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The reflectance [Formula: see text] and transmittance [Formula: see text] of Si and GaAs wafers irradiated by a 6 ns pulsed, 532 nm laser have been studied for s- and p-polarized 250 GHz radiation as a function of laser fluence and time. The measurements were carried out using precision timing of the [Formula: see text] and [Formula: see text] signals, allowing an accurate determination of the absorptance [Formula: see text] where [Formula: see text]. Both wafers had a maximum reflectance above 90% for a laser fluence [Formula: see text]. Both also showed an absorptance peak of ~50% lasting ~2 ns during the risetime of the laser pulse. Experimental results were compared with a stratified medium theory using the Vogel model for the carrier lifetime and the Drude model for permittivity. Modeling showed that the large absorptance at the early part of the rise of the laser pulse was due to the creation of a lossy, low carrier density layer. For Si, the measured [Formula: see text] and [Formula: see text] were in very good agreement with theory on both the nanosecond time scale and the microsecond scale. For GaAs, the agreement was very good on the nanosecond scale but only qualitatively correct on the microsecond scale. These results may be useful for planning applications of laser driven semiconductor switches.

The phase stability of a 140GHz, 1kW pulsed gyro-amplifier system and the phase dependence on the cathode voltage were experimentally measured. To optimize the measurement precision, the amplifier was operated at 47 kV and 1 A, where the output power was ∼ 30W. The phase was determined to be stable both pulse-to-pulse and during each pulse, so far as the cathode voltage and electron beam current are constant. The phase variation with voltage was measured and found to be 130±30°/kV, in excellent agreement with simulations. The electron gun used in this device is non-adiabatic, resulting in a steep slope of the beam pitch factor with respect to cathode voltage. This was discovered to be the dominant factor in the phase dependence on voltage. The use of an adiabatic electron gun is predicted to yield a significantly smaller phase sensitivity to voltage, and thus a more phase-stable performance. To our knowledge, these are the first phase measurements reported for a gyro-amplifier operating at a frequency above W-band.

[Image: see text]

Metamaterials (MTMs) are structured materials with subwavelength features that can be engineered to have some unique properties not found in nature, such as negative refractive index, reversed Doppler effect, and reversed Cherenkov radiation. Based on these novel MTMs, several important research groups have made great attempts to develop novel MTM-inspired vacuum electron devices (VEDs) and accelerators. Just as solid-state power devices are innovated by incessant emerging of new semiconductor materials, VEDs can also be inspired by MTMs to have very remarkable advantages, such as smaller size, higher power, higher efficiency, and/or larger gain relative to conventional VEDs, such as traveling-wave tubes, backward-wave oscillators, and klystrons. Similarly, relative to conventional accelerators, MTM-inspired accelerators have obvious advantages, such as smaller size and higher accelerating gradient. Furthermore, MTM-inspired devices have promising applications in areas such as radar, communication, electronic warfare, microwave heating, and imaging.

The effect of reflection is studied experimentally and theoretically on a high-power 110 GHz gyrotron operating in the TE(22,6) mode in 3 μs pulses at 96 kV, 40 A. The experimental setup allows variation of the reflected power from 0 to 33 % over a range of gyrotron operating conditions. The phase of the reflection is varied by translating the reflector along the axis. Operating at a higher efficiency point, at 4:40 T with 940 kW of output power, reflected power exceeding 11% causes a switch from operation in the TE(22,6) to simultaneous operation in the TE(22,6) and TE(21,6) modes with a large decrease of the total gyrotron output power. This switching effect is in good agreement with simulations using the MAGY code. Operating at a more stable point, 4:44 T with 580 kW of output power, when the reflection is increased, the output power remains in the TE(22,6) mode but it decreases monotonically with increasing reflection, dropping to 200 kW at 33% reflection. Furthermore, at a reflection above 22%, a power modulation at 25 to 30 MHz is observed, independent of the phase of the reflected wave. Such a modulated signal may be useful in spectroscopic and other applications.

We report the recent measurement of high gradient and rf breakdown rates (BDRs) in a millimeter-wave accelerating cavity powered by a megawatt gyrotron. The cavity is powered by 10 nanoseconds rf pulses, chopped from 3 microsecond gyrotron pulses using a laser-driven silicon switch. The highest achieved accelerating gradient is 230 MV/m corresponding to a peak surface electric field of 520 MV/m after processing the cavity with more than 105 pulses. The initial BDR is about 10−3[1/pulse] and it has not reached a steady state yet. We expect the BDR to further decrease after extended running time.

The effect of reflection is studied experimentally on a 110 GHz, 1.2 MW short-pulse gyrotron. The experimental setup allows variation of the reflection from 0 to 35% over a range of gyrotron operating conditions. For high-efficiency operating points, the reflection is causing a decrease of the gyrotron power and is responsible for the excitation of an unwanted transverse mode, suppressing the excitation of the main mode.

This paper details recent progress on the experimental demonstration of a W-band klystron amplifier completed at the MIT. The amplifier utilizes a square lattice photonic bandgap (PBG) structure that permits the use of a highly oversized beam tunnel of diameter ~λ/4. Cold test measurements of the PBG klystron cavities revealed successful fabrication of the device. In hot test, a small-signal gain of 26 dB was measured at 93.7 GHz, with a saturated output power of 30 W.

A 17 GHz single cell, standing wave, copper accelerator structure with an axisymmetric elliptical central cell sidewall was tested for internal and downstream dark current as a function of gradient up to 93 MV/m. The elliptical sidewall was predicted to suppress the internal dark current and the lower order multipactor modes as compared with a previously tested structure having a straight sidewall. During the conditioning phase of the elliptical sidewall structure, strong internal dark current generated by an N=1 multipactor mode was observed at gradients in the 10 to 20 MV/m range. After conditioning with 2.2×10^{5} pulses to 93 MV/m, the N=1 mode was completely suppressed and no multipactor resonances were observed. The internal dark current was reduced to a comparatively low level, much smaller than in the previously studied, straight sidewall structure, in good agreement with simulations. The energy spectrum of the electrons colliding with the sidewall was measured using an isolated side dark current monitor and a bias voltage. As the conditioning progressed, the electron energy spectrum showed an increase in the concentration of lower energy electrons, also in good agreement with simulations. Studies of internal dark current may help to understand the rf conditioning and ultimate performance of high gradient accelerator structures., United States. Department of Energy. High Energy Physics (Grant DE-SC0015566)

The elliptical polarization produced by a pair of sinusoidally profiled miter-bend grating polarizers has been measured experimentally and found to be in excellent agreement with theory. The polarizers were designed to be used at 170 GHz in the miter bends of a 63.5 mm corrugated metallic waveguide for creating the arbitrary elliptically polarized microwave beam needed for electron cyclotron resonance heating of plasma. Using a vector network analyzer to generate a linearly polarized HE11 incident mode, each polarizer was individually tested to measure the rotation $\alpha $ and ellipticity $\beta $ of the elliptically polarized reflected microwave beam as a function of the grating rotation angle. The grating polarizers were then tested together to measure the elliptical polarization as a function of the angles of the combination of the two polarizers arranged sequentially on the transmission line. The map of the ellipticity versus grating angles agreed very well with numerical simulations using high-frequency structure simulator. Numerical simulations show that up to eight combinations of rotation angles of the two grating polarizers can provide the same ellipticity but with the varying ohmic loss, thus allowing the choice of settings to minimize the ohmic loss, a necessary consideration to minimize heating effects in megawatt power level systems.

We report the design and experimental demonstration of a frequency tunable terahertz gyrotron at 527 GHz built for an 800-MHz dynamic nuclear polarization enhanced nuclear magnetic resonance (DNP-NMR) spectrometer. The gyrotron is designed at the second harmonic ( $\omega = {2}\omega _{c}$ ) of the electron cyclotron frequency. It produces up to 9.3-W continuous microwave (CW) power at 527.2-GHz frequency using a diode type electron gun operating at ${V} = {16.65}$ kV, ${I}_{b} = {110}$ mA in a TE11,2,1 mode, corresponding to an efficiency of ~0.5%. The gyrotron is tunable within ~0.4 GHz by combining voltage and magnetic field tuning. The gyrotron has an internal mode converter that produces a Gaussian-like beam that couples to the HE11 mode of an internal 12-mm i.d. corrugated waveguide periscope assembly leading up to the output window. An external corrugated waveguide transmission line system is built including a corrugated taper from 12- to 16-mm i.d. waveguide followed by 3 m of the 16-mm i.d. waveguide The microwave beam profile is measured using a pyroelectric camera showing ~84% HE11 mode content.

This Letter reports the successful experimental demonstration of amplification of subterahertz radiation in a klystron with photonic crystal cavities. The klystron has six cavities, with each cavity having a series of oversized photonic crystal cells made up of a $5\ifmmode\times\else\texttimes\fi{}3$ array of square posts. The center post is removed from each cell to form a highly oversized ($0.8\text{ }\text{ }\mathrm{mm}\ensuremath{\sim}\ensuremath{\lambda}/4$) beam tunnel, with power coupling from cell to cell through the tunnel. The pulsed electron beam is operated at 23.5 kV, 330 mA in a 0.5 T solenoidal field. At 93.7 GHz, a small-signal gain of 26 dB and a saturated output power of 30 W are obtained. Experimental results are in very good agreement with the predictions of a particle-in-cell code. The successful achievement of high gain operation of a photonic crystal klystron amplifier is promising for the future extension of klystron operation well into the terahertz frequency region.

We report initial high-gradient testing results of a 110 GHz single-cell standing wave accelerating cavity powered by a 1 MW gyrotron. The cavity is fed with 10 ns, 100s of kilowatt pulses, and achieved a field gradient up to 225 MV/m. We also report the observation of rf breakdowns and cavity processing for $\lt /p\gt \gt 10 ^{5}$ pulses.

We describe the design and construction of a modular, triple-resonance, fully balanced, DNP-MAS probe based on transmission line technology and its integration into a 500 MHz/330 GHz DNP-NMR spectrometer. A novel quantitative probe design and characterization strategy is developed and employed to achieve optimal sensitivity, RF homogeneity and excellent isolation between channels. The resulting three channel HCN probe has a modular design with each individual, swappable module being equipped with connectorized, transmission line ports. This strategy permits attachment of a mating connector that facilitates accurate impedance measurements at these ports and allows characterization and adjustment (e.g. for balancing or tuning/matching) of each component individually. The RF performance of the probe is excellent; for example, the 13C channel attains a Rabi frequency of 280 kHz for a 3.2 mm rotor. In addition, a frequency tunable 330 GHz gyrotron operating at the second harmonic of the electron cyclotron frequency was developed for DNP applications. Careful alignment of the corrugated waveguide led to minimal loss of the microwave power, and an enhancement factor e = 180 was achieved for U-13C urea in the glassy matrix at 80 K. We demonstrated the operation of the system with acquisition of multidimensional spectra of cross-linked lysozyme crystals which are insoluble in glycerol-water mixtures used for DNP and samples of RNA.

A laser-driven semiconductor switch (LDSS) employing silicon (Si) and gallium arsenide (GaAs) wafers has been used to produce nanosecond-scale pulses from a 3 μs, 110 GHz gyrotron at the megawatt power level. Photoconductivity was induced in the wafers using a 532 nm laser, which produced 6 ns, 230 mJ pulses. Irradiation of a single Si wafer by the laser produced 110 GHz RF pulses with a 9 ns width and >70% reflectance. Under the same conditions, a single GaAs wafer yielded 24 ns 110 GHz RF pulses with >78% reflectance. For both semiconductor materials, a higher value of reflectance was observed with increasing 110 GHz beam intensity. Using two active wafers, pulses of variable length down to 3 ns duration were created. The switch was tested at incident 110 GHz RF power levels up to 600 kW. A 1-D model is presented that agrees well with the experimentally observed temporal pulse shapes obtained with a single Si wafer. The LDSS has many potential uses in high power millimeter-wave research, including testing of high-gradient accelerator structures.

We have built a frequency tunable 527 GHz gyrotron for an 800 MHz DNP-NMR spectrometer. The gyrotron is designed at the second harmonic $(\pmb{\omega=2}\pmb{\omega}_{\mathbf{c}})$ of the electron cyclotron frequency. It produces ~10 W continuous microwave power at 527.12 GHz frequency using a diode type electron gun $(\mathbf{V}\sim \pmb{16.5}\mathbf{kV},\ \mathbf{I}_{\mathbf{b}}\sim \pmb{110}\mathbf{mA})$ . The gyrotron is tunable within ~ 0.5 GHz by combining voltage and magnetic field tuning. The gyrotron comprises an internal periscope assembly to internally convert a Gaussian beam to an HE ll mode in a 12 mm i.d. corrugated waveguide. The corrugated waveguide transmission line system is built including 3 m long 16 mm i.d. waveguide and a corrugated taper from 12 mm to 16 mm i.d. waveguide.

The nonlinear interaction of electrons with a high intensity, spatially localized, Gaussian, electro-magnetic wave packet, or beam, in the electron cyclotron range of frequencies is described by the relativistic Lorentz equation. There are two distinct sets of electrons that result from wave-particle interactions. One set of electrons is reflected by the ponderomotive force due to the spatial variation of the wave packet. The second set of electrons are energetic enough to traverse across the wave packet. Both sets of electrons can exchange energy and momentum with the wave packet. The trapping of electrons in plane waves, which are constituents of the Gaussian beam, leads to dynamics that is distinctly different from quasilinear modeling of wave-particle interactions. This paper illustrates the changes that occur in the electron motion as a result of the nonlinear interaction. The dynamical differences between electrons interacting with a wave packet composed of ordinary electromagnetic waves and electrons interacting with a wave packet composed of extraordinary waves are exemplified.

We report the experimental demonstration of a mm-wave electron accelerating structure powered by a high-power rf source. We demonstrate reliable coupling of an unprecedented rf power—up to 575 kW into the mm-wave accelerator structure using a quasi-optical setup. This standing wave accelerating structure consists of a single-cell copper cavity and a Gaussian to TM01 mode converter. The accelerator structure is powered by 110 GHz, 10-ns long rf pulses. These pulses are chopped from 3 ms pulses from a gyrotron oscillator using a laser-driven silicon switch. We show an unprecedented high gradient up to 230 MV/m that corresponds to a peak surface electric field of more than 520 MV/m. We have achieved these results after conditioning the cavity with more than 105 pulses. We also report preliminary measurements of rf breakdown rates, which are important for understanding rf breakdown physics in the millimeter-wave regime. These results open up many frontiers for applications not only limited to the next generation particle accelerators but also x-ray generation, probing material dynamics, and nonlinear light-matter interactions at mm-wave frequency.

We report the measurement of the internal dark current in a 17 GHz, high gradient accelerator cavity and its comparison with theory. The cavities were fabricated from copper and had a sidewall that was either uncoated or coated with diamondlike carbon or TiN. The dark current was monitored by a downstream detector and by detectors behind two small slits made in the cavity sidewall. With an increasing gradient, the downstream current increased monotonically, as expected for field emission. The variation of the internal, side dark current was not monotonic but showed the onset of peaks at gradients near 45 and 65 MV/m. These were identified as the N=2 and N=1 single point multipactor resonances. The total internal dark current was estimated at ∼15–30 A. The magnitude of the internal dark current and its dependence on the gradient were in good agreement with simulations using the cst code as well as an in-house code. Processing to a higher gradient, ∼90 MV/m, eliminated the N=2 mode, but the N=1 mode persisted. The coated sidewall cavities showed the same multipactor resonances as the uncoated structure. However, at the highest gradient achieved in testing, the coated structures showed a modest reduction in the internal dark current., United States. Department of Energy (Grant DE-SC0015566)

We show high power test progress for 110 GHz high-gradient accelerating structures. The purpose of this work is to study the physics of particle accelerators in the mm-wave regime, as well as ultrahigh vacuum RF breakdown in such structures. The setup under investigation consists of a single-cell standing-wave accelerating cavity operated in the n-mode and fed via the TMm mode in a circular waveguide, which is in turn fed through a Gaussian horn. The cavities will be powered with nanosecond pulses from a MW gyrotron oscillator, allowing it to reach a peak accelerating gradient of 400 MeV/m.

We present the first demonstration of high-power, reversed-Cherenkov wakefield radiation by electron bunches passing through a metamaterial structure. The structure supports a fundamental transverse magnetic mode with a negative group velocity leading to reversed-Cherenkov radiation, which was clearly verified in the experiments. Single 45 nC electron bunches of 65 MeV traversing the structure generated up to 25 MW in 2 ns pulses at 11.4 GHz, in excellent agreement with theory. Two bunches of 85 nC with appropriate temporal spacing generated up to 80 MW by coherent wakefield superposition, the highest rf power that metamaterial structures ever experienced without damage. These results demonstrate the unique features of metamaterial structures that are very attractive for future high-gradient wakefield accelerators, including two-beam and collinear accelerators. Advantages include the high shunt impedance for high-power generation and high-gradient acceleration, the simple and rugged structure, and a large parameter space for optimization.

Prior research regarding the development of a W-band extended interaction klystron (EIK) with a photonic bandgap (PBG) based interaction circuit suggests that conventional PBG geometries, which have been demonstrated with larger dimensions are not the ideal choice for smaller, millimeterwave EiKs. In this presentation, a new, practical PBG geometry, which may be directly machined is presented. A 2D finite element Eigenmode solver is developed and utilized to calculate the electromagnetic properties of the PBG, including the global frequency bandgaps.

The phase stability of a 140 GHz, 1 kW pulsed gyro-amplifier system was measured and found to be stable both pulse-to-pulse and during each pulse. The gyro-amplifier has been developed at MIT for pulsed Dynamic Nuclear Polarization Nuclear Magnetic Resonance (DNP-NMR) spectroscopy1. It uses a confocal geometry to achieve an overmoded interaction circuit with reduced mode competition and is designed to operate without severs. The confocal gyro-amplifier demonstrated a peak circuit gain of 35 dB, a bandwidth of 1.2 GHz, and a peak output power of 550 W at 140.0 GHz.

We present the experimental results of a high power microwave source with a metamaterial structure designed at 2.4 GHz. The structure is a waveguide loaded with two metamaterial plates consisting of complementary split ring resonators placed in the waveguide with reverse symmetry. Two backward wave modes can propagate in the waveguide and interact with an electron beam of up to 490 kV, 84 A. Full microsecond long output microwave pulses up to 2.9 MW were measured with an efficiency of 9% from the Cherenkov-cyclotron instability.

Metamaterials are subwavelength structure material engineered to have some unique properties that are not found in nature, such as reversed Cherenkov radiation (RCR). Based on the RCR, some important research groups in the world have made great attempts to develop novel metamaterial-inspired vacuum electron devices. These research results clearly show that metamaterial-inspired vacuum electron devices have very remarkable advantages, such as small size, high power, high efficiency, and high gain. These brand new metamaterial-inspired devices have promising applications in radar, communications, electronic warfare, microwave heating, accelerators, imaging, and many other areas.

This report summarizes the progress of experimental efforts for the development a 94 GHz extended interaction klystron (EIK) with a photonic bandgap (PBG) based interaction structure. Input return loss measurements of the interaction structure are presented, demonstrating the successful fabrication of the input and output cavities of the EIK. Additionally, input return loss and insertion loss measurements of microwave-vacuum feedthrough windows are also presented. Finally, a review of the complete W-band experimental apparatus is given. Hot test experimental efforts for this study are expected to be completed by the 2018IVEC meeting.

A mm-wave disk-loaded waveguide (DLWG) traveling wave tube (TWT) amplifier is being designed for operation at 250 GHz. This design, in which coupling from cell to cell is via a highly ovesized beam tunnel, is attractive due to the ease in fabrication, even at high frequencies. The design has been conducted using analytic Pierce theory and time-domain simulations, which predict high gain. However, particle-in-cell (PIC) simulations show hybrid modes that de-bunch the electron beam and deteriorate the device performance when the structure length is increased to achieve very high gain. Additional design optimization is needed to avoid these unwanted modes.

Simple analytical formulae are presented for the design of linear tapers with very low mode conversion loss in overmoded corrugated waveguides. For tapers from waveguide radius a 2 to a 1, with a 1

A 94-GHz overmoded traveling-wave tube (TWT) has been designed, fabricated, and successfully tested. The TWT operates in the rectangular TM31 mode of the cavity, while lower order modes are suppressed using selectively placed strips of lossy dielectric. The 87-cavity TWT circuit was directly machined from Glidcop, a dispersion-hardened copper. The TWT was tested in a 0.25 T solenoidal magnetic field in 3- $\mu \text{s}$ pulses. Operating at a voltage of 30.6 kV with 250 mA of collector current, the TWT was zero-drive stable and achieved 21 ± 2 dB linear device gain with 27-W peak output power. Taking into account 3 dB of loss in both the input and output coupling circuits, the gain of the TWT circuit itself is 27 ± 2 dB with 55 W of saturated circuit output power. Using the 3-D particle-in-cell code CST Particle Studio, the linear circuit gain was estimated to be 28 dB and the saturated output power 100 W, in good agreement with the experimental results. The measured bandwidth of 30 MHz was significantly smaller than the predicted value of 250 MHz. The overmoded TWT is a promising approach to high-power TWT operation at W-band and to the extension of the TWT to terahertz frequencies.

The design and experimental results of a 140-GHz gyro-amplifier that uses a dielectric-loaded, severless confocal waveguide are presented. The gyro-traveling wave amplifier uses the HE06 mode of a confocal geometry with power coupled in and out of the structure with Vlasov-type, quasi-optical couplers. Dielectric loading attached to the side of the confocal structure suppresses unwanted modes allowing zero-drive stable operation at 48 kV and 3 A of beam current. The confocal gyro-amplifier demonstrated a peak circuit gain of 35 dB, a bandwidth of 1.2 GHz, and a peak output power of 550 W at 140.0 GHz.

We report the amplification of 250 GHz pulses as short as 260 ps without observation of pulse broadening using a photonic-band-gap circuit gyrotron traveling-wave-amplifier. The gyrotron amplifier operates with a device gain of 38 dB and an instantaneous bandwidth of 8 GHz. The operational bandwidth of the amplifier can be tuned over 16 GHz by adjusting the operating voltage of the electron beam and the magnetic field. The amplifier uses a 30 cm long photonic-band-gap interaction circuit to confine the desired TE03-like operating mode while suppressing lower order modes which can result in undesired oscillations. The circuit gain is >55 dB for a beam voltage of 23 kV and a current of 700 mA. These results demonstrate the wide bandwidths and a high gain achievable with gyrotron amplifiers. The amplification of picosecond pulses of variable lengths, 260–800 ps, shows good agreement with the theory using the coupled dispersion relation and the gain-spectrum of the amplifier as measured with quasi-CW input pulses.

The linear and nonlinear theory of a gyroamplifier using a confocal waveguide is presented. A quasi-optical approach to describing the modes of a confocal waveguide is derived. Both the equations of motion and the mode excitation equation are derived in detail. The confocal waveguide circuit has the advantage of reducing mode competition, but the lack of azimuthal symmetry presents challenges in calculating the gain. In the linear regime, the gain calculated using the exact form factor for the confocal waveguide agrees with an azimuthally averaged form factor. A beamlet code, including velocity spread effects, has been written to calculate the linear and nonlinear (saturated) gain. It has been successfully benchmarked against the MAGY code for azimuthally symmetric cases. For the confocal waveguide, the beamlet code shows that the saturated gain is reduced when compared with results obtained using an azimuthally averaged form factor. The beamlet code derived here extends the capabilities of nonlinear gyroamplifier theory to configurations that lack azimuthal symmetry.

Low power measurements were done on a set of miter-bend polarizers for converting a linearly polarized microwave beam into an arbitrary elliptically polarized beam at 170 GHz for Electron Cyclotron Resonance Heating in ITER. The miter-bend polarizers were designed and constructed by General Atomics (GA). The measured data are compared with numerical simulations performed using the vector theory of diffraction gratings and with High Field Structure Simulator (HFSS) software for the designed sinusoidal groove shapes. A full polarization map is obtained, both experimentally and numerically, by calculating the phase shifts of TE and TM modes of the grating surface. Excellent agreement is obtained between the theoretical predictions and the experimental results. In addition to the polarization properties, mode conversion in the polarization rotator miter-bend was studied experimentally by measuring the radiated fields from the waveguide aperture as a function of grating rotation angle.

The challenge in manufacturing Traveling Wave Tubes (TWTs) at high frequencies is that the sizes of the structures scale with, and are much smaller than, the wavelength. This requires advanced nano-machining techniques or sheet-beam devices and puts strict limits on the peak output power that can be safely handled by the devices. Furthermore, the diameter of the electron-beam tunnel limits the amount of beam current, requires high magnetic fields for beam compression and creates difficulties in alignment.