The Q-Machine is an instrument used primarily for studies of waves and instablilities in fully ionized plasmas. Following the primary research efforts in controlled fusion in the late 1950s, it became apparent that not enough was known about plasma instabilities to confine fusion plasmas. This, in addition to the infant field of space plasma physics, led to the development of the first Q-machines.
The main problem in studying basic plasma and wave modes in fusion research machines and glow discharges was the very unstable nature of these types of plasmas. It was believed that development of a quiet, steady plasma source that would avoid complications introduced by large currents and magnetic fields was a worthwhile endeavor. In 1960, two independent groups, one led by Nathan Rynn and Nick D'Angelo at Princeton University, and the other by Knechti and Wada at the Hughes Research Laboratories, were successful in developing sources of magnetically confined alkali plasmas, or Q-machines. The letter Q, meaning "quiescent" was chosen by the Princeton group in the hope that a thermally produced plasma would be quiescent, or free from low-frequency instabilites. This expectation was not met, but allowed the study and discovery of basic low-frequency plasma wave modes. The wave modes first observed in a Q-machine are now the focus of study in plasma processing reactors, space plasmas, and neary anywhere a "plasma" is to be found.
The Q-machine in the form developed by Rynn and D'Angelo, and now used by the Experimental Plasma Group, is in principle a very simple device. The plasma source is a "hot plate", which is heated to ~2300 Kelvin by electron bombardment, from a filament located behind the plate. The hot plate is usually made of tungsten, or a metal with similar properites, for both its work function properties, as well as its ability to survive high temperatures. A diagram of a single ended Q-machine is given below:
The plasma is formed in a vacuum chamber, most typically pumped down to a base pressure of 10E-6 Torr. Once the chamber is evacuated, a small atomic "oven", containing an alkali metal (in this case, cesium, but we commonly use potassium as well) and Calcium Chloride is heated, forcing a chemical reaction that produces gaseous cesium. Once the Cesium atoms strike the hot plate, the relation between the Work Function of the tantalum hot plate, and the ionization potential of the cesium, forces the cesium to surrender an electron to the hot plate, creating a source of cesium ions. In addition, the hot plate emits thermionic electrons due to its high temperature, giving the basic constituents of a plasma, ions and electrons.
Once the ions and electrons are liberated, a powerful magnetic field confines the electrons, and their motion is limited to a small column running down the length of the chamber. At the end of the chamber, the plasma column either terminates at a cooled end plate, recombining the ions and electrons into neutral cesium atoms (the single-ended Q-machine, pictured above), or is reflected off of another hot plate, which serves as a second plasma source. (a double-ended Q-machine)
At present, two Q-machines are operable at the University of Iowa, both capable of double-ended operation. The Q-machines have been used for studies of basic waves and instabilities in typical plasmas, plasmas with negative ions, and other subjects, most recently, the Q-machine has proven useful in studies of wave modes in dusty plasmas, as well.
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