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Short-wavelength, low-frequency, frequently coherent fluctuations regulate transport across the plasma boundary in all steady-state tokamak confinement regimes of interest for a reactor. In the ideal case, these fluctuations expel particles, especially impurities, without significantly degrading energy confinement. The Shoelace antenna is a novel tool employed on the Alcator C-Mod tokamak designed to couple to such fluctuations inductively.

Signal-to-noise is a key ingredient in successful NMR experiments, a fact that has stimulated the development of dynamic nuclear polarization (DNP). In solid state magic angle spinning (MAS) NMR experiments, we have observed signal enhancements of 50–400 at 140 and 250 GHz operating frequencies, depending on a variety of experimental conditions. These experiments are possible because of the development of gyrotrons for generating the high frequency microwaves needed for DNP/NMR. We report progress in the development of tunable gyrotrons at 330 and 460 GHz for DNP/NMR spectroscopy. The successful recent demonstration of DNP/NMR enhancements using the 460 GHz gyrotron with a 700 MHz NMR spectrometer makes it the highest frequency DNP/NMR system currently in operation worldwide. A 527 GHz gyrotron is now under development for application in an 800 MHz NMR system. We present an overview of the systems and related THz technologies developed at MIT for DNP/NMR spectroscopy.

Corrugated waveguide made with a tap can significantly lower fabrication costs. A 330 GHz, 19 mm i.d., 4.5 m long brass circular waveguide was tapped and tested. Transmission measurements were compared with models that take into account the shape of the corrugation.

We report amplification of pulses as short as 400 ps in a 1 kW, 140 GHz gyro-TWT amplifier. To our knowledge, this is the first observation of picosecond pulse amplification in a gyro-amplifier. The pulses show measurable broadening due to two distinct phenomena: group velocity dispersion and spectral narrowing due to the finite gain bandwidth. Theoretical analysis is in good agreement with the observed broadening of the pulses. Picosecond pulses are useful for applications in spectroscopy and advanced radar.

High power millimeter wave and terahertz sources are used in DNP/NMR spectroscopy to greatly enhance the NMR signal. A key issue is the efficient coupling of the source power to the sample. We present HFSS calculations showing that the MIT 250 GHz DNP/NMR system provides a B 1 value of 13 µT/W1/2. The simulation shows the importance of the diffraction of radiation passing through the RF coil, leading to non-uniform illumination of the sample. A simple single pass model yields insight into the origin of the observed B 1 value. Methods of optimizing the B 1 value are described.

The levitated dipole experiment (LDX) is investigating a magnetic dipole field configuration similar to planetary magnetospheres as an alterative confinement concept for fusion plasmas. Recent experiments have confirmed that stationary, highly peaked plasma density profiles (> 50 core to edge) are formed by the conservation of the product of plasma density and differential flux tube volume (V = ∮dl/B). These natural peaked profiles are maintained by ambient plasma turbulence, contrary to experience with plasmas inside a set of coils where turbulence acts to flatten profiles. LDX uses a 1.1 MA, 34 cm mean radius, 560 kg superconducting coil (F-coil) that is freely floated by a 280 kA levitating coil for over 2 hours between cryogenic recoolings. Plasmas are started and sustained by electron cyclotron heating (ECH) on closed flux surfaces encircling the F-coil that cross the outer midplane radius between 67 and 177 cm. The magnetic field strength varies from 0.007 to 3.2 Tesla around the F-coil on these flux surfaces corresponding to EC resonance between 0.2 to 90 GHz. Consequently, ECH in a magnetic dipole is possible with a multiplicity of sources at many frequencies.

We report the first experimental studies of picosecond pulse amplification and broadening in a gyrotron amplifier. These studies allow investigation of the amplification process in a new physical regime, where wave dispersion and finite gain bandwidth play a major role. The experiments were performed on a 140 GHz gyro-traveling wave tube (Gyro-TWT) operating in a higher order mode of a confocal waveguide. We observed significant pulse broadening for pulses shorter than 1 ns when the operating point of the amplifier was close to the waveguide cut-off. For example, a 580 ps pulse at 137.30 GHz, a frequency near cutoff, was measured to be broadened by 100 %, while pulses at higher frequency, further from cutoff, showed much smaller broadening. The results of these experimental studies show that for the picosecond pulse amplification necessary in applications such as Electron Paramagnetic Resonance (EPR), the operating point of the gyro-TWT should be chosen to be at least a few percent above the cut-off to avoid pulse distortion.

The ITER transmission lines (TLs) must be designed to deliver 20 MW from a 24 MW, 170 GHz gyrotron system. Miter bends are the main source of loss for these highly overmoded, corrugated, cylindrical waveguide TLs. Previous estimates have used only a pure HE 11 mode for the analysis of the loss due to a miter bend, however higher order modes (HOMs) must be considered for a practical analysis. For the linearly-polarized, Gaussian-like beam from a gyrotron, the LP mn mode basis set should be used to describe the fields in the corrugated waveguide. The HOM content greatly affects the propagation of HE 11 content in a miter bend, with a large emphasis placed on the percentage of HOMs and the phase difference between HE 11 and each HOM. By considering LP modes, a complete basis set is used to investigate the HOM effects on HE 11 loss in a miter bend. We also present a new conservation theorem relating the power centroid offset and propagation angle due to any two LP mn modes propagating in the corrugated waveguide.

The design and experimental study of a 140 GHz, 1 kW gyro-traveling wave tube (Gyro-TWT) operating in the HE 06 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.

Millimeter waves are ideally suited for sensing and diagnosing materials, devices, and processes that are broadly important to energy, environment, and national security. Thermal return reflection (TRR) techniques that detect and use thermal emission as a probe to diagnose materials and systems make possible accurate noncontact thermal analysis measurements that can resolve emissivity and temperature simultaneously. Scientific basis of TRR, 2-D and potentially 3-D measurements, and applications of TRR are discussed.

Mode conversion in miter bends and polarizers is the main contributor of loss in the transmission lines (TLs) for the ITER 170 GHz ECH system, which should transport one megawatt of power with the smallest possible loss. Previous loss estimates assumed that the power in the TL was carried by a pure HE11 mode; however, in practice, there is significant power in higher order modes (HOMs). It is shown that the mode conversion loss of the power in an HE11 mode at a miter bend is greatly altered by the presence of even a small proportion of HOMs in the TL, and is a strong function of both the magnitude of the HOM and its phase relative to that of the HE11 mode. The resulting total loss in the ITER transmission lines is expected to be very different from the loss previously predicted using single mode theory.

The transmission lines (TLs) for ITER electron cyclotron heating require extremely low losses, since the 24 MW of power generated by the 170 GHz gyrotrons should be delivered to the plasma with an efficiency of 83% or more. We are developing cold-test techniques to precisely measure the losses in the TL components for ITER, before installation. Experimental results on the measurement of the loss in some TL systems with coherent and incoherent techniques are presented and compared. We report a preliminary measurement of loss in the prototype TL components for the ITER 170 GHz transmission line.

Dynamic nuclear polarization enhanced nuclear magnetic resonance (DNP/NMR) requires a high power continuous-wave (CW) electromagnetic wave source in the submillimeter wavelength range to obtain significant enhancements in spin spectra from biological samples. Having the ability to generate very high power at submillimeter wavelengths, gyrotrons are well suited for use in DNP/NMR spectrometers1. A 460 GHz CW gyrotron for DNP enhanced 700 MHz NMR spectroscopy has been developed", and day-long stable operation has been demonstrated1 at MIT. In this presentation, an upgraded design of the 460 GHz gyrotron will be discussed. To produce higher and more stable output with the present 12 kV electron gun, TE11.2 mode is selected. This design is capable of lowering the starting oscillation current to 26 mA with the 30 mm long cavity. From simulations using a nonlinear gyrotron code, MAGY, above 60 watts of output power is predicted with the beam current of 100 mA. A Vlasov-tvpe launcher with a helical cut is designed for the coupling of the whispering-gallery mode. High power radiation with good spectral and spatial characteristics from this tube should provide NMR spectrometers with high signal enhancement. Besides experimental progress to implement the DNP/NMR spectrometer, other possible applications to utilize this powerful CW submillimeter-wave source, will also be discussed.

Summary form only given. A critical need exists for confined fast ion diagnostics in tokamak fusion experiments, particularly for fusion product alpha particles in ITER and future fusion burning experiments. To develop this diagnostic capability and in support of current fast ion plasma physics research, collective Thomson scattering (CTS) diagnostics have been implemented at TEXTOR and ASDEX Upgrade (AUG) tokamaks using available gyrotron infrastructure with the addition of sensitive scattered signal receiver systems. At TEXTOR a 180 kW, 110 GHz gyrotron and a 42 channel. 6 GHz bandwidth heterodyne receiver has achieved up to 100 CTS scattered spectra per plasma shot with 4 ms time and 10 cm spatial resolution. Large scattering angles (~160deg) with steerable optics enable observation of fast ion spatial and field orientation anisotropies. Studies of fast ion dynamics behavior with neutral beam injection (NBI) and ion cyclotron heating have commenced, resulting in unique observations of fast ions redistributions during sawteeth and slow down after NBI turn off. At AUG a 1 MW, 105 GHz mode of a two-frequency gyrotron with a 50 channel, 10 GHz bandwidth receiver is becoming operational for CTS diagnostics with resolutions similar to TEXTOR. Precise gyrotron frequency measurements, notch filter timing, transmission line alignments, and receiver field of view mappings inside the tokamak have been accomplished using novel beam profile instrumentation. AUG-CTS commissioning progress will be presented. Plasma measurements in AUG are expected to provide new insights into fast ion physics and to further validate gyrotron CTS as a fast ion diagnostic tool for ITER.

We report microsecond pulse and CW results of a gyrotron operating near 460 GHz and 230 GHz at the second electron cyclotron harmonic and fundamental, respectively. Peak power levels of up to 70 W at the fundamental and 3 W at the second harmonic have been obtained in operation at up to 13 kV and 150 mA. The gyrotron has been operated continuously to achieve 3 W CW at the second harmonic. The gyrotron was also demonstrated as a wideband continuous frequency tunable oscillator by magnetically exciting fundamental higher order longitudinal modes. The gyrotron oscillator can be used for dynamic nuclear polarization (DNP) studies in conjunction with a 700 MHz (/sup 1/H), 16.5 T nuclear magnetic resonance (NMR) spectrometer.

Diagnostics of materials in high-temperature, optically inaccessible environments can be reliably achieved with millimeter-wave methods and components. A 137 GHz heterodyne receiver using a thermal return reflection (TRR) technique and ceramic waveguides tested to 1500/spl deg/C have been used to demonstrate the capability to measure temperature, emissivity, viscosity, and salt layer formation in laboratory and pilot scale melter tests.

Millimeter-wave (MMW) techniques and technologies provide robust new diagnostic tools for glass melt measurements that are addressing needs for vitrification of high level and low activity nuclear wastes in glass media at the Defense Waste Processing Facility (DWPF) in Savannah River and the Waste Treatment Plant (WTP) under construction at Hanford. Over the past seven years, we have demonstrated real-time measurement of several glass melt parameters, e.g., viscosity, cold-cap temperature, foaming, and pouring. An overview of MMW diagnostics for vitrification plants is presented.

A 250 GHz HE/sub 11/ corrugated transmission line with a sampling directional coupler for multiwatt CW gyrotron radiation has been successfully implemented for dynamic nuclear polarization experiments. A second transmission line at 460 GHz is being built. Trade offs between smooth walled dielectric and corrugated waveguides are evaluated.

137 GHz heterodyne receiver measurements with hollow ceramic HE/sub 11/ waveguides are being applied to nuclear waste glass melt monitoring of temperature, emissivity, and surface fluctuations/flow in both laboratory and engineering scale melters at temperatures as high as 1500/spl deg/C. New insights are being provided on molten glass/salt interaction and glass pour dynamics.

Millimeter-wave technologies can provide novel and reliable on-line monitoring capability for many important parameters inside high temperature process environments such as in the manufacture of glass, metals, and waste remediation. Important parameters include temperature, emissivity, density, and viscosity, which often cannot be monitored reliably by conventional techniques. The physical and analytical basis for millimeter-wave monitoring of high temperature processes is presented along with experimental results at temperatures up to 1500/spl deg/C.

A 3 MW (upgradeable to 4 MW) 4.6 GHz Lower Hybrid Current Drive (LHCD) system is being implemented on Alcator C-Mod to control and sustain current profile evolution. The LHCD low-power microwave and active control system, using fast vector modulators, will provide a phase and amplitude controlled driver for each of twelve 4.6 GHz, 250 kW klystrons. Resulting phase and power outputs of each klystron will be monitored by I-Q detectors and made available for closed-loop control of the klystron phase and amplitude during plasma shots. Synchronized digital controllers will be implemented for each control loop and operation will be coordinated with external fast protection circuitry provided in the LHCD transmitter and coupler protection systems.

A radiometric method for resolving the emissivity, /spl epsiv/, and temperature, T, in thermal emission measurements is derived and experimentally demonstrated. The viewed source's own emission is utilized as an incoherent probe beam by means of a beamsplitter and return mirror. Measurements of several refractory materials at 137 GHz to 1500/spl deg/C were carried out.

Localized alpha-particle velocity distribution and density, ion temperature, DT fuel ratio, and internal magnetic field pitch angle can all be potentially diagnosed by a collective Thomson scattering system. Relativistic electron cyclotron calculations and TORAY ray tracing for 6 tesla ITER parameters indicate that 90 GHz is an optimum frequency for this diagnostic. With 400 kW at the plasma and a 90/spl deg/ scattering angle, signal to noise ratios approaching 100 are possible for 0.5% n/sub e/, alpha-particle fractions and 100 ms integration times. A millimeter wave system would be robust and adaptable to the ITER environment and access.

summary form only given. A microwave induced plasma device is being developed for sensitive continuous emission monitoring of trace metals in furnace exhausts using atomic emission spectroscopy. The plasma is created in air at atmospheric pressure, and maintained by a 1.5 kW, 2.45 GHz microwave source. A diagnostic system, consisting of a fiber optic cable, lenses, a spectrometer, and a linear Si diode detector array, provides information about the light emitted from the plasma with a maximum resolution of 0.01 nm in the range of 190-600 nm. In this work we describe the experimental setup for the temperature profile reconstruction, the conceptual method for data analysis and some preliminary results.

Summary form only given, as follows. An atmospheric pressure microwave plasma torch has been shown to be a sensitive and accurate atomic excitation medium for stack exhaust compliance monitoring of trace metals air pollution. High electron excitation energies are needed for minimizing the detection limits of some important metal pollutants such as mercury and arsenic. A study has been made of the effects of power and airflow on the electronic excitation temperature, T/sub exc/, of a 0.9 - 1.5 kW, 2.45 GHz plasma torch. The plasma is sustained in a 28 mm internal diameter fused quartz tube which penetrates through the wide walls of a tapered and shorted WR-284 (72/spl times/17 mm cross-section) waveguide. Abel inversion of radial profile chord averaged Fe I emission lines in the 370 to 377 nm range have been used to obtain localized T/sub exc/ profiles inside the waveguide excitation region. Microwave power has little effect on temperature, but increased airflow from 14 lpm to 28 lpm constricts the plasma and increases temperature by about 13 %. Temperature profiles are relatively flat with a maximum central T/sub exc/ observed of 6550 K +/- 350 K at 28 lpm. The axially peaked temperature profiles and almost no microwave reflection (< 1%) are consistent with skin depth of order 10 mm and an electron density of /spl sim/ 10/sup 13/ cm/sup -3/.

Summary form only given. A 1.5 kW atmospheric microwave plasma at 2.45 GHz is being developed as an excitation source for real-time detection of hazardous metals in smokestack exhaust. There is currently an important need for metal continuous emissions monitors (CEMs) to meet current and future clean air regulations. A number of plasma generation methods for metal atomic emission spectroscopy are being tested for this application including inductively coupled plasmas (ICPs), laser sparks, DC electrode sparks, and microwave discharges. The microwave plasma has a significant advantage to continuously operate robustly in large volumes of fast flowing (/spl ges/14 1/minute) air or undiluted stack exhaust. Experimental measurements will be presented of the performance of this microwave plasma with Hg, Cd and As as a function of oxygen additive. An attempt will be made to explain the behavior in terms of the possible UV absorption and atomic excitation mechanisms in the plasma.

Possibilities for use of high temperature superconductors in microwave/far infrared resonators and waveguides are described. Relative to conventional superconductors, the use of high temperature superconductors could remove the constraint of relatively low energy gaps and provide larger thermal margins and more effective cooling. The higher critical magnetic fields may also be important for an increased range of operation. These features could be used to improve performance of present superconducting microwave cavities and to provide new applications at both microwave frequencies and at much higher frequencies. Constraints that may inhibit development include a need for superconducting properties over a high fraction of the surface, limits on critical RF fields and the effect of D.C. magnetic fields.