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... 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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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... 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.
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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... 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.
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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... 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.
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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... 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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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... 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.
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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... 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 ...
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... 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.
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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... 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.
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We designed and experimentally demonstrated the operation of a novel quasi-optical gyrotron oscillator. The core of the device is an overmoded confocal cavity which can effectively suppress undesired modes, and therefore has extremely low... more
We designed and experimentally demonstrated the operation of a novel quasi-optical gyrotron oscillator. The core of the device is an overmoded confocal cavity which can effectively suppress undesired modes, and therefore has extremely low mode density. Stable single-mode single-frequency operations at 140 GHz has been achieved with peak RF output power exceeding 30kW. The results and experience obtained on this tube will benefit further developments of confocal gyro-TWT amplifiers. Conventional high-power millimeter-wave gyrotron oscillators and gyro-amplifiers utilize cylindrical cavities and waveguides. A few have studied gyrotrons based on slotted, almost cylindrical, cavities. The novel quasi-optical concept we propose uses structures made of two cylindrical mirrors forming a confocal cavity (mirror curvature radii are equal to the distance between mirrors). It has the quality of having a low mode density. By properly choosing the mirror width, one can selectively choose a desired mode with little loss while spurious mode are suppressed through diffraction. Such qualities are extremely important in the development of gyro-TWT amplifiers in which mode competition is a severe problem. As a precursor to the amplifier experiment, a gyrotron oscillator with confocal cavity is designed to study the characteristic of confocal structures.
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We have derived the global band gaps for general two-dimensional (2D) photonic band gap (PBG) structures formed by square or triangular arrays of metal posts. Such PBG structures have many promising applications in active and passive... more
We have derived the global band gaps for general two-dimensional (2D) photonic band gap (PBG) structures formed by square or triangular arrays of metal posts. Such PBG structures have many promising applications in active and passive devices at microwave, millimeter wave, and higher frequencies. A coordinate-space, finite-difference code, called the photonic band gap structure simulator (PBGSS), was developed to calculate complete dispersion curves for lattices for a series of values of the ratio of the post radius (r) to the post spacing (a). The fundamental and higher frequency global photonic band gaps were determined numerically. These universal curves should prove useful in PBG cavity design. In addition, for very long wavelengths, where the numerical methods of the PBGSS code are difficult, dispersion curves were derived for the transverse-magnetic (TM) mode by an approximate, quasi-static approach. Results of this approach agree well with the PBGSS code for r/a<0.1. The present results are compared with experimental data for transverse-electric (TE) and TM mode PBG resonators built at Massachusetts Institute of Technology (MIT) and the agreement is found to be very good.
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A gyrotron oscillator with a single cylindrical cavity has produced output powers up to 645 kW and efficiencies up to 24 percent at 140.8 GHz, and step-tunable single-mode operation between 126 and 243 GHz. Mode stability and suppression... more
A gyrotron oscillator with a single cylindrical cavity has produced output powers up to 645 kW and efficiencies up to 24 percent at 140.8 GHz, and step-tunable single-mode operation between 126 and 243 GHz. Mode stability and suppression of nearby competing modes are found to persist even for operation in very high-order cavity modes with severe mode competition. These results greatly improve prospects of developing CW megawatt gyrotrons relevant to the heating of fusion plasmas.
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Detailed studies of the spectral characteristics and spatial mode structure of a single-mode, high-power Raman free-electron maser (FEM) oscillator operating with a Bragg resonator are reported. The FEM oscillator generated approximately... more
Detailed studies of the spectral characteristics and spatial mode structure of a single-mode, high-power Raman free-electron maser (FEM) oscillator operating with a Bragg resonator are reported. The FEM oscillator generated approximately 1 MW of microwave power in a single axial mode at 27.47 GHz, with an electron beam energy of 320 keV and a transmitted current of 30 A, yielding an efficiency of 10.3%, in good agreement with nonlinear simulations. These results indicate that a Raman free-electron maser can operate at high power and efficiency with stable single-mode output in the long pulse (equilibrium) regime.
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Various papers on space sensing, communications, and networking are presented. Individual topics addressed include: multiple satellite network architectures based on laser communications, mitigation of scintillation for high data rate... more
Various papers on space sensing, communications, and networking are presented. Individual topics addressed include: multiple satellite network architectures based on laser communications, mitigation of scintillation for high data rate communications links, space communication network issues for air defense, telecommunications and navigation systems design for manned Mars exploration missions, effect of double-ended beam tracking errors on optical PPM communication, estimation and tracking for deep-space optical communications, development of air-to-air laser communications, imaging atomic line filter for satellite tracking, BER testing of quadrant photodetectors, liquid crystal devices for free-space optical communications, semiconductor laser for laser communications applications, demonstration of an optically butted IR sensor, directive detection of neutrinos and antineutrinos, high-resolution scintillating fiber gamma-ray telescope, discrimination of multiple transient pointlike sources, submillimeter wavelength space-based imaging radar, all-optical three-dimensional matched filter for moving target detection, real time AI approach to discrimination.
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When using overmoded corrugated waveguide transmission lines for high power applications, it is necessary to control the mode content of the system. Ideally, overmoded corrugated transmission lines operate in the fundamental HE11 mode and... more
When using overmoded corrugated waveguide transmission lines for high power applications, it is necessary to control the mode content of the system. Ideally, overmoded corrugated transmission lines operate in the fundamental HE11 mode and provide low losses for long distances. Unwanted higher order modes (HOMs), particularly LP11 and HE12, are often excited in the experimental systems due to practical misalignments in the transmission line system. This paper discusses how the unwanted modes propagate along with the fundamental mode in the transmission line system by formulating an equation that relates the center of power offset and angle of propagation of a beam (for the HE11 and LP11 modes) or the waist size and phase front radius of curvature of a beam (for the HE11 and HE12 modes). By introducing two miter bend correctors into the transmission system-miter bends that have slightly angled or ellipsoidal mirrors-the HOMs can be precisely manipulated in the system. This technique can be used to eliminate small quantities of unwanted modes, thereby creating a nearly pure fundamental mode beam with minimal losses. Examples of these applications are calculated and show the theoretical conversion of up to 10% HOM content into the fundamental HE11 mode with minimal losses.
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The design and experimental study of a 140 GHz, 1 kW gyro-traveling wave tube (Gyro-TWT) operating in the HE06 mode of a confocal waveguide is presented. A combination of quasioptical gain sections with dielectric loaded severs enables... more
The design and experimental study of a 140 GHz, 1 kW gyro-traveling wave tube (Gyro-TWT) operating in the HE06 mode of a confocal waveguide is presented. A combination of quasioptical gain sections with dielectric loaded severs enables high gain operation with mode selectivity. A nanosecond-scale 120 mW pulse driver at 140 GHz was demonstrated. This input driver pulse will be amplified by the Gyro-TWT and the resulting output high-power short pulse will be transmitted to an EPR spectrometer probe. A quasioptical mode converter utilized to transform the higher order operating confocal mode into a Gaussian-like beam was designed and cold tested. The simulated and measured Gaussian beam patterns of the output mode converter are in good agreement.
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Photonic crystal structures are attractive for use in high-power microwave applications. Experiments have demonstrated use of cavities based on metallic lattices for millimeter wave/THz generation and high-gradient particle acceleration.... more
Photonic crystal structures are attractive for use in high-power microwave applications. Experiments have demonstrated use of cavities based on metallic lattices for millimeter wave/THz generation and high-gradient particle acceleration. Due to unique dispersion properties and the large number of materials available, dielectric lattices are particularly flexible for engineering of frequency band gaps; for example, they allow the design of overmoded cavities that have no lower-order mode competition. The use of such oversized cavities offers critical advantages for experiments at high frequencies, addressing key issues such as pulsed heating and parasitic higher-order mode (HOM) excitation. We present a specific design of an accelerator cavity operating in a TM02-like mode at 17 GHz, formed by a 2D dielectric lattice between metal plates. The use of dielectric rods reduces magnetic pulsed heating on the inner rods, which has been shown to cause breakdown in metal-rod structures. Wakefield simulations show greatly reduced HOM excitation relative to a conventional pillbox cavity. We discuss high-power testing at 17 GHz at MIT and future topics of study, including 3D photonic crystal microwave devices, exotic dielectric materials, and combined metal and dielectric lattices.
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A brief discussion of optically pumped submillimeter (SMM) lasers is presented. The laser consists of two components: an SMM or FIR laser. The CO2 pump laser is step tuned by a grating to particular wavelength within its 9.2-10.8 micron... more
A brief discussion of optically pumped submillimeter (SMM) lasers is presented. The laser consists of two components: an SMM or FIR laser. The CO2 pump laser is step tuned by a grating to particular wavelength within its 9.2-10.8 micron band. Both CW and pulsed operation are possible, and the laser performance is quite different in these two regimes. The SMM laser transition is between adjacent rotational levels within the excited vibrational level; SMM laser lines in the 40-2000 micron wavelength range have been obtained by optical pumping. Attention is given here to SMM laser polarization and efficiency and to the differences between CW and pulsed operation.
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The operation of a high-power, tunable gyrotron oscillator at wavelengths extending into the submillimeter-wave regime is reported. Using a 14-T Bitter magnet, frequencies from 141 GHz (TE15,2,1 mode) up to 328 GHz (TE27,6,1 mode) have... more
The operation of a high-power, tunable gyrotron oscillator at wavelengths extending into the submillimeter-wave regime is reported. Using a 14-T Bitter magnet, frequencies from 141 GHz (TE15,2,1 mode) up to 328 GHz (TE27,6,1 mode) have been measured. As in earlier experiments, it was possible to step tune through a sequence of TEm,p1 modes over this range. From 198 GHz to
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We report the successful cold test and hot test demonstrations of a metal Ku-band PBG accelerator structure. The 17.140 GHz 6-cell PBG accelerator structure with reduced long-range wakefields was designed for the experiment. The copper... more
We report the successful cold test and hot test demonstrations of a metal Ku-band PBG accelerator structure. The 17.140 GHz 6-cell PBG accelerator structure with reduced long-range wakefields was designed for the experiment. The copper structure was electroformed and cold-tested. Tuning was performed through chemical etching of the rods. Final cold test measurements were found to be in very good agreement with the design. The structure was installed on the beam line at the accelerator laboratory at Massachusetts Institute of Technology and powered with 2 MW of peak power from the 17.14 GHz klystron. The electron beam was accelerated by 1.4 MeV inside the PBG accelerator, which is in excellent agreement with the predicted accelerating gradient of 35 MV/m.
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High frequency gyrotrons suitable for heating fusion plasmas are presently being investigated at MIT. In tests with a conventional tapered cavity, powers up to 0.84 MW were obtained in the TE23,6,1 mode at 113.2 GHz, for an efficiency of... more
High frequency gyrotrons suitable for heating fusion plasmas are presently being investigated at MIT. In tests with a conventional tapered cavity, powers up to 0.84 MW were obtained in the TE23,6,1 mode at 113.2 GHz, for an efficiency of 23%. It was found that the most severe mode competition was with other azimuthal TE modes. In later experiments, a mode
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A 1.5 MW, 110 GHz gyrotron is under development for electron cyclotron resonance plasma heating at DIII-D. Research conducted at MIT in short pulse operation is aimed at providing data on the operation of the gyrotron at very high... more
A 1.5 MW, 110 GHz gyrotron is under development for electron cyclotron resonance plasma heating at DIII-D. Research conducted at MIT in short pulse operation is aimed at providing data on the operation of the gyrotron at very high efficiency. The 1.5 MW gyrotron design is based on previous successful results from the 1 MW gyrotrons built by Communication and Power Industries (CPI). A TE_22,6 mode cavity is utilized with an electron beam voltage of 96 kV and a beam current of 40 A. Recently we have successfully run the gyrotron in the axial configuration, and the experimental peak power was 1.44 MW at 37 % efficiency, when operating in the TE_22,6 mode. We have rebuilt this experiment in a configuration which has an internal mode converter and depressed collector, which should enhance the efficiency (> 50 %). The internal mode converter of the TE_22,6 mode to a Gaussian beam consists of an irregular waveguide launcher and four quasi-optical mirrors. A new cavity that provides grea...
We report results from a 1.5 MW, 110 GHz gyrotron experiment. Operating in three microsecond pulses at 96 kV and 40 A, 1.4 MW was obtained in the TE22,6 mode when power was taken along the axis. New results with an internal mode converter... more
We report results from a 1.5 MW, 110 GHz gyrotron experiment. Operating in three microsecond pulses at 96 kV and 40 A, 1.4 MW was obtained in the TE22,6 mode when power was taken along the axis. New results with an internal mode converter and depressed collector are presented.
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High power gyrotrons used for ECH heating of the DIII-D Tokamak at General Atomics have been able to produce 1 MW at 110 GHz for close to 10 s. The next generation of these tubes is being designed and built to produce 1.5 MW. The gyrotron... more
High power gyrotrons used for ECH heating of the DIII-D Tokamak at General Atomics have been able to produce 1 MW at 110 GHz for close to 10 s. The next generation of these tubes is being designed and built to produce 1.5 MW. The gyrotron design includes a 96 W, 40 A MIG electron gun to achieve a microwave efficiency of 39% in the TE22,6 mode. The physics and engineering features of the 1.5 MW, 110 GHz gyrotron are being examined at MIT, where a short (3 μs) pulse experimental version of the gyrotron is used for testing the gun and cavity design. In this study we present the most recent results of these experiments.
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The ultrastructure of rabbit cornea endothelial cells was examined by scanning electron microscopy (SEM) in freeze-cleaved corneas using a Hitachi S-570 scanning electron microscope in the high resolution mode (HRSEM). In order to study... more
The ultrastructure of rabbit cornea endothelial cells was examined by scanning electron microscopy (SEM) in freeze-cleaved corneas using a Hitachi S-570 scanning electron microscope in the high resolution mode (HRSEM). In order to study phototoxic effects in vitro, rabbit corneas (experimental) were cultured as organ culture in the presence of 5 micrograms/ml chlorpromazine (CPZ) and irradiated. For comparison, control 1
Use of microwave photonic bandgap (PBG) structures is a promising approach to improving the performance of rf accelerator structures in terms of better control of wakefields. We have developed the Photonic Band Gap Structure Simulation... more
Use of microwave photonic bandgap (PBG) structures is a promising approach to improving the performance of rf accelerator structures in terms of better control of wakefields. We have developed the Photonic Band Gap Structure Simulation (PBGSS) code to study bulk properties of electromagnetic TE and TM wave propagation in two-dimensional (2D) PBG structures. The eigenmodes are calculated and global photonic bandgaps are determined. Rf accelerator applications of PBG cavities are discussed
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A novel over-moded metallic structure based on sub-wavelength meta-waveguides patterned by complementary metamaterials (C-MTM) is proposed as a potential electromagnetic structure for particle acceleration and radiation generation. One of... more
A novel over-moded metallic structure based on sub-wavelength meta-waveguides patterned by complementary metamaterials (C-MTM) is proposed as a potential electromagnetic structure for particle acceleration and radiation generation. One of the advantages of this structure is planar fabrication which is advantageous for short-wavelength applications. A transmission line theory of the mode propagation and interaction with the electron beam is developed. It is shown that the structure supports a negative-index accelerating mode that can resonantly interact with a relativistic electron beam. In the context of radiation generation, the proposed structure can be utilized as a Backward Wave Oscillator. Using COMSOL simulations, it is shown that the beam-mode interaction results in the mode's exponential growth. Mode competition and the excitation of deleterious transverse wakes will also be discussed. Both positive and negative index TM modes strongly interacting with the beam are ident...