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Plasma Science, IEEE Transactions on

Issue 5 • Date Oct 1994

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Displaying Results 1 - 25 of 60
  • Three-dimensional simulation analysis of a 3 cm wavelength free-electron laser afterburner

    Publication Year: 1994 , Page(s): 649 - 653
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    We have simulated a 3 cm wavelength free-electron laser afterburner (FEL Afterburner) using two sets of parameters: one is for a 3-cm period wiggler and the other is for a 5.4 cm period wiggler. For the 3 cm period wiggler, the input beam energy is 112.5 keV, and for the 5.3 cm period wiggler the beam energy is increased to 290 keV to make the FEL Afterburner operate at the same frequency. It is found, from the simulations, that the FEL Afterburner with a longer period wiggler has a higher power conversion efficiency: larger than 16% $ for the 5.4 cm wiggler while only about 9% for the 3 cm wiggler. It is also shown that to enhance the interaction efficiency in the slow wave cavity, the slow wave number should be a little larger than the sum of the fast wave number and the wiggler wave number View full abstract»

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  • High-harmonic slotted gyroklystron amplifier: linear theory and nonlinear simulation

    Publication Year: 1994 , Page(s): 920 - 931
    Cited by:  Papers (2)
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    A fluid theory and two self-consistent particle-tracing simulation codes are described for designing low voltage, sth-harmonic slotted gyroklystron amplifiers, in which axis-encircling electron beams are in resonance with the sth-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/vz=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 View full abstract»

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  • High performance, 70 kV third-harmonic smooth-bore gyroklystron amplifier

    Publication Year: 1994 , Page(s): 932 - 938
    Cited by:  Papers (1)
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    The design of a 95 GHz third-harmonic smooth-bore gyroklystron amplifier utilizing an axis-encircling electron beam is presented. Using a self-consistent simulation code, a parameter search was performed to determine the optimum device characteristics for a gyroklystron operating in the circular TE31 mode. For a 70 kV, 5 A electron beam with v/v||=2 and Δv z/vz=10%, the three-cavity gyroklystron is predicted to yield 70 kW with 20% efficiency, 0.2% bandwidth, and a saturated gain of 32 dB. CW operation would yield a tolerable wall loading of 2 kW/cm2 View full abstract»

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  • Chaos and gyrotron-type of interaction

    Publication Year: 1994 , Page(s): 834 - 849
    Cited by:  Papers (1)
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    The theory of chaotic behaviour of dynamical systems has been used to investigate the basic features of interaction in fast-wave devices such as gyrotrons. A simple model of the interaction process seems to indicate that gyrotrons readily resist the onset of chaotic behaviour even at large amplitudes of oscillations View full abstract»

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  • A free electron laser experiment on "angular steering"

    Publication Year: 1994 , Page(s): 654 - 658
    Cited by:  Papers (1)
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    A paper by Jerby (1990) has discussed a number of mechanisms whereby FEL radiation may be directed electronically into different radiation patterns. We have devised an experiment (1992) to test this concept using the Columbia microwave FEL, which amplifies radiation at 24 GHz to a level <1 MW. A 4 mm dia, electron beam (580 kV) is propagated in a guiding field of 0.8 T inside an overmoded 24 mm dia. cylindrical waveguide. A TE11 mode is grown in a 33 cm long first undulator section (period 3.36 cm), and upon entering the following undulator section (period 2.26 cm, length 40 cm), the electron bunches convert to TM11 radiation which is further amplified. The far-field pattern of the TM11 emitted power is distinct from the TE11 pattern. Numerical and experimental studies are described in this paper showing the resulting radiation pattern. View full abstract»

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  • Stable high-power TE01 gyro-TWT amplifiers

    Publication Year: 1994 , Page(s): 585 - 592
    Cited by:  Papers (8)
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    The stability of high power gyro-TWT amplifiers operating in the low-loss TE01 mode of cylindrical waveguide has been studied, Linear theory has been used to determine the threshold start-oscillation beam current for absolute instability in the operating mode and the critical section lengths for the dominant gyro-BWO interactions occurring at various cyclotron harmonics in other waveguide modes. The performance of the amplifier was evaluated with a nonlinear, self-consistent slow-timescale simulation code. Utilizing interaction sections whose lengths are less than the threshold start-oscillation length and are separated by attenuating severs for isolation, two stable three-section devices have been designed which are predicted to yield: (1) a peak output power of 230 kW at 35 GHz with an efficiency of 23%, a saturated gain of 46 dB and a constant-drive bandwidth of 6% for a 100 kV, 10 A electron beam with an α=νz =1.0 and an axial velocity spread Δνzz=5% and (2) 105 kW at 94 GHz with 21% efficiency, 45 dB saturated gain and 5% constant-drive bandwidth for a similar 5 A electron beam. In addition, the design of the 0 dB input/output couplers and the MIG electron gun are given. Due to the low loss of the TE01 mode, both of these amplifiers can be operated continuously View full abstract»

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  • Measurement of the temporal and spatial phase variations of a pulsed free electron laser amplifier

    Publication Year: 1994 , Page(s): 659 - 665
    Cited by:  Papers (4)  |  Patents (1)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (464 KB)  

    We report temporal and spatial phase measurements of a pulsed free electron laser amplifier (FEL) operating in combined wiggler and axial guide magnetic fields. The system uses a mildly relativistic electron beam (750 kV, 90-300 A, 30 ns), and is excited by a high power 33.39 GHz magnetron source. The phase during the voltage pulse is measured by an interferometric technique from which frequency chirping is determined View full abstract»

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  • Theory of phase-locked gyrotrons operating at cyclotron harmonics

    Publication Year: 1994 , Page(s): 818 - 824
    Cited by:  Papers (5)
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    The theory of phase-locked gyrotrons operating at cyclotron harmonics is developed, with particular attention given to the effects of electron velocity spread. Extensive studies are made for operation at the second harmonic in which phase locking is achieved by subharmonic prebunching. One result of these studies is that we are able to predict, for a range of velocity spreads, the parameters that yield the highest efficiency. More important, we find that for electron beams with velocity spread and a finite transit angle in the drift region, the bandwidth of stable operation is drastically reduced. This effect can be mitigated by properly tapering the external magnetic field there. Such tapering is necessary in devices where the bandwidth is required to be a reasonable fraction of 1/Q View full abstract»

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  • Electron beam behavior in an axially-extracted virtual cathode oscillator

    Publication Year: 1994 , Page(s): 939 - 944
    Cited by:  Papers (10)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (536 KB)  

    High-power microwave generation in an axially-extracted virtual cathode oscillator has been studied experimentally. The microwave emission of 120 MW at frequencies in the range of 10-12 GHz is observed in the presence of annular electron beam operated at a 300 kV and 20 kA. The electron beam current in the diode region is well characterized by the electron space-charge-limited current in bipolar flow, which is well below the critical current defined for diode pinching. The observed microwave emission is accompanied by the strongly pinched electron beam. Essential feature of such a pinching effect for the microwave generation is confirmed by the absence of emission when the pinch of electron beam is suppressed by the application of the axial magnetic field View full abstract»

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  • Experimental results and numerical simulations of a high power 140 GHz gyrotron

    Publication Year: 1994 , Page(s): 861 - 870
    Cited by:  Papers (7)
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    The design of a 0.5 MW 140 GHz gyrotron with axial RF output operating in the TE10,4 mode is presented. Experimental results and numerical simulations are compared. In particular the effects of RF reflections at the output window, velocity spread of the helical electron beam and startup of oscillations by simultaneous increase of pitch factor α and relativistic factor γ are discussed. In short pulse operation (⩽5 ms) an output power of 690 kW has been obtained with an electronic efficiency of 31% and a mode purity of about 99%. The experiments have shown that it is possible to use for step frequency tuning the azimuthal neighbours of the TE10,4 mode, TE9.4 at 132.6 GHz (420 kW) and TE 11,4 at 147.3 GHz (300 kW) as working modes by decreasing and increasing the magnetic field, respectively View full abstract»

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  • High-power, single-beam plasma wave tube

    Publication Year: 1994 , Page(s): 593 - 599
    Cited by:  Papers (3)
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    Short-pulse, ultra-broadband sources of RF radiation are needed for a variety of new applications. To meet this demand, we have developed and optimized a single-beam Plasma Wave Tube (PWT), The PWT is a unique microwave/millimeter-wave source which utilizes the interaction between beamexcited electron plasma waves to generate kilowatt-power (~10 kW) radiation at microwave to millimeter-wave frequencies with a beam-to-radiation conversion efficiency of ⩾0.4%. In a single-beam PWT, an electron beam (⩽40 kV, ⩾200 A, 5-to-20-μs pulse width) is injected into a gas-filled (e,g., hydrogen) cylindrical waveguide. The beam first ionizes the gas to generate a plasma, and then nonlinearly interacts with the plasma to generate radiation from 6-to-60 GHz. Slew rates of up to 7 GHz/μs have been measured during a single beam pulse. The radiation has a wide instantaneous bandwidth, typically 10 GHz or wider. Electron-beam transport through the waveguide is accomplished with no externally applied magnetic fields because the beam space charge is cancelled by the background plasma View full abstract»

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  • Tunable compact high power far-infrared grating free-electron laser

    Publication Year: 1994 , Page(s): 666 - 673
    Cited by:  Papers (1)
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    The linear and nonlinear operation of a multiwave grating free-electron laser is described. The radiation is generated by the passage of an annular electron beam through a coaxial waveguide, the central conductor being in the form of a corrugated cylinder. The model includes the effects of self-field forces, beam emittance and gyromotion of electrons in a guide magnetic field. The efficiency is determined by numerical simulation and compared with analytical estimates. A compact source utilizing, for example, a high brightness beam from a carbon fibre electron gun, that is tunable in the wavelength band 35-55 μm with output in the kilowatt range, is shown to be feasible View full abstract»

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  • Phase locking of two long-pulse, high-power magnetrons

    Publication Year: 1994 , Page(s): 616 - 625
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    We have phase locked a pair of long-pulse, high-power, secondary-emission magnetrons to an RF-isolated low-power driver via a waveguide directional coupler. With the 3 dB hybrid coupler, the peak and average power sent back towards the driver were lower than the total power by factors of 20 and 100, respectively. A dual-channel, thyratron-switched line-type modulator was used to power the magnetrons in parallel. In this experiment the total microwave power from the pair of phase-locked high-power magnetrons was limited to 48 MW for 700 ns pulses and 46 MW for 900 ns pulses because one of the high-power magnetrons would only operate at or below half power. Experimental results from this repetitively-pulsed (10 Hz) phase-locked magnetron pair are presented which show excellent phase stability of several degrees both during the pulse and from pulse-to-pulse View full abstract»

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  • Experimental studies of stability and amplification in a two-cavity second harmonic gyroklystron

    Publication Year: 1994 , Page(s): 825 - 833
    Cited by:  Papers (20)
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    We report the operating characteristics of a sequence of two-cavity second harmonic gyroklystrons which are derived in part from a previous fundamental tube and utilize output cavities which resonate at twice the drive frequency. We present results from the design simulations as well as details of the stable range of operating parameters. While the harmonic tube is somewhat more susceptible to spurious oscillations and more sensitive to parameter variations than the fundamental device, there is still considerable parameter space available for amplifier operation. Peak powers above 30 MW are obtained with efficiencies greater than 28% and large signal gains of 27 dB. These results depend critically on the magnetic field profile which has a slight up-taper at the optimum operating point. The nominal beam parameters include a pulse length of 1 μs, a voltage near 450 kV, a current in the range 235-245 A, and a perpendicular to parallel velocity ratio (α=v/vz) near one View full abstract»

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  • Influence of sideband oscillations on gyrotron efficiency

    Publication Year: 1994 , Page(s): 871 - 877
    Cited by:  Papers (5)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (476 KB)  

    We report the observation of sideband mode effects on the efficiency of overmoded gyrotron operation. Two cavities were designed and operated which differed in the presence of sideband modes. In one version of the cavity, parasitic backward waves modes were observed and efficiencies were approximately 22% at 40 A beam current. With the use of a multi-mode self-consistent nonlinear code, a modified design was generated which eliminated the sideband modes. Experiments were conducted with this new cavity which produced efficiencies of approximately 33% at 30 A and 27% at 40 A beam current, but with a slightly higher velocity ratio than seen with the earlier cavity. An additional cavity, also with no sideband modes but with a longer cavity length and therefore higher Q obtained powers up to 1.3 MW with an efficiency of 39% at a 40 A beam current View full abstract»

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  • Beam-cavity interaction physics for mildly relativistic, intense-beam klystron amplifiers

    Publication Year: 1994 , Page(s): 730 - 739
    Cited by:  Papers (10)
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    We examine beam-cavity interaction physics relevant to mildly relativistic, intense-beam klystron amplifiers. This is an interesting but difficult regime of operation, because of the combination of high beam current and low voltage. The advantage of this regime is that it is relatively easy to access high beam powers (and potentially high microwave output powers) at relatively low beam energy. We calculate the effect of the extremely high beam loading in the input and idler cavities. The output cavity's shunt impedance must match the low beam impedance in order to prevent high output gap voltages that will reflect electrons back upstream. This leads to very low cavity Q factors (<10). We derive expressions for mode purity for such low-Q cavities and show that acceptable cavities can be designed View full abstract»

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  • Performance prediction of proposed Indian MHD retrofit channel

    Publication Year: 1994 , Page(s): 973 - 978
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    A mathematical model for the channel analysis of MHD generators based on nonlinear fluid dynamics and Maxwell's equations has been presented. The fluid properties of the seeded combustion products of coal burnt with oxygen enriched/preheated air, needed for the present analysis, have been obtained by using a computer program based on the principle of minimization of Gibb's free energy (NASA-SP-273). This has been used to study the variation of different flow parameters inside the MHD channel, Thus, the power generated by the Indian MHD retrofit channel (having segmented geometry) has been estimated under different operating conditions. The limitation imposed due to electrical stress on the insulating walls and due to slagging have also been considered for limitation of power output from the channel View full abstract»

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  • Nonlinear gain computation for the sheet beam crossed-field amplifier

    Publication Year: 1994 , Page(s): 626 - 637
    Cited by:  Papers (5)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1004 KB)  

    When the RF amplitude in a crossed field device is much smaller than the external DC voltage, the energy exchange between an electron and the wave is given by the change in the potential energy of the electron guiding center. The shift of the beam center of charge follows the space bunching into “spokes” caused by the RF-induced drift. A nonlinear estimate for the gain is derived and applied to the linear format crossed-field amplifier fed by a continuous sheet beam. The adiabatic approximation for the guiding center trajectories in the low gain regime determines the fraction of trapped/streaming particles and the energy exchange for each group. The radiation gain equals the change in the electron potential energy resulting from the shift in the beam center of charge across the anode-cathode voltage. The drift kinetic energy is approximately conserved, opposed to other microwave devices converting kinetic energy into radiation. The theory accounts for the symmetry of the response curve relative to the frequency detuning ω-ω0, and the flat top near resonance. The analytic predictions agree with the experimental measurements of the gain versus frequency response View full abstract»

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  • Effects of tapered tubes on long-pulse microwave emission from intense e-beam gyrotron-backward-wave-oscillators

    Publication Year: 1994 , Page(s): 578 - 584
    Cited by:  Papers (5)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (536 KB)  

    Experiments are reported on tapered-tube versus uniform-tube, gyrotron-backward-wave-oscillators (4.5-6 GHz) driven by an intense electron beam with parameters: 0.8 MV, 1-4 kA, and pulselength (0.5-1 μs). Results show that, compared to a uniform interaction tube, a gyro-BWO with a 10% downtapered tube produces the following effects: 1) highest microwave peak-power (up to about 100 MW in internal tube), a factor of 2 higher than the uniform tube, 2) more reproducible long-pulse (400-500 ns) emission, and 3) the largest inferred-integrated energy (factor of 2.5-3 increase). Experiments show high power microwave spikes with lower power plateaus. Experimental observations are in qualitative agreement with MAGIC code simulations of uniform and tapered-tube gyro-BWO's View full abstract»

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  • A re-examination of the traveling wave interaction

    Publication Year: 1994 , Page(s): 499 - 503
    Cited by:  Papers (5)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (496 KB)  

    A re-examination of the traveling wave interaction (in the helix traveling wave tube and the free-electron laser) is performed using three-dimensional linear stability analyses for an idealized annular electron beam model in order to investigate the validity of the well-known scaling laws in which the gain varies as the cube (fourth) root of the current when space-charge effects are negligible (dominant). The results indicate that these scaling laws are simplistic generalizations which break down for broad bandwidth interactions, and that the actual variation of the pain, with the current can be more complex. A three-dimensional nonlinear analysis of a free-electron laser using a more realistic electron beam model is also discussed in which these scaling laws are also shown to break down, View full abstract»

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  • Results from an X-band coaxial extended length cavity

    Publication Year: 1994 , Page(s): 504 - 510
    Cited by:  Papers (4)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (628 KB)  

    Experiments and simulations demonstrate high-power microwave generation at 9 GHz in a coaxial geometry. The 9 cm diameter annular electron beam is propagated between inner and outer drift tube conductors, a configuration which increases the beam current and reduces the structure fields from existing high-power sources, Since the TEM mode of the coaxial guide reduces the quality factor of small-gap cavities, especially at high frequency, the interaction is provided by an extended length cavity loaded with dielectric. A single 16 cm cavity generates 200 MW of power from the 400 keV, 7 kA electron beam. Although the cavity can oscillate at a number of resonances, a single mode is selected with 10-30 kW of input power from a magnetron. A coupler samples 25 MW of the power from the interaction region, precisely measured using a single-shot calorimeter. Simulations indicate that the efficiency of the device is limited to 7% by saturation effects, and can be improved by reducing the length of the cavity View full abstract»

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  • Broadband operation of a Ka-band tapered gyro-traveling wave amplifier

    Publication Year: 1994 , Page(s): 536 - 543
    Cited by:  Papers (23)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (660 KB)  

    A wideband low-voltage millimeter-wave gyro-traveling wave tube (gyro-TWT) amplifier operating in the TE10 rectangular waveguide mode at the fundamental cyclotron frequency is under investigation, The device incorporates precise axial tapering of both the magnetic field and the interaction circuit for broadband operation. Experimental results of a wide (33%) instantaneous bandwidth with a small signal gain in excess of 20 dB and saturated efficiency of ~10% were achieved and shown to be in good agreement with the theory. Reflective instability due to multi-pass effects by mismatches was observed and characterized. Gain and efficiency have been limited by this reflective instability rather than by absolute instabilities which limit the performance of gyro-TWT's with uniform cross-section. The start-oscillation current in terms of the relevant experimental parameters such as the beam velocity ratio (α), magnetic field detuning and reflection coefficient has been measured and compared with theory. Measurements of the phase variation in terms of the RF frequency have shown that the phase varies ±30° from fitted linear phase line View full abstract»

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  • A theoretical analysis of relativistic klystron oscillators

    Publication Year: 1994 , Page(s): 706 - 712
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    An electron beam pre-modulated at the first cavity in a klystron enters the second cavity opening, exciting it. Induced voltage at the second cavity in a high-power klystron forms a virtual cathode momentarily, sending back a part of the beam toward the first cavity. The relationship between the induced voltage and the return current at the first cavity is investigated. The boundary between the amplifier and oscillator operation regions is described in the parameter space defined by the return current strength and inter-cavity distance. Nonlinear saturation mechanism of the first cavity excitation is investigated for the oscillator operation region. One of the key saturation mechanisms of the cavity excitation is that the location at which the maximum current modulation occurs starts to shift toward the first cavity, if the induced voltage at the first cavity increases significantly. The saturation amplitude of the induced voltage is an increasing function of the beam intensity and energy, but it is independent of the return current View full abstract»

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  • Helical Cerenkov effect, a novel radiation source

    Publication Year: 1994 , Page(s): 526 - 529
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    The observability of the helical Cerenkov effect as a novel radiation source is discussed. Depending on the value of the index of refraction of the medium, the strength of the uniform magnetic field, and the electron beam energy, helical Cerenkov radiation can occur in the same spectral regions as the ordinary Cerenkov effect, that is, from microwave to visible wavelengths. From the kinematics point of view, I argue that for a microwave wavelength of 10-1cm this effect should be observable in a medium with an index of refraction of 1.4, with a beam energy of 3 MeV, and a uniform magnetic field of 4 T. On the specific level, however, for the sake of simplicity, I discuss the observability of this effect for visible light with the central wavelength of 5×10-5 cm which can be achieved with 2 MeV in beam energy, silica aerogel as a medium (with an index of refraction of 1.075), and uniform magnetic fields from 5 to 10 T. For a 10-T magnetic field, I calculate that in the visible region of 250 to 750 nm an electron will produce a photon per 10 cm of traveled length. As to the stimulated helical Cerenkov emission, I estimate that respectable gains are possible even if the beam passes close to the dielectric rather than through it. In addition to being potentially a new radiation source, the helical Cerenkov effect could possibly be used as a detector of radiation by energetic electrons that are trapped in a medium by strong magnetic fields View full abstract»

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  • Design of a high power X-band magnicon amplifier

    Publication Year: 1994 , Page(s): 785 - 795
    Cited by:  Papers (11)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1000 KB)  

    We present a design study for an X-band frequency-doubling magnicon amplifier driven by a 500 keV, 172 A beam from a field-emission diode. This study makes use of steady-state particle simulations employing the realistic fields of magnicon cavities connected by beam tunnels, and includes the effects of finite electron beam diameter. The simulations propagate an electron beam through a sequence of deflection cavities at 5.7 GHz, followed by an output cavity that operates at 11.4 GHz. The deflection cavities and the output cavity contain synchronously rotating TM modes. The deflection cavities progressively spin up the beam transverse momentum, until α≡v/vz >1, where v and vz are the velocity components perpendicular and parallel to the axial magnetic field. The output cavity uses this synchronously gyrating beam to generate microwave radiation at twice the drive frequency. Self-consistency of the simulation is achieved by iteration until power balance exists in each cavity, and until the optimum RF phase in each cavity is determined. The final magnicon circuit should produce 20 to 50 MW at 11.4 GHz, depending on initial beam diameter, with a drive power of 1 kW at 5.7 GHz View full abstract»

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