Grand challenges in low-temperature plasma physics

INTRODUCTION A plasma is a hot ionized gas and classification of this fourth state of matter can be initially done using the basic concept of temperature. A prime example is the Sun which exhibits a very hot plasma in its core with temperatures of about 1.5 × 107 K (a result of fusion reactions with a proton density of ∼1026 cm−3) and cooler surface temperatures of about 6000 K [1]: the Solar wind originates from the Solar Corona, expands into the universe and impacts the Earth’ magnetosphere and ionosphere, the two plasma layers surrounding Earth’s gaseous atmosphere. Aurorae in the Northern and Southern skies near the magnetic poles are examples of this interaction. The electron density in the ionosphere is low (104–106 cm−3) and the background neutral gas density is also low (about 108 cm−3 or 3 × 109 Torr at 300 km altitude) approaching the “space-like” environment created in laboratories to develop and test hardware for space use (i.e., satellites and payloads). At the surface of the Earth lightning strikes of a storm are naturally occurring plasmas operating near atmospheric pressure (neutral density of about 2 × 1019cm−3) with large electron densities (about 1015– 1017 cm−3) characteristic of streamers, arcs and filamentary discharges. The temperatures and densities of neutral and charged particles are critical parameters affecting the physical mechanisms within the plasma such as particle transport and collisional processes [2] and their range spans many orders of magnitude, opening doors to an extremely wide range of plasma applications. Plasmas in which fusion reactions take place are often referred to as “hot” plasmas. The high-temperature plasma community has a well-defined aim of triggering and controlling fusion plasmas (as can be found in the Sun’s core) for energy production, with various worldwide large scale programs or experiments in place (i.e., the International Thermonuclear Experimental Reactor ITER, the National Ignition Facility ICF. . .) [3]. Hot plasmas in space also include relativistic plasmas (highest electron temperatures) and quantum plasmas (highest electron densities). All other plasmas are classified as low-temperature or “cold” plasmas: those gaseous plasmas or electrical discharges have been successfully harnessed and studied in the laboratory since the 1920s and in space using satellites since the 1960s. The latter two plasma examples essentially sit at the opposite ends of the “cold” plasma spectrum. The wide range of available plasma parameters has largely contributed to the long and expanding list of plasma applications as a result of both scientific and economic drivers: the temperature range of the neutral and/or ionized species allow heat and particle control to burn, melt, cut, coat, grow materials from the macroscopic to the microand nanoscale via “so called” plasma processes. Amongst thousands of plasma applications processes, a few examples are presented to show the past, present and future key role cold plasma physics must play in addressing the challenges facing the modern world: predicted energy crisis, environmental issues and ecosystems, global climate variation, population growth and biomedical concerns. In addressing those issues we must better understand the physics of the atmosphere, ionosphere and magnetosphere, the physics of the plasma interacting with a boundary and develop new clean sources of energy.