10 kW microwave source now in operation on CNT

Photos of argon plasmas heated by the new microwave source. Left: plasma in the vicinity of the launch antenna. Right: view toward the center of CNT.

A new microwave generator and waveguide launch system has now been commissioned for operation on CNT. The generator creates microwaves at 2.45 GHz using a magnetron, the same technology used in kitchen microwave ovens. The microwaves are then sent through a waveguide system and down an antenna that brings them right up to the edge of the plasma. The plasma is created when the microwaves resonate with electrons gyrating in the magnetic field in the chamber, causing them to accelerate. This phenomenon is known as electron cyclotron resonant heating, or ECRH. The accelerated electrons then collide with neutrals, creating ions and more free electrons.

The launch antenna, essentially a cylindrical metal tube that guides the microwaves toward the plasma, was custom-made for CNT. It enters at an oblique angle through an access port and emits the waves through a vacuum-tight fused-silica window. The waves are launched near the plasma edge to increase the amount of power that the plasma can absorb on the first pass of the microwave beam (before it reflects off the chamber walls). Upstream on the transmission line, a twistable, flexible rectangular waveguide allows the polarization of the microwaves to be rotated to an any angle. This flexibility allows the field to be polarized for optimal plasma absorption in multiple magnetic configurations.

The microwave generator has the capacity to generate a 10 kilowatt beam, which is ten times more powerful than the previous microwave source. This should lead to denser, hotter plasmas, bringing CNT a step closer to its long-term goal of studying plasmas with high beta (i.e., the ratio of kinetic pressure to magnetic pressure). High-beta plasmas are of great interest in fusion energy research because the plasmas in fusion power plants will need to attain high beta values in order for fusion to occur.

Interior of the CNT vacuum chamber. The cylindrical microwave launch antenna can be seen emerging into the chamber on the upper right, terminating at a glass window. The two interlocking rectangular tubes house the coils that help to generate the magnetic field that confines the plasma. The brown rod on the lower right is part of the electron gun used for flux surface measurements. The microwave generator prior to installation. It creates microwaves by accelerating electrons through a vacuum tube and collecting the radiation they emit. This radiation is then sent out through the rectangular waveguide on the lower left. In the current setup, a series of connected waveguides deliver the microwaves to the vacuum chamber and into the plasma.
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