Plasma Science and Fusion Center
Massachusetts Institute of Technology
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WAves & beams
| Megawatt Gyrotron Oscillator | Millimeter and Sub millimeter Wave Gyrotrons | Millimeter Wave Gyrotron Amplifiers |
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Mode Converter Research | Thermionic Cathode Emission Studies |
High Power Microwave Sources
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| A prototype of a 1.5 MW gyrotron oscillator at 110 GHz for plasma heating applications. |
A prototype of a 1.5 MW, 110 GHz gyrotron oscillator was designed and built for studying the physics of megawatt class gyrotron oscillators. We are investigating novel resonator designs which favor strong single mode operation and improve efficiency. Depressed collector operation to enhance the overall efficiency is also being studied. In the future we plan to build a 2 MW, 120 GHz gyrotron for plasma heating and conduct research on novel step tunable megawatt gyrotrons.
An industrial version of the 1.5 MW, 110 GHz tube has been built by Communication and Power Industries, Palo Alto, CA for use Electron Cyclotron Resonance Heating (ECRH) and Current Drive (ECCD) at the DIII-D tokamak at General Atomics in San Diego.
Phase Retrieval of Gyrotron Beams
Millimeter and Sub millimeter Wave Gyrotrons
We are investigating novel concepts for building millimeter and sub millimeter wave gyrotrons for a variety of applications such as plasma diagnostics, material processing etc. Millimeter and Sub millimeter wave gyrotrons are now being employed for spin-polarizing samples used in Dynamic Nuclear Polarization (DNP) experiments in Nuclear Magnetic Resonance (NMR) spectroscopy. A 250 GHz gyrotron oscillator capable of producing up to capable of 25 W continuous wave operation in DNP experiments at the Francis Bitter Magnet Laboratory at MIT. A CW gyrotron oscillator at 460 GHz is capable of record operation with over 8 W CW for applications in dynamic nuclear polarization experiments (DNP) in nuclear magnetic resonance (NMR) studies. The gyrotron operates in the second electron cyclotron harmonic and will be used in the highest field DNP experiments to date (700 MHz, 16 T) at the Francis Bitter Magnet Laboratory at MIT.
Corrugated Waveguide and Directional Coupler for CW 250 GHz Gyrotron DNP Experiments
A novel quasioptical resonator with very high mode selectivity was successfully demonstrated in a 140 GHz gyrotron experiment. Up to 80 kW of peak power was observed without any competition from spurious modes.
A gyrotron with a Photonic Band Gap (PBG) resonator was tested at 140 GHz. The gyrotron demonstrated an unprecedented single mode operation over a frequency range of 35 % while operating in a very high order mode. This idea of using PBG resonators can be used in all classes of microwave tubes.
Physical Review Focus: Lattice sends a crystal clear signal
Millimeter Wave Gyrotron Amplifiers
We are conducting research on a variety of novel gyrotron amplifiers in the millimeter wave band. Recently, we designed, built and tested a novel high average power capable gyrotron traveling wave tube amplifier. The amplifier produced up to 30 kW of peak power in 3 microsecond pulsed operation at 140 GHz. A linear gain of up to 29 dB and unsaturated efficiency of 11 % was measured in experiments. The amplifier used a novel overmoded yet mode selective quasioptical interaction structure which allowed stable single mode operation in the TE(0,3) like mode. Such an interaction structure can handle up to 100 kW average-power operation in the W-Band (94 GHz)
A 140 GHz, 100 W gyrotron amplifier is currently being designed for use in DNP experiments. We intend to build even higher frequency gyrotron amplifiers for use in DNP and Electron Paramagnetic Resonance (EPR) experiments.
Mode Converter Research
In gyrotrons it is also critical to have a high efficiency mode converter that can transform the output mode of the gyrotron into a Gaussian beam in free space. In recent years, we have been studying the measurement of the phase distribution in microwave beams using a phase retrieval algorithm based on the measured field amplitude on a series of planes. Recently, We have completed the first demonstration of a new approach to determining the phase distribution in the beam. This method uses the measured field amplitude on a series of planes but relies on calculated moments of the field amplitude to determine the parameters of a polynomial expansion of the spatial phase distribution. This irradiance moment method, which has been previously used at optical wavelengths, is being applied to microwave frequencies for the first time at MIT. It has the advantage of allowing internal consistency checks of the phase data. The method will be applied to design single mirror correctors for the gyrotrons at General Atomics in San Diego.
Thermionic Cathode Emission Studies
A uniform electron beam with minimal velocity spread is essential for high interaction efficiency in electron beam devices. We are conducting a variety of experiments to study the origins and the effects of nonuniform emission from large thermionic cathodes used in vacuum electron devices. The nonuniformity is measured under near operating conditions and it effects on the quality of the beam are also determined by various diagnostics. We are also modeling the effects of nonuniform emission on critical beam parameters using advanced gun simulation codes such as Michelle
