• Photonic Band Gap (PBG) Structures

Photonic Band Gap (PBG) structures are periodic lattices of either dielectric or metal structures. The periodic nature of the structure gives rise to stop and pass -bands in the electromagnetic transmission spectrum. While frequencies in the stop-band are strongly reflected, the frequencies in the pass-band propagate almost without attenuation. This property can be used to build electromagnetic interaction structures which permit the existence of a single mode.

• Quasioptical Open Waveguides

Quasioptical open waveguides, rely on the diffraction from the open ends of the structure for reducing the mode population. These structures, such as a confocal waveguide which consists of two finite aperture cylindrical mirrors separated by a distance equal to their radius of curvature have a mode population that is π times more sparse than a corresponding closed cylindrical resonator.

At least one proof-of-principle experiment has been already demonstrated for each of these classes of novel interaction structures. Two new experiments are in progress.

• 140 GHz PBG Gyrotron Oscillator

• 140 GHz Confocal Gyrotron Oscillator

• 280 GHZ Second Harmonic Confocal Gyrotron Oscillator

• 140 GHz Confocal Gyrotron Traveling Wave Tube Amplifier

HIGH FREQUENCY GYROTRON RESEARCHJ. R. Sirigiri, M. A. Shapiro, I. Mastovsky, and R. J. TemkinPlasma Science and Fusion Center, Massachusetts Institute of Technology

We present the results from the high frequency

gyrotron research at MIT which is aimed at the

development of high frequency and high average

power gyrotrons. Four different experiments at 140

GHz are described including a gyro-TWT with a highly

overmoded yet mode-selective interaction structure.

The recent successful operation of a novel gyrotron

with a Photonic Band Gap resonator is also presented.

Waves and Beams Division

The size of the electromagnetic interaction structure scales inversely with operating frequency in all microwave devices. This makes the size of the interaction structures a fraction of the wavelength for fundamental mode operation which

• limits average power handling capability

• increases fabrication complexity

At millimeter and sub-millimeter wave (> 30 GHz) frequencies. Overmoded structures, which have transverse dimensions much larger than a wavelength offer

• high average power (> 10 kW) capability

• simpler fabrication

however, they suffer from serious mode competition.

At the Massachusetts Institute of Technology, multifaceted research on various novel electromagnetic structures is being conducted to develop which are

• overmoded yet mode selective

PHOTONIC BAND GAP GYROTRON

TE04-like mode confined in a defect surrounded by a triangular lattice of metal rods

TE03-like mode confined between confocal mirrors

The PBG resonator is made from a triangular lattice of metal rods which has a band gap for TE-modes at the design frequency of 140 GHz. A cavity is formed by removing the innermost 19 rods to create a defect whose size matches that of a conventional cylindrical resonator at 140 GHz in the TE041 mode. Only the TE041 design mode is confined by the lattice while all the competing modes which are frequency offset leak through the lattice which appears transparent at frequencies other than 140 GHz.

CAD drawing of the PBG resonator. The small aperture on the lower end plate forms the input cutoff section and the bigger hole on the upper end plate is used to extract the radiation from the cavity

The magnitude of the electric field of the TE041-like eigenmode in a cross section of the PBG resonator. The simulations were performed using HFSS.

140 GHz PHOTONIC BAND GAP GYROTRON

The PBG resonator used in the 140 GHz TE041-like mode gyrotron.

• Frequency 139.98 GHz

• Cavity length = 8 wavelengths

• Ohmic Q ~ 13 500

• Diffractive Q ~ 16 000

• The size of the resonator should pose no problems for fabrication in the W – band (94 GHz)

• Initial lattice dimensions chosen using SUPERFISH and final design optimized on HFSS.

The PBG Gyrotron

SUMMARY• Unprecedented range of single

mode operation over a 40 % band around the design frequency

• 25 kW peak power

• Efficiency (7%) limited by diffractive Q in the proof-of-principle design

REFERENCES1. Photonic-Band-Gap Resonator

Gyrotron, J. R. Sirigiri, K. E. Kreischer, J. Machuzak, I. Mastovsky, M. Shapiro and R. J. Temkin, Phys. Rev. Lett., vol. 86, no. 24, p. 5628, 11 June 2001).

2. Lattice Sends a Crystal Clear Signal, Physical Review Focus, 7 June 2001.

140 GHz CONFOCAL GYROTRON

A confocal resonator with open sidewalls was designed to support a TE03-like mode.

The 140 GHz confocal resonator used in TE03-like mode gyrotron experiments.

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SUMMARY• 83 kW peak power at 136 GHz

• Reduced Mode competition

• Successful pilot project for the 140 GHz confocal gyro-TWT

• Efficiency of 18 % can be enhanced by using a azimuthally asymmetric electron beam

EXPERIMENTAL RESULTS

REFERENCES1. 140 GHz Gyrotron Experiments

Based on a Confocal Cavity, W. Hu, M. A. Shapiro, K. E. Kreischer, R. J. Temkin, IEEE Trans. Plasma Sci., vol. 26, no. 3, p. 366, 1998.

The mode population in such a resonator is πtimes more sparse than that in a conventional cylindrical resonator. The aperture of the mirrors was chosen to cause additional diffractive losses on the lower order modes and improve mode-selectivity.

The 280 GHz confocal cavity attached to the beam tunnel

of the gyrotron

By choosing narrow mirror aperture the confocal waveguide can confine a TE0,2n-like mode while the TE0,n-like which has a wider waist at the mirrors suffers significant diffraction losses from the open walls.

A 280 GHz, TE0,6-like mode gyrotron has been designed and built and is currently being tested. Competition from the fundamental mode resonance occurring from the TE0,3-like mode is eliminated by substantially decreasing the transverse Q factor by choosing narrow mirror aperture.

The Q –factor of the TE0,6-like mode is higher than that of the TE0,3-like mode thus lowering the starting current of the second harmonic mode below the fundamental mode.

The TE06-like mode at 280 GHz inside the confocal

resonator. This mode is used to interact with the second

beam harmonic (s=2)

The confocal cavity attached to two output uptapers to match it to the collector/output waveguide.

SUMMARY• Initial experiments in

progress

• Development of higher harmonic (s >2), high frequency (>500 GHz) gyrotrons

• Precursor to a high harmonic gyro-TWT

140 GHz CONFOCAL GYRO-TWT

High average power (>10 kW) amplifiers in the W-band (94 GHz) and higher frequencies need to have large interaction structures for sustaining the thermal load below 1 kW/cm2. The confocal waveguide is an excellent candidate for an overmoded interaction structure due to its sparse mode population and good mode selectivity.

A confocal gyro-TWT experiment has been designed and built at MIT. It will be tested shortly.

SUMMARY• Gyro-TWT is ready for testing

• Potential for second harmonic operation.

– a 280 GHz second harmonic confocal gyrotron is being tested

• Cold test of the interaction structure complete

• Novel quasioptical sever with high average power (>10 kW)capability

• Successful operation opens up possibilities of building high frequency (>300 GHz) amplifiers

REFERENCES1. J. R. Sirigiri, K. E. Kreischer, M. A.

Shapiro and R. J. Temkin, Novel Quasioptical W-Band Gyro-TWT, Int. Vacuum Electron. Conf. (IVEC), Monterey, CA, May 2000.

2. J. R. Sirigiri, Theory and Design of a Novel Quasi-Optical Gyrotron Traveling Wave Amplifier, M. S. Thesis submitted to the Dept. of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, October 1999.

• Simple geometry

• Larger size enhances thermal capacity for high average power operation

• Gaussian like mode

• Diffraction losses can be used for parasitic mode suppression

• Can be used for high harmonic operation

• Novel quasioptical sever has potential for 100 kW average power operation

• > 3 GHz bandwidth

• Successful gyrotron oscillator experiments

The interaction structure of the 140 GHz confocal gyro-TWT. The section of the interaction structure with very narrow aperture works as the sever.

HFSS simulations showing the action of the sever.

HFSS model for analyzing the sever section of the interaction structure.

A CAD drawing of the 140 GHz confocal gyro-TWT.

Cold test of the input transmission line of the amplifier

ACKNOWLEDGEMENTSThis research is funded by the Department of Defense under the auspices of the MURI- Innovative Microwave Vacuum Electronics program and the Department of Energy under the Fusion Sciences Program.

The authors wish to thank William Mulligan for his help in running the experiments and Chiping Chen and Evgenya Smirnova for their support with the theory of PBG structures.

MOTIVATION

MODE-SELECTIVE STRUCTURES

ABSTRACT

SECOND HARMONIC CONFOCAL GYROTRON

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Frequency = 140.04 GHzVoltage = 67.52 kVCurrent = 5.10 A

An atlas of band gaps for a triangular lattice of metal posts in air. The operating point of the PBG gyrotron resonator is shown as a blue dot.

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Second Harmonic at 280 GHz

Fundamental at 142.86 GHz

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Spatial power profile in the confocal interaction structure at 140 GHz.

Gain-Bandwidth characteristics of the confocal gyro-TWT

Cross section of the confocal waveguide supporting a TE03-like mode at 140 GHz. The gyrating annular electron beam is also shown.

5.7 mm

6.8 mm

3.6 mm

Frequency 141GHz Peak Output Power 122kW Saturated Gain 38dB Peak Efficiency 27.5% Saturated Bandwidth 2.9% Beam Voltage 65kV Beam Current 7A Velocity Pitch Factor 1.2 Longt. Momentum Spread 8% Beam Radius 1.8mm Peak Magnetic Field 5.35T Cyclotron Harmonic Fundamental Operating Mode HE06 Confocal

Mode spectrum of the confocal waveguide

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Waveguide modes

1.71 m

1.71 m