Gyrotron Processing of Materials
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Figure 1. Gyrotron tube.
trons that can operate at various millimeter wavelengths, in a continuous mode, and with power outputs up to tens of kilowatts. Energy conversion into microwaves is relatively efficient, up to 40%. These gyrotrons generate microwave energy in the form of a beam that can be focused, spread, rastered, or directed with metal mirror optics onto the material or part of the material to be processed. A photograph of a gyrotron tube is shown in Figure 1. The tube shell is constructed of stainless steel. High voltage is applied to the cathode, which is located at the top of the tube as shown (Figure 1); the window, through which the microwave beam exits the unit, is within the flange, facing the viewer, and is located just below the midpoint. The tube is approximately 165 cm long and weighs about 60 kg. During operation, the tube is mounted inside a shielding chamber and fits inside a superconducting magnet. Scientists at the E.O. Paton Electric Welding Institute (PWI) in Kyyiv (Kiev), Ukraine, have developed numerous applications for this microwave tool. Some of these applications have been implemented in industrial production settings. Figure 2 shows a research gyrotron installation. Figure 3 shows the work chamber. The microwave beam enters the work chamber from the right and is deflected down onto the workpiece by the metal mirror mounted within the chamber. Shielding (removed for the photograph) and safety interlocks protect the operator. This article focuses on some of these new developments and opportunities in gyrotron microwave processing. Properties of Gyrotron Microwave Radiation The intense microwave beam generated by the gyrotron has unique properties. First, it is an efficient source for heating nonmetallic materials such as ceramics, glasses, polymers, and other organic substances. When such materials are exposed to gyrotron radiation, they can be heated to their melting points in seconds or even fractions of a second. Second, in addition to heating materials very rapidly, gyrotron radiation heats the bulk of the material at a penetration depth of several millimeters to tens of millimeters. These penetration depths correspond to the dimensions commonly encountered during industrial processing. No other known heat source provides such heating. Lasers, plasmas, and infrared radiation penetrate only to the depth of a tenth or a hundredth of a millimeter, and all the material is heated from the surface. Centimeter microwave radiation
MRS BULLETIN/NOVEMBER 1993
Gyrotron Processing of Materials
Figure 2. Research gyrotron installation.
Figure 3. Gyrotron work chamber.
MRS BULLETIN/NOVEMBER 1993
sources do not generate microwave energy as a beam. The material processed is "bathed" by the radiation, which penetrates deeply into the material with comparatively low interaction efficiency. Thus, gyrotron radiation can heat a cubic centimeter of aluminum oxide to its melting point in a fraction of a second; centimeter wavelength radiation generated by a magnetron or klystron will take
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