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Heating non-uniformity has been a major hurdle to numerous applications in microwave heating. The exposure to a non-uniform field is a well-known cause for this difficulty. There are, however, some much less-understood causes, which are inherent in nature in that they persist even in a perfectly uniform field. The current study addresses two such causes in water heating. The first one concerns the effects of field shielding by microwave-induced polarization charges, and the second one on the effects of microwave resonances in water.

The dimer under study is a dielectric structure formed of two identical sub-units. Dimer interactions with electromagnetic waves are widely studied in connection with electromagnetic properties of complex systems. The dimer in the form of two closely spaced grapes is also a subject of high public interest because its gap region sparks curiously in a household microwave oven. A recent paper presented the first scientific interpretation of this long-standing puzzle. It attributed the sparking phenomenon to an electromagnetic hotspot in the gap created by electromagnetic resonances in the dimer. This study has opened up a fertile ground for further research as well as motivating the current study on an independent mechanism. Our simulation and experimental results consistently point to an electrical origin for the sparks. The triggering mechanism is a two to three orders-of-magnitude buildup of localized electric field in the narrow gap, which results from the mutual enhancement of polarization charges on opposite sides of the gap. Consequently, sparks are observed at the frequency of 27 MHz, at which no electromagnetic resonance in the dimer is possible. Results also indicate a broad frequency range of the electrical mechanism, which persists even when the dimer is in strong electromagnetic resonance with the first few higher-order modes. These quantitative characterizations of basic dimer properties, in particular the broad frequency range of the polarization-charge enhancement effect, may be helpful for the understanding of collective behavior of multi-particle systems under electromagnetic radiations.

Motivated by numerous applications, active research is being conducted worldwide to explore the terahertz (THz) region of the electromagnetic spectrum. Waveguides are key components for the generation, propagation, and utilization of THz waves. This paper addresses two types of commonly-used waveguides in their simplest implementation: the single-wire and hollow metallic waveguides. Research on these two types of waveguides has appeared rather separately in literature. Yet they are based on a common formalism. This suggests a treatment on the same theoretical footing for an examination of their basic properties in terms of the field behavior, power flow, dispersion relation, and attenuation rate. Here, we present a comparative study on their similarities, differences, and relative merits, along with physical interpretations.

Because of the complicated geometry of the slotted structure, analytical theories of such structures are inevitably developed on the basis of simplifying assumptions. On the other hand, the accuracy of the theory is of importance to the design of microwave interaction structures. In this study, modes of the slotted waveguide are investigated analytically and simulated with the HFSS code. It is shown that, in spite of the approximations made, the dispersion relation and field patterns of the standard analytical theory are in excellent agreement with the HFSS simulations over the complete range of the slot depth. Modes not built into the theory will also be noted.

We present the theory and experimental observation on the polarization-charge shielding effects in microwave heating. A wood-stick sample is irradiated by a uniform traveling wave in an anechoic chamber. As a result of polarization-charge shielding, the heating rate is shown to be highly dependent on the orientation of the sample with respect to the wave electric field.

Over a span of five decades, the electron cyclotron maser (ECM) has undergone a remarkably successful evolution from basic research to a powerful millimeter, sub-millimeter, and THz wave generator called the gyrotron, while continuously being enriched by major advances. Below we discuss the ECM principle and its significance, followed by an overview of recent progresses made in two specific areas: THz gyrotron oscillators and millimeter-wave gyro-TWT amplifiers. Topics of the talk will include selected examples of device performances, ongoing/potential applications, and relevant physics issues.

The electron cyclotron maser (ECM) is based on a stimulated cyclotron emission process involving energetic electrons in gyrational motion. It constitutes a cornerstone of relativistic electronics, a discipline that has emerged from our understanding and utilization of relativistic effects for the generation of coherent radiation from free electrons. Over a span of four decades, the ECM has undergone a remarkably successful evolution from basic research to device implementation while continuously being enriched by new physical insights. By delivering unprecedented power levels, ECM-based devices have occupied a unique position in the millimeter and submillimeter regions of the electromagnetic spectrum, and find use in numerous applications such as fusion plasma heating, advanced radars, industrial processing, materials characterization, particle acceleration, and tracking of space objects. This article presents a comprehensive review of the fundamental principles of the ECM and their embodiment in practical devices.

Experimental results are presented on the first W-band gyrotron Traveling-Wave Tube (gyro-TWT) developed to exploit the 94 GHz atmospheric window for long-range, high-resolution radar applications. The gyro-TWT is designed to operate in the higher order TE01 mode and is driven by a 100 kV, 5 A electron beam with a pitch angle of v⊥/vz=1 and velocity spread of Δvz/vz=5%. Large-signal simulations predict 140 kW output power at 92 GHz with 28% efficiency, 50 dB saturated gain, and 5% bandwidth. The stability of the amplifier against spurious oscillations has been checked with linear codes. To suppress the potential gyro-BWO interactions involving the TE02, TE11, and TE21 modes, the interaction circuit with a cutoff frequency of 91 GHz has been loaded with loss so that the single-path, cold-circuit attenuation is 90 dB at 93 GHz. A coaxial input coupler with 3% bandwidth is employed with a predicted and measured coupling of 1 dB and 2 dB, respectively. The operating voltage is limited to below 75 kV because of oscillations encountered at higher voltages in this initial embodiment. Preliminary test at Vb=60 kV and Ib=3.7 A yielded 59 kW saturated output power at 92.2 GHz with 42 dB gain, 26.6% efficiency, and a 3 dB bandwidth of 1.2 GHz (1.3%).

Self-consistent effects on the starting current of gyrotron oscillators are examined. Field profiles in the open cavity are shown to be sensitive to the interaction dynamics. This can either significantly raise or lower the oscillation threshold, particularly for the low-Q modes. The transition from resonant-mode oscillations at the low magnetic field to backward-wave oscillations at the high magnetic field is demonstrated.

The gyrotron traveling-wave tube (gyro-TWT) is a millimeter-wave amplifier based on the electron cyclotron maser instability. It is a device of increasing importance because of its power and bandwidth capabilities. The current paper presents a brief overview of the gyro-TWT research over the past quarter of a century. Advances made by different groups employing various schemes are discussed and achieved performances are surveyed.

A high-power gyrotron traveling-wave amplifier operating in the low-loss TE/sub 01/ mode has been constructed at the University of California, Davis that will be driven by a 100-kV, 5-A electron beam with a pitch angle (/spl upsi//sub /spl perp////spl upsi//sub z/) of unity and velocity spread of 5%. The amplifier is predicted by large-signal simulations to generate 140 kW at 92 GHz with 28% efficiency, 50-dB saturated gain and 5% bandwidth. The stability of the amplifier from oscillation has been investigated with linear codes. The threshold current for the absolute instability of the TE/sub 01/ operating mode for the chosen operating parameters is predicted to be 10 A. To suppress the potential gyro-backward-wave oscillator interactions, the interaction circuit with a cutoff frequency of 91 GHz has been loaded with distributed loss so that the single-pass attenuation is 90 dB at 93 GHz. The coaxial input coupler has a predicted and measured coupling of 1 and 2 dB, respectively.

Radio frequency (RF) and microwave heating of dielectric objects is often susceptible to an excessive temperature spread due to uneven energy deposition. The exposure to a nonuniform field is a well-studied cause for this difficulty encountered in numerous applications. There are, however, some less-understood causes, which are inherent in nature in that they persist even in a perfectly uniform field. We present an experimental study on three main inherent causes with rice grains as samples. Of these cause, somewhat less obvious is the polarization charge shielding effect1 2. Exposed to the wave electric field, electrical charges in the dielectric object react in a way to partially shield the wave electric field. As a result, the dielectric object's interior electric field can be significantly smaller than the incident electric field, resulting in a much reduced power deposition. Experiments are conducted in an applicator, in which samples are irradiated by a 24 GHz microwave. High radiation uniformity (~99%) and polarization control allow a quantitative examination of each cause. Their individual and collective effects are found to be highly significant. In particular, polarization-charge shielding alone can result in a temperature spread of~ 18.2% for the samples examined. Physical interpretations are given and an effective method for its mitigation is demonstrated.

Relativistic electronics research in recent years has produced powerful millimeter waves on the MW level, while also extending the frequency range into the terahertz (THz) region and beyond. These developments have opened up new horizons in applications. The current study is motivated by the associated need for high-power absorbers not readily available at such frequencies. Our focus is on effective absorber schemes which can handle high power while also possessing a structural simplicity for easy implementation. In and above the THz region, the electrical conductivity can no longer be treated as a real constant. We begin with a derivation of the field penetration depth applicable to all frequencies. Requirements to meet the intended criteria are then determined from the wave penetration and reflection properties. Design examples in the 1–1000 GHz range are illustrated, which consist of a thin lossy conducting layer on the surface of a pyramidal shaped metal base. It is shown in theory that such structures can function effectively in the millimeter and THz regions.

Modeling a gyrotron cavity in 3D and time-domain is very challenging due to the open-end features of cavity structure and higher-order mode excitation employed to achieve high efficiency. In this work, a 3D conformal finite-difference time-domain (CFDTD) method has been developed to perform a cold test of a gyrotron cavity. Arbitrarily high order modes can be launched in a gyrotron cavity in time domain to determine the open cavity resonant characteristics. Our preliminary results show that the 3D CFDTD method could provide an alternative modeling tool for designing and validating a gyrotron cavity configuration for further hot tests.

Physical processes in the gyrotron backward-wave oscillator (gyro-BWO) are investigated theoretically. Results indicate highly current-sensitive field profiles and hence sharply contrasting linear and saturated behaviors. The linear field extends over the entire structure length, whereas the saturated profile depends strongly on the energetics in the internal feedback loop. It is shown that this distinctive feature substantially influences the basic properties of the gyro-BWO including the start-oscillation current, efficiency, power scaling, and stability of tuning.

We report experimental investigation of a novel scheme for efficient interaction between a linear electron beam and an electromagnetic wave in a complex extended interaction structure. The structure of interest consists of a high R/Q, five-gap, coupled-cavity resonator that incorporates a coaxial section of a quarter plasma wavelength placed between the first and second cavities. In the coaxial section, beam and wave propagate in separate channels. The first cavity, strongly coupled to the other cavities through the wave channel of the coaxial section, serves as a buncher cavity. An inner channel running through the center conductor of the coaxial section provides a cutoff drift space for ballistic bunching of electrons, an effect that is shown to significantly enhance the interaction efficiency. Oscillation power of 2.2 kW at 16.6 GHz was demonstrated with an interaction efficiency of 30%. The total efficiency was further increased to 41% by incorporation of a two-stage depressed collector.

Physics and technology issues of importance to the high-gain gyrotron traveling wave amplifier (gyro-TWT) are investigated in theory and experiment. The gyro-TWT is known to be highly susceptible to spurious oscillations, especially in high gain operations. In the current study, oscillations of various origins are classified and characterized with detailed theoretical modeling. They are shown to be intricately connected to the interplay between the absolute/convective instabilities, circuit losses, and reflective feedback. Knowledge of these processes leads to the concept of an ultra high gain scheme which employs distributed wall losses for the suppression of spurious oscillations. A proof-of-principle Ka-band gyro-TWT experiment stable at zero drive has produced 93 kW saturated peak power at 26.5% efficiency and 70 dB gain, with a 3 dB saturated output power bandwidth of 3 GHz. The saturated gain is more than 30 dB beyond that previously achieved.

Generation of a uniform density plasma has been a key consideration in the design of microwave/plasma processing systems. A circularly polarized wave can generate a plasma with good azimuthal uniformity as well as provide strong resonant interaction with the plasma electrons. In this article, we report the development of a three-port polarization converter which efficiently converts the TE10 wave of a standard rectangular waveguide into a circularly polarized TE11 wave of a cylindrical waveguide. Employing the same principle for mode conversion, the converter is also made to function as a protective device of the high power microwave source by providing a separate port for the return and damping of the reflected wave. Such a converter has been analyzed, constructed, and tested. At the operating frequency of 2.45 GHz, test results indicate a 25 dB return loss, 97% end-to-end polarization conversion efficiency, and better than 20 dB rejection of the reflected wave. The structural simplicity of the device allows high power operation as well as easy construction for system research and applications.

Mode competition in the gyrotron traveling wave amplifier is shown to be intricately connected to the interplay between the absolute/convective instabilities, circuit losses, and reflective feedback. Physical origins of spurious oscillations are analyzed and characterized. Fundamental understanding of these processes leads to a device concept which provides zero-drive stability at ultrahigh gain. The scheme was verified in a proof-of-principle experiment in the $\mathrm{Ka}$ band, producing 93 kW saturated peak power at $26.5%$ efficiency, 70 dB gain, and a 3 dB bandwidth of 3 GHz.

For open-end cavities commonly used in gyrotrons, the lack of a well-defined geometrical boundary gives rise to distinctive features as well as physics issues that are basically different from the closed cavity. There is a question concerning the consistency between the time- and frequency-domain interpretations of the resonant frequency. There are also inherent uncertainties in cold testing an open-end cavity by the transmission method in which one probe is located inside the cavity. Further, properties of the cavity can be significantly altered by a slightly mismatched load. We address these issues by developing a spectral model based on an integrated quantity, the total field energy in the cavity, which properly accounts for effects due to the open boundary. Resonant frequencies determined from the field-energy spectrum are shown to be fully consistent with those of the temporal model. The field-energy spectrum can be constructed from the power of the reflected waves, thus allowing cold test by the reflection method. In the cold test, calibration modification and phase averaging have been employed to minimize the effects of external coupling. Results of the cold test are compared with single-mode code and high-frequency structure simulator (HFSS) calculations. Measured data are found to be in good agreement with the HFSS predictions, while the single-mode code considerably overestimates the Q value at large output taper angles.

The magnetron injection gun (MIG) has been the predominant choice as the electron-beam generator in gyrodevices and it can be tuned by externally installed magnetic-field trim coils. The double-anode MIG, traditionally employed in most experiments, has the intermediate anode as an internal tuning mechanism. The single-anode design, on the other hand, is simpler to construct but lacks an internal beam tuning mechanism. The current paper describes a method to allow mechanical tuning of the MIG. Since the tip of the cathode is located in the region of the highest electric-field gradient, its extension and retraction relative to the rest of the cathode assembly provide an effective means for local electric-field profile modification and, therefore, a tuning mechanism for the electron-beam property and quality. It is demonstrated, in computer simulations and experimental tests, that the tuning mechanism adds a useful degree of freedom to the single-anode MIG for beam optimization. This method may also be applied to the double-anode design.

This study addresses some theoretical issues of interest to surface plasmon propagation along a metal-wire waveguide at terahertz (THz) frequencies. The conductor is characterized by a shallow field penetration depth at microwave frequencies and the transparency to frequencies above the ultraviolet. Falling between these two regimes, THz surface plasmons are thus particularly sensitive to the electron dynamics in the metal wire. Also, with evanescent fields both inside and outside, surface plasmons are known to be loosely bound to the metal wire, which further complicates the behavior of surface plasmons. These issues are examined here in a comparative study under AC- and DC-conductivity models. Depending on the wire radius, the frequency dependence of the conductivity has either a very significant effect on the Ohmic loss or no effect at all, while an opposite trend is found for the wave attenuation rate. Extensive data on the wave attenuation constant is also presented and interpreted. The results show ...

This paper investigates the inadequacy of using the growth rate from the dispersion relation to represent the bandwidth of a cyclotron autoresonance maser (CARM) amplifier. It is shown that the dispersion relation can accurately predict the amplifier`s bandwidth, but at least three waves that the electron beam couples to must be retained in the calculation. Interference between these waves must be included. The bandwidth of the coupling coefficient for the forward growing wave can be much narrower than the bandwidth of this wave`s growth rate. In addition, the input signal has been observed to be almost totally absorbed by the electron beam at a specific frequency which is a function of the operating conditions. This phenomenon is caused by the beating between the forward constant amplitude wave and the forward growing wave. It is analogous to the Kompfner dip in a conventional linear beam traveling wave tube and can, therefore, become a useful diagnostic for determining the experimental parameters of a CARM amplifier.

The gyrotron is a class of powerful radiation source based on the electron cyclotron maser instability [1, 2]. It plays a prominent role in the electromagnetic spectrum by delivering unprecedented power levels in the millimeter and sub-millimeter wavelength regime and hence opening up a wide range of applications. Motivated by still more applications which require moderate to high power terahertz (THz) sources, there have been extensive worldwide research efforts to develop sub-THz and THz gyrotrons, some of which have already broken the THz barrier.

A fluid theory and two self-consistent particle-tracing simulation codes are described for designing low voltage, s/sup th/-harmonic slotted gyroklystron amplifiers, in which axis-encircling electron beams are in resonance with the s/sup th/-order azimuthal modes of a series of magnetron-type cavities, allowing the gyrotron amplifier's required magnetic field to be reduced by a factor of s. The linear fluid theory yields a convenient closed-form expression for gain and the nonlinear simulation code determines the large-signal device performance, while the much faster linear simulation code allows thorough, multi-dimensional parameter searches to be performed quickly. The simulation codes self-consistently account for shifts of the cavity's resonant frequency and quality factor due to beam loading. The three theoretical approaches, which agree in the small-signal regime for weak beam loading, were used to design a 95 GHz, three-cavity, slotted twelve-vane, sixth-harmonic gyroklystron amplifier utilizing a 70 kV, 10 A, v/sub /spl perp///v/sub z/=2, axis-encircling beam and a 6.1 kG magnet. The nonlinear self-consistent simulation code predicts that the sixth-harmonic gyrotron amplifier with an ideal beam will yield an electronic efficiency of 20% and a saturated gain of 37 dB, while the more realistic device with a 10% axial velocity spread will generate a peak output power of 84 kW with 12% efficiency, a saturated gain of 27 dB and a 0.2% constant-drive bandwidth. >

The concept that the relatively weak harmonic gyro-TWT interactions allow high values of electron beam current for stable operation has been extended to design two extremely high power, 140 GHz, third-harmonic TE/sub 31/ gyro-TWT amplifiers. One device is driven by an axis-encircling electron beam from a cusp gun and the other employs a magnetron injection gun (MIG). These devices are predicted by a self-consistent nonlinear numerical simulation code to yield, respectively, output powers of 775 kW and 937 kW with 15.5% and 18.7% efficiency, saturated gains of 27 dB and 30 dB, and saturated bandwidths of % and 6.5%. The stability of the amplifiers is ensured by limiting the length of the interaction section(s) to the shortest starting oscillation length as determined by linear theory. The cylindrical waveguide circuits of both amplifiers have been sliced to suppress modes without a threefold azimuthal symmetry. The amplifier utilizing a MIG yields superior performance because the dominant competing interaction is minimized for the choice of the beam's guiding center radius. The advantages as well as limitations of this approach for high power microwave generation are also addressed. >

Cyclotron harmonic interactions are a key physics issue of critical importance to the generation of terahertz radiation via the electron cyclotron maser instability for practical magnetic field strengths. We present an inherent mechanism, as well as a deciding factor, which governs the competition between low- and high-harmonic interactions. Multimode simulations reveal the physical process in which a significant advantage develops for the lower-harmonic interaction, which eventually dominates in the fully nonlinear stage. The results also suggest a start-up scenario for persistent higher-harmonic operation.

Summary form only given. We present a theoretical study of the competition between fundamental and harmonic interactions in the gyrotoron oscillator. It is shown through the particle-in-cell simulation that, in the nonlinear stage, the fundamental harmonic interaction possesses a significant advantage over the harmonic interaction, which can become a deciding factor in the competition process. The beam-wave coupling coefficient is given by Hsm=J2s-m(kmnrc)J'2S(kmnrL), where s is the cyclotron harmonic number, Jn(x) is a Bessel function of order n, xmn = kmn/rw, and xmn is the nth nonzero root of Js'(kmnrL). Hsm of a higher harmonic mode decreases much faster than a lower harmonic mode as the electron Larmor radius decreases. Hence, in saturated operation, Hsm (averaged over all electrons) of a higher har-monic mode decreases rapidly along the interaction structure as the electron transverse energy is depleted, while Hsm of a fundamental harmonic mode remains near its original value. This suggests an inherent advantage for fundamental harmonic mode.

The solenoidal guide field of Raman free electron lasers can be tuned to resonantly enhance a desired harmonic component of the electron quiver velocity. The feasibility of a harmonic FEL based on this effect is investigated. Computer simulations demonstrate that the gain and efficiency of the second harmonic generation can be comparable to that of the fundamental generation.

We present a theoretical study of the competition between fundamental and harmonic interactions in the gyrotoron oscillator. It is shown through the particle-in-cell simulation that, in the nonlinear stage, the fundamental harmonic interaction possesses a significant advantage over the harmonic interaction, which can become a deciding factor in the competition process. A physical interpretation is given.

Efficiency enhancement in a gyrotron backward wave oscillator (gyro-BWO) with a nonlinearly tapered waveguides is investigated with a self-consistent code. It is shown that the interaction efficiency can be enhanced significantly to a value as high as 61%. Mechanism of the efficiency enhancement is interpreted physically via phase space diagrams.

Performance of a fundamental cyclotron harmonic, TE01-mode gyrotron traveling-wave amplifiers (gyro-TWTs) is studied theoretically under the marginal stability criterion. The interaction circuit is shown in Fig. 1. As is illustrated in Fig. 2 for the parameters in Table I, absolute instabilities on both lower and higher order modes and at the fundamental and second cyclotron harmonic frequency present severe constraints to the stable operating current. General methods for stabilizing these unwanted oscillations are outlined, on the basis of which the parametric dependence of gain, power, bandwidth and efficiency on the beam velocity ratio, circuit lengths, wall resistivity, and velocity spread is obtained under the marginal stability criterion. The results allow a tradeoff among gain, power, bandwidth, and efficiency to reach a desired design.

We present a theoretical study of the competition between fundamental and harmonic interactions in the gyrotoron oscillator. It is shown through the particle-in-cell simulation that, in the nonlinear stage, the fundamental harmonic interaction possesses a significant advantage over the harmonic interaction, which can become a deciding factor in the competition process. A physical interpretation is given.

For pt.I, see ibid., vol.20, no.3, p.155-162 (1992). Based on an analytical study of the stability problems of gyrotron traveling wave amplifiers (gyro-TWTs), an extremely high power second-harmonic gyro-TWT has been designed, evaluated and optimized with a self-consistent nonlinear numerical simulation code. The design, which is based on the magnetron-injection-gun (MIG)-type beam, is presented. Using a 100 kV, 25 A MIG beam with alpha =1 and an axial velocity spread of 5%, nonlinear self-consistent analysis of a three-stage second-harmonic gyro-TWT amplifier predicts a peak output power of 533 kW, peak efficiency of 21.3% and a 7.4% saturated bandwidth, which verifies the theoretical predictions that a stable harmonic gyro-TWT can generate power levels an order of magnitude higher than those possible from a fundamental gyro-TWT. It is shown that the positioning of the electron beam is very important. A multistage structure is used to recover the loss in gain resulting from shortening the interaction sections to ensure stability. >

Stability properties of both the fundamental and second harmonic gyrotron travelling wave amplifier (gyro-TWT) are examined with multi-mode particle simulations. The second harmonic cyclotron interaction with an axis-encircling electron beam is found to be more stable to oscillations and can yield significantly greater power than the fundamental harmonic gyro-TWT. A multiple stage interaction structure based on a marginal stability criterion is proposed and illustrated with examples of a 128kW fundamental gyro-TWT and a 532 kW second harmonic gyro-TWT, Stable amplification at much higher power levels is in principle possible.

We present a theoretical study of the competition between fundamental and harmonic interactions in the gyrotron backward-wave oscillator (gyro-BWO). It is shown through particle-in-cell simulation that, in the nonlinear stage, the fundamental harmonic interaction possesses a significant advantage over the harmonic interaction, which can become a deciding factor in the competition process. A physical interpretation is given.

The virtual cathode oscillator (vircator) is a widely investigated device for the generation of very high-power microwaves (HPM) with pulse lengths (≪200ns) limited by diode closure. Because of its structural simplicity and frequency tuneablity, the vircator has become the most readily available HPM sources. The vircator radiation results from the multi-frequency oscillatory behavior of the virtual cathode plasma and the reflecting electrons trapped in the potential well between the real and virtual cathodes. Thus, the radiation is generally incoherent and broadband in nature in the absence of any control measures such as an external cavity, a feedback reflector, and pre-modulation of electrons. In the experimental investigation of the vircator, the output power is commonly measured with a calibrated crystal detector. There is, however, an inherent difficulty in this method; namely, the output signal has a broad spectrum while each of the detector components has a frequency dependence. Thus, the observed signal is distorted from the actual signal. This leads to erroneous interpretation of the instantaneous power as well as the temporal power profile. Here, we overcome this difficulty through a time frequency analysis (TFA) of the raw signal data. On the basis of the TFA data and the sensitivity of the measuring system, the time-resolved frequency spectrum of the actual signal is reconstructed (Fig. 1), which allows an accurate determination of the temporal power profile. It also reveals a stepwise spectral variation during the pulse and hence suggests an ever changing physical process during the wave generation and a modal structure characterized by a set of discrete oscillation frequencies.

A driven-oscillator model is developed to examine the orbital-velocity harmonics of off-axis electrons in combined helical wiggler and axial guide magnetic fields. The gyroresonance effect is found to occur at harmonics of the wiggler frequency in addition to the well-known fundamental-harmonic resonance. Resonance at the first negative harmonic is potentially important for providing an explanation of the unexpected observations of a recent free-electron-laser experiment. Exploration of the gyroresonance erect as a harmonic selection mechanism for the free-electron laser is discussed

Mode interactions in the gyrotron traveling wave tube amplifier (gyro‐TWT) are examined with particle simulations and compared with experimental observations. Studies focus on the competition between the absolute and convective instabilities. Detailed diagnostics on mode structures, field growth profiles, and evolution of electron distribution in momentum and energy spaces allow an in depth look at the basic physical processes, such as the mechanism of mode competition and suppression. Mutual verifications between the theory and experiment have strengthened understanding of the multimode behavior and added to the predictability of high‐power gyro‐TWT’s.

The cyclotron autoresonance maser (CARM) is shown to be capable of extremely high power efficient operation with relatively weak magnetic field requirements. A general description of the CARM traveling wave amplifier is presented. The linear theory is applied to study the stability of the device and a self-consistent numerical simulation code is used to predict the performance. It is found that grazing intersection often gives superior performance, especially for high current electron beams. The designs of two CARM amplifiers for the applications of a high gradient rf linac and electron cyclotron resonance plasma heating as well as a proof of principle experiment are presented.

By delivering unprecedented power and gain, the gyrotron traveling-wave amplifier (gyro-TWT) offers great promise for advanced millimeter wave radars. However, the underlying physics of this complex nonlinear system is yet to be fully elucidated. Here, we report a new phenomenon in the form of nonlinearly driven oscillations. A zero-drive stable gyro-TWT is shown to be susceptible to a considerably reduced dynamic range at the band edge, followed by a sudden transition into driven oscillations and then a hysteresis effect. An analysis of this unexpected behavior and its physical interpretation are presented.

A high-duty extended-interaction klystron (EIK) is built to power a high-Q cavity for materials processing. The EIK is comprised of a single-gap input cavity, 5 buncher cavities, and a 2-gap output cavity. The cavities can be stagger tuned for broad bandwidth. The gaps are made of CuNi alloy to increase the cavity loading for further bandwidth improvement. A compact neodymium-iron-boron permanent magnet system is employed for electron beam focusing. A depressed collector is employed for overall efficiency enhancement.

Improving the electron beam power and quality is essential to increasing the power and performance of the gyro-TWT. We have designed, and tested, a new magnetron injection gun (MIG) with nominal alpha=1.5 and calculated axial velocity spread (via EGUN) of 3.9% (neglecting cathode surface roughness, emission non-uniformity, and other defects). The gun is designed to nominally operate at 100 kV with ~8 Amp beam current at a cathode current density of 30 A/cm2 and uses Scandate impregnated cathodes. Beam analyzer tests show that the gun performs with excellent beam quality and that the design is flexible with respect to maintaining the beam quality with variable operating parameters such as voltage, current, and compression. This is a low compression (only 12) thin beam design. The surface quality of advanced high current density cathodes can be inferred from the beam quality measurements. Advanced gyro-TWT design using the improved MIG is discussed.

A scheme for harmonic wave generation using a prebunched electron beam has been demonstrated. The prebunched electron beam has been used to further increase the efficiency of the authors' axis-encircling high-harmonic gyrotron. The proof-of-principle experiment was performed at the third harmonic with a TE/sub 312/ mode at 27.7 GHz. The conversion power of 6.7 kW was significantly greater than that used in the nonprebunched experiment. Also, mode competition was effectively suppressed. As expected, the unsaturated output power is proportional to the square of the electron beam current and the start of oscillation current is essentially zero. A linear theory, derived by taking into account the spread of the guiding center and the spread of the axial velocity, gives good agreement with the experimental results. >

A saturated instantaneous bandwidth (FWHM) of at least 10% in a Ka-band gyrotron traveling-wave tube (gyro-TWT) with uniform interaction waveguide and magnetic field was obtained. A relatively low alpha ( identical to nu /sub perpendicular to // nu /sub ///) beam has resulted in stable saturated operation and broad bandwidth. This, together with the overall performance of the tube (18.4-kW maximum power at 18.6% efficiency and 18-dB gain), strongly reinforces the possibility of developing a new generation of millimeter traveling-wave amplifiers based on the gyro-TWT. >

Selective suppression of high order axial modes of the gyrotron backward-wave oscillator (gyro-BWO) is investigated in theory and in experiment. The gyro-BWO interaction is much more efficient in a down-tapered interaction structure, while it is also more susceptible to the problem of axial mode competition in such a structure. Because higher order axial modes (at a higher oscillation frequency) penetrate deeper into the interaction structure, application of distributed wall loss at the downstream end of the interaction structure is shown in theory to be effective for selective suppression of these modes without degrading the efficiency of the desired fundamental axial mode. A stable gyro-BWO operating in a single mode throughout the entire beam pulse is demonstrated on the basis of this principle. Theoretical and experimental results are found to be in good agreement.

Summary form only given. The design of a new magnetron injection gun (MIG) and axial energy beam analyzer for the purpose of testing new advanced cathodes for high emission density capability and emission uniformity has been done. The goal of this thin beam design was to reduce magnetic compression in order to reduce the detrimental effects of manufacturing and assembly errors, and test operation at much higher than normal emission densities, e.g. 20 to 50 A/cm2. It should be possible to see in beam analyzer tests, and gyro-TWT experiments, the effects of cathode surface roughness and emission non-uniformity on the electron beam produced by high emission density cathodes. The final result of the design (by EGUN), assuming a perfectly smooth cathode, was an axial velocity spread of 3.9%, a transverse spread of 1.7%, for transverse to axial velocity ratio (alpha) of 1.5 with a cathode current density of 30 A/cm2 with a total current of 7.7 amp. Other parameters are a cathode magnetic field of 3.0 kG, final field of 35.6 kG, and voltage of 100 kV. A compression of only ~12 is unusually low for a MIG for the final operating magnetic field of ~36 kG, compared to 25 to 35 for other MIG designs in use. Other notable features are a relatively steep cathode angle 50 degrees, desirable to reduce transverse misalignments, and a blunt nose. The peak electric field on the cathode is 147 kV/cm and on the edge of the nose is 205 kV/cm, satisfactory for pulse operation, but apparently necessary as efforts to reduce the fields resulted in poorer performance. Present cathodes generally have surface roughness in the range of 2.5-5 microns (particle radius) where estimates show that it is desirable to have roughness of ~0.5 microns or less to not introduce significant additional velocity spread. The surface roughness will be inferred by the beam analyzer that directly measures the axial energy distribution of the electrons with a voltage depressing electrode. The mechanical design of the MIG utilizes e-beam heated cathode buttons that can be readily changed to allow fast testing of conventional and the new Scandate types. The new MIG and beam analyzer are under construction. First tests will be of the old gyro-TWT MIG, used to produce 140 kW peak at 94 GHz at only 2.2% bandwidth, which is suspect of very high velocity spread. With the new MIG and improved stability measures being taken in a new W-band gyro-TWT, we anticipate output powers in the range of 250-500 kW.