Richard Temkin

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Continuous-wave submillimeter-wave gyrotrons

Proceedings of Spie the International Society For Optical Engineering, 2006

Recently, dynamic nuclear polarization enhanced nuclear magnetic resonance (DNP/NMR) has emerged ... more Recently, dynamic nuclear polarization enhanced nuclear magnetic resonance (DNP/NMR) has emerged as a powerful technique to obtain significant enhancements in spin spectra from biological samples. For DNP in modern NMR systems, a high power continuous-wave source in the submillimeter wavelength range is necessary. Gyrotrons can deliver tens of watts of CW power at submillimeter wavelengths and are well suited for use in DNP/NMR spectrometers. To date, 140 GHz and 250 GHz gyrotrons are being employed in DNP spectrometer experiments at 200 MHz and 380 MHz at MIT. A 460 GHz gyrotron, which has operated with 8 W of CW output power, will soon be installed in a 700 MHz NMR spectrometer. High power radiation with good spectral and spatial resolution from these gyrotrons should provide NMR spectrometers with high signal enhancement through DNP. Also, these tubes operating at submillimeter wavelengths should have important applications in research in physics, chemistry, biology, materials science and medicine.

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Coherent Transition and Smith Purcell Radiation Experiments

AIP Conference Proceedings, 2006

We review results from radiation experiments done at MIT on the Haimson Research Corporation (HRC... more We review results from radiation experiments done at MIT on the Haimson Research Corporation (HRC) 17 GHz linear accelerator. Both Smith-Purcell radiation (SPR), and Coherent Transition radiation (CTR) were observed. To understand SPR, an Electric Field Integral Equation (EFIE) method was developed and confirmed with an experiment. Because our linac produces a train of bunches, radiation was only observed at integer multiples of the RF frequency. New measurements made on CTR show excellent agreement with EFIE expectations, on an absolute scale, and also provide a bunch length measurement. The possibility for an absolute scale bunch length measurement is confirmed with a proof of principle experiment. Future extension of the EFIE code and corollary experiments are discussed.

Erratum to: Experimental Results on a 1.5 MW, 110 GHz Gyrotron with a Smooth Mirror Mode Converter

Journal of Infrared Millimeter and Terahertz Waves, Jul 8, 2011

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The Design of Helical Antennas for Whispering Gallery Mode Gyrotrons

Piers Progress in Electromagnetics Research Symposium, 1989

Progress on a Smith Purcell Radiation Bunch Length Diagnostic

20Th Particle Accelerator Conference, 2003

A Smith-Purcell radiation diagnostic to measure the micro bunch length of a 20MeV, 17 GHz electro... more A Smith-Purcell radiation diagnostic to measure the micro bunch length of a 20MeV, 17 GHz electron bunch train is being implemented at MIT. The bunch length can be determined by measuring the frequency and angular distribution of the emitted radiation. The beam is produced by a 17 GHz, 50 MeV/m traveling wave accelerator built by Haimson Research Corporation (HRC). The high operating frequency of this accelerator allows for the production of ultra-short bunches of about 180 femtoseconds. At this time we are assembling the experiment and we will present an update on the progress.

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Experimental and theoretical investigation of high gradient acceleration. Progress report, June 1, 1991--February 1, 1992

Observation of Wakefields in a 17 GHz Metallic Photonic Bandgap PBG Structure

2007 Ieee Particle Accelerator Conference, Jun 25, 2007

Preliminary results are reported on experimental wakefield measurements made on a 6 cell, 17.14 G... more Preliminary results are reported on experimental wakefield measurements made on a 6 cell, 17.14 GHz metallic photonic band gap (PBG) accelerator structure. Wakefields were observed using a variety of detectors and methods. The PBG structure is open, containing no outer wall, and radiation has been observed through a window in the surrounding vacuum vessel. The output coupler port has also been used with a vacuum window to observe radiation coupling out of the port. Estimations of radiation are made using HFSS and basic wakefield theory. Measurements have been made using video diode detectors and a heterodyne receiver. Plans are discussed for further experiments.

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Operation of a gyrotron at the fundamental and second harmonic

A formalism is developed for determining the possibility to convert gyrotron operation from an op... more A formalism is developed for determining the possibility to convert gyrotron operation from an operation at the fundamental frequency to one at the second harmonic. This possibility can be studied by making use of the adiabatic theory of gyrotron electron guns along with simple resonator theory. It is found that second harmonic operation must be restricted to a range of magnetic fields with values which are close to the value of the original field. Attention is given to solutions with optimized beam location, parasitic modes in whispering gallery mode gyrotrons operating at harmonics, parasitic modes in azimuthally symmetric mode gyrotrons, and an example in which the fundamental frequency is 140 GHz and the gyrotron operates in the range from 240 to 320 GHz.

Frequency tuning and efficiency enhancement of high-power far-infrared lasers

In Infrared and Millimeter Waves Volume 12 Orlando Fl Academic Press Inc 1984 P 195 278, 1984

Plasma structures observed in gas breakdown using a 1.5 MW, 110 GHz pulsed gyrotron (7 pages)

Regular two-dimensional plasma filamentary arrays have been observed in gas breakdown experiments... more Regular two-dimensional plasma filamentary arrays have been observed in gas breakdown experiments using a pulsed 1.5 MW, 110 GHz gyrotron. The gyrotron Gaussian output beam is focused to an intensity of up to 4 MW/ cm 2 . The plasma filaments develop in an array with a spacing of about one quarter wavelength, elongated in the electric field direction. The array was imaged using photodiodes, a slow camera, which captures the entire breakdown event, and a fast camera with a 6 ns window. These diagnostics demonstrate the sequential development of the array propagating back toward the source. Gases studied included air, nitrogen, SF 6 , and helium at various pressures. A discrete plasma array structure is observed at high pressure, while a diffuse plasma is observed at lower pressure. The propagation speed of the ionization front for air and nitrogen at atmospheric pressure for 3 MW/ cm 2 was found to be of the order of 10 km/s.

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Internal mode converter mirror shaping from measured field intensity

25Th Anniversary Ieee Conference Record Abstracts 1998 Ieee International Conference on Plasma Science, Jun 1, 1998

Experimental Charge Density of Copper

Phys Rev B, 1972

The first six charge-density form factors of crystalline copper have been measured by Bragg scatt... more The first six charge-density form factors of crystalline copper have been measured by Bragg scattering of CuKα and MoKα x rays from copper-powder samples. Detailed studies of both the samples and the x-ray beam parameters have reliably established the experimental error at about 1%. The measured form factors are in best agreement with an augmented-plane-wave (APW) self-consistent-field calculation of Snow using Xα Slater exchange for a value of α between 0.70 and 0.75. They are in poor agreement with an APW calculation using the Chodorow potential but agree well with a calculation by Wakoh using Slater exchange and a self-consistent procedure. It is concluded that form-factor measurements of high accuracy are a sensitive and useful test of band wave functions and crystalline potentials.

Spatial dispersion in metamaterials with negative dielectric permittivity and its effect on surface waves

Optics Letters, Jul 1, 2006

The effect of spatial dispersion on the electromagnetic properties of a metamaterial consisting o... more The effect of spatial dispersion on the electromagnetic properties of a metamaterial consisting of a three-dimensional mesh of crossing metallic wires is reported. The effective dielectric permittivity tensor γij(ω,k) of the wire mesh is calculated in the limit of small wavenumbers. The procedure for extracting the spatial dispersion from the ω versus k dependence for electromagnetic waves propagating in the bulk of the metamaterial is developed. These propagating modes are identified as similar to the longitudinal (plasmon) and transverse (photon) waves in a plasma. Spatial dispersion is found to have the most dramatic effect on the surface waves that exist at the wire mesh-vacuum interface.

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Calculation of a Hyperbolic Corrugated Horn Converting the TEM 00 Mode to the HE 11</s

J Infrared Millim Terahertz W, 2011

Corrugated waveguide transmission lines are in use to transmit high power mm-wave radiation from ... more Corrugated waveguide transmission lines are in use to transmit high power mm-wave radiation from gyrotrons to the plasma for electron cyclotron plasma heating in tokamaks such as ITER. The coupling efficiency of the gyrotron output radiation formed as a quasi-Gaussian beam to the waveguide mode is a critical issue. A hyperbolic corrugated horn serves as a converter of the TEM00 Gaussian mode to the HE11 mode of a corrugated waveguide. We report the design of a hyperbolic horn for application in the ITER transmission line at 170 GHz. The theoretical conversion efficiency of the horn is higher than 0.995.

Tenth International Conference on Infrared and Millimeter Waves, heldat Lake Buena Vista, Fla., USA, 9 - 13 December 1985. Conference digest

Long-Pulse, High Power X-Band Relativistic Traveling-Wave Tube Amplifier

A high-power X-band traveling-wave tube (TWT) amplifier has been designed. A disk-loaded circular... more A high-power X-band traveling-wave tube (TWT) amplifier has been designed. A disk-loaded circular waveguide is used as the slow wave structure (SWS) for wave interaction with a 450 kV, 80 A solid electron beam. From Pierce theory, the predicted gain at 9.7 GHz is 1.94 dB/cm with an instantaneous bandwidth of 5% and a phase advance of n/2 for the TM01 mode. Cold dispersion measurements show good agreement with the simulation.

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DURIP Instrumentation for Advanced Source Development at 95 GHz

Characterization of Mode Content and Losses in the ITER ECH Transmission Lines

Estimation of overall losses in the transmission line (TL) due to ohmic loss, inaccurate coupling... more Estimation of overall losses in the transmission line (TL) due to ohmic loss, inaccurate coupling of the quasi-Gaussian beam to the overmoded waveguide and the mode conversion becomes vital to characterize the ITER ECH system which uses 24 MW RF power at 170 GHz. Components in the TL such as 63.5-mm diameter corrugated waveguides, plane mirrors and polarizers at the miter-bends (MBs) must be characterized for these losses. Tilt and offset of the gyrotron output beam wrt the TL result in excitation of higher order modes (particularly LP11 mode) and ...

Experiments on dielectric window breakdown at atmospheric pressure using a 1.5 MW, 110 GHz gyrotron

It is increasingly important to know the power threshold for breakdown of high-power microwave (H... more It is increasingly important to know the power threshold for breakdown of high-power microwave (HPM) transmission through a window as the output powers of HPM devices continue to rise. One of the frequency ranges where breakdown threshold data are scarce is the W-band (75 to 111 GHz). We report preliminary experimental results on window breakdown at atmospheric pressure using the near-Gaussian output beam generated from a 1.5 MW, 110 GHz gyrotron with a pulse length of 3 microseconds. Successful breakdowns were achieved by focusing the beam down to approximately 6-mm radius at a polycarbonate window. The threshold power density and peak electric field were determined to be roughly 2 MW/cm^2 and 50 kV/cm for 50% breakdown probability. Also, we have observed periodic plasma array structures in the time-integrated photographs of breakdown plasmas, and the origin of this periodicity is currently under investigation. These results will be compared with extensive data taken at Texas Tech Univ. at lower frequency, in S-Band.

Initial operation of a wideband 140 GHz, 1kW confocal gyro-traveling wave amplifier

2007 Joint 32nd International Conference on Infrared and Millimeter Waves and the 15th International Conference on Terahertz Electronics, Sep 1, 2007

The present experimental results of a zero-drive stable, short pulse 140 GHz gyro-traveling wave ... more The present experimental results of a zero-drive stable, short pulse 140 GHz gyro-traveling wave tube amplifier are reported. The amplifier consists of three amplifying sections employing a novel high order HE(0,6) operating mode of a quasi-optical confocal waveguide in order to reduce mode density and achieve circuit loss through diffraction rather than absorption in localized dielectric materials. The confocal waveguide preferentially provides higher loss to the lower order competing modes thus allowing operating in a higher order mode. Two quasi-optical severs are used to suppress backward wave oscillations. At present, the amplifier has achieved a linear gain of 34 dB in experiment, and peak output powers up to 400W have been observed at a beam voltage of 39.5kV, beam current of 1.65A, and beam pitch factor of 0.9. Bandwidths over 1 GHz have been observed.

Richard Temkin

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