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TheColumbia Non-neutral Torus (CNT) is a smallstellarator at theColumbia UniversityPlasma Physics Laboratory designed by Thomas Sunn Pedersen with the aid of Wayne Reiersen and Fred Dahlgren of thePrinceton Plasma Physics Laboratory to conduct the first investigation of non-neutral plasmas confined on magnetic surfaces. The experiment, which began operation in November 2004, is funded by theNational Science Foundation and theUnited States Department of Energy in the form of a Faculty Early Career Development (CAREER) award.[1]
CNT, which is housed in a cylindricalvacuum chamber made of 316stainless steel, measures 60 inches in diameter and stands 75 inches tall. The empty chamber is capable of reaching a pressure of 2x10−10Torr.
CNT is unique in its simple geometry. Magnetic surfaces are created using only fourelectromagnetic coils – two interlocking coils inside the chamber, and two poloidal field coils outside the chamber. The two interlocking coils have a radius of .405m, and the angle between them can be manually selected to be 64°, 78°, or 88°, allowing for different shear and rotational transform values, and magnetic surface configuration. The poloidal field coils have a radius of 1.08 m. The coils are powered by a 200 kW power supply and are capable of producingmagnetic fields of 0.01–0.2T. The configuration of CNT creates a very low aspect ratio of 1.9, the lowest of any stellarator built.
| Parameter | Value |
|---|---|
| ne | 1012−1014 m−3 |
| Te | 1–100 eV |
| B | 0.01–0.2 T |
| R | 0.3 m |
| a | 0.1 m |
| P | 10−10 Torr |
Thomas Sunn Pedersen is the principal investigator of CNT, which studies several areas of theoretical and experimental non-neutral plasma physics. These include the equilibrium ofnon-neutral plasmas,transport and confinement, and ion-relatedinstabilities. The CNT theory program is run by Pedersen and Prof. Allen Boozer, also at Columbia University.
First studies on CNT showed the successful creation of magneticsurfaces with the simple four coil design. At sufficiently low neutral pressures and sufficiently high magnetic field strengths, the plasmas are essentially pure electron plasmas and aremacroscopically stable with confinement times of up to 20 ms. Transport is driven by collisions with neutrals as well asE x B drift along insulatingrods inserted into the plasma. At higher neutral pressures (10−7 Torr and above), an ion related instability is observed, with a frequency in the 10–50 kHz range, and a poloidal mode number m = 1.
The CNT group installed a conducting boundary in August 2007 to study its effects on confinement times, and to allow measurements in the absence of internal rods. Future plans for CNT include the study of electron-positron plasmas confined on magnetic surfaces and further studies of partly neutralized plasmas.
For his efforts, Pedersen has been recognized by theNational Science Foundation's Faculty Early Career Development (CAREER) Program for ground-breaking work in plasma physics. CAREER offers the National Science Foundation's most prestigious award to the early career-development activities of teacher-scholars who most effectively integrate research and education within the context of the mission of their organization. The award translates to$800,000 over five years and is expected to begin in September.
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