as a Research Assistant Professor at Tufts University involves the study and use of non-equilibrium plasmas. These plasmas are gaseous mixtures of positively-charged, negatively-charged, and neutral particles. Typically plasmas of positive ions and free electrons are generated by applying large electric fields to a gas. From a technological perspective, much of the utility of plasmas derives from the ability manipulate these charged particles via applied and induced electric and magnetic fields. The study of plasmas is an essential component of materials processing, astrophysics, and fusion energy research.
The non-equilibrium nature of some plasmas refers to differences in temperatures between the charged and neutral species. Typically the electrons are much more energetic than the ions and neutral species. Hot electrons (which initiate reactions by breaking chemical bonds) can drive reactions at rates that would otherwise require gas temperatures of tens of thousands of Kelvins, while the ions and neutral gas remain near room temperature.
Historically, most non-equilibrium plasmas have been generated at gas pressures from 10-6 to 10-3 atmospheres. At higher pressures, collisions lead to faster energy transfer between electrons and other species, drawing their temperatures closer. Careful design of the discharge system and drive circuitry is required to keep the plasma "cold". One design approach is to keep the plasma quite small (typically less than 1 mm3), increasing the surface area-to-volume ratio and therefore increasing cooling. Such plasmas are loosely referred to as microplasmas.
Microwave circuit design
Microwave-frequency electric fields are much more efficient for plasma generation compared to lower-frequency power, as the field reverses direction before many of the accelerated electrons can reach an electrode and be removed from the plasma. The high frequency also limits the motion of heavier ions, preventing them from carrying significant energy to the electrodes which in turn allows extremely robust and long-lived devices.
The use of microwave frequencies also enables the construction of modestly-sized resonant structures that can be used in the discharge design. Resonant structures can be simply created using microstrip geometries on standard substrates (such as the quarter-wave resonator shown with plasma above), making the design and fabrication of new discharge designs fast and relatively simple. Such resonators can develop the moderate voltage amplitudes (a few hundred volts) required to ignite microplasmas and then dynamically reduce that voltage as the plasma itself loads the resonator.
The manipulation of these resonant structures opens up even more design possibilities. Arrays of resonators with identical resonant frequencies efficiently share power. The power distribution can then be altered by shifting the resonant frequencies of selected resonators using added solid-state circuitry, enabling logic control over the parameters of individual microplasmas in large arrays.
Measurement of plasma properties is critical to understand and manipulate plasma behavior and to characterize its interaction with the drive circuitry. Due partly to the small size of microplasmas, our diagnostics are primarily based on optical techniques. Non-equilibrium plasmas naturally emit photons from the decay of excited neutral atoms and molecules as well as charged-particle collisions. We employ a variety of models to extract information about the plasma from spectroscopic data. To complement passive emission spectroscopy, we use active laser-based techniques such as absorption, induced fluorescence, and scattering measurements. In addition to employing well-understood diagnostics, we also adapt and benchmark techniques from other fields to the highly non-equilibrium plasma environment.
Applications of atmospheric-pressure Plasmas
Much of the design work and diagnostics of microplasmas are driven by specific applications. These applications come from a diverse range of industries and topics including materials deposition, manipulation of electromagnetic energy, environmental sensing, laser systems, medical treatments, lighting, and aerodynamic flow control. Such applications most commonly take advantage of the ability of microplasmas to generate high densities of electrons and active chemical species, and to efficiently generate photons. For specific examples, see the list of some recent projects.