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Considered is superradiant undulator radiation emitted at grazing resonance by intense electron bunches periodically injected into a resonating cavity. Compared to a single bunch in a long waveguide, cumulative interaction of periodic bunches in a cavity allows much higher saturated efficiency induced by superradiant stimulated emission of radiation. Simulations in the saturation regime show that the self-field of intense bunches tends to reduce the efficiency by obstructing nonlinear bunching. When energy gain is balanced by cavity loss, the emitted radiation finds a steady state in which the radiation pulse persists with no dispersion. At that state, the cavity works as a stable oscillator, supplying periodic bursts of radiation via output coupling.

Having the merits of conceptual simplicity, high-output-power capacity, and wide tuning characteristics, the vircator (virtual cathode oscillator) has been investigated as a high-power microwave (HPM) generator. In axial vircator, the power, frequency, and pulsewidth are all sensitive to the cathode shape, and the frequency depends strongly on the anode-cathode gap distance. The efficiency of the vircator is reported to only a few percent. Therefore, we devised the new type cathode to enhance the output power. The new type cathode is adding the ring to the cathode with velvet, so-called cathode-wing. The electron beam is only emitted from cathode with velvet, while a cathode-wing forms the electric fields to prevent the backstream electrons due to virtual cathode from leaking to the diode chamber wall. We investigated the axial vircator with the cathode-wing by the HPM generator ldquoChundoongrdquo (max 600 kV, 88 kA, 60 ns). It is found that the cathode-wing contributes to enhancement of the output microwave power efficiency to 7% in comparison with 3% for a conventional axial vircator without cathode-wing.

We have generated the Ar plasma in the diode chamber based on the established coaxial electrode type and investigated the emitted visible light for emission spectroscopy. The optical emission spectrum data have been obtained for the focused plasma in the cylindrical diode chamber under the input voltage of 4.5 kV and pressure of 40 mtorr. We observed the nine emission lines of Ar II: 407.3, 410.6, 422.4, 426.8, 427.9, 435.0, 438.1, 444.5, and 488.0 nm. The downstream electron temperature and density have been measured by the Boltzmann plot and Stark broadening, respectively, from the assumption of local thermodynamic equilibrium. The length of the acrylic insulator has been changed to 30, 40, and 45 mm, respectively, which is covered on the inner cathode surface, to investigate the influence of insulator length on the downstream electron temperature and density in this experiment. Our results show that the downstream electron temperature and density of focused plasma are 2.5 eV and 1.6times1016 cm-3, respectively, for the coaxial plasma focus with cylindrical electrodes at the insulator length of 40 mm.

Numerical simulation studies of a coaxial virtual cathode oscillator with and without a bar reflector have been performed by the three-dimensional particle-in-cell (PIC) code, MAGIC. Also, the experimental studies are performed in our high-power microwave pulser, "Chundoong" to compare with numerical results. It consists of a cylindrical anode-mesh, a coaxial type cathode, and a bar reflector. Results show that the output microwave mode without a reflector is shown to be a mixture of TE and TM modes and the output microwave frequency has a narrow bandwidth and a main frequency is about 3.38 GHz from a coaxial vircator without a bar reflector. But the TM01 mode with its frequency of 3.34 GHz is shown to be dominant for the coaxial vircator with a bar reflector. It is found in this experiment that the output microwave power sensitively depends upon the position of a bar reflector

A numerical simulation study of the high-power microwave generation from the vircator (virtual cathode oscillator) is carried out for coaxial diode structure by using a three-dimensional particle-in-cell (PIC) code called MAGIC. The coaxial vircator has a centered annular cathode body, cathode ring and cylindrical meshed-anode in order to enlarge the active interaction region in the virtual cathode space. In order to enhance a conversion efficiency of the output microwave power, ring-type reflector was installed to coaxial-type diode structure. The simulation results show that the output microwave frequency has a narrow bandwidth and its output microwave power sensitively depends upon the position and width of ring-type reflector. The maximum output microwave power is obtained by adopting a ring-type reflector 10 mm in width and 40 mm in the distance from the intense relativistic electron beam to the ring-type reflector. The output microwave mode without a ring-type reflector is a mixture of TM and TE mode. However, the TM/sub 01/ mode with its resonance frequency of 2.2 GHz is dominant for the ring-type reflector.

When an electron beam is generated from a cathode, an electrode sheath plasma is also generated. Hence, the perveance of the diode is varied and changes the diode output characteristics. In our pulsed-power system "Chundoong," the perveance characteristics have been experimentally investigated, from which the sheath-plasma expansion speed has been measured to be about 3 cm//spl mu/s inside the diode. It is shown that the sheath-plasma expansion inside the diode gives the influence on the electron-beam current density. The vircator microwave output has been shown to be sensitively dependent on the anode-cathode (A-K) gap distance. The highest microwave power occurs at the initial A-K gap distance d=4 mm with peak power of 200 MW and a frequency of about 5.5 GHz in this experiment.

An axially extracted virtual cathode oscillator has been fabricated. This ultra-high-power microwave generator is powered by the "Chundoong" intense relativistic electron beam pulser (Max: 600 kV, 88 kA, and 60 ns duration). In this experiment, a solid electron beam with a peak diode current of 21 kA is obtained from the pulser operated at a peak diode voltage of 300 kV. It is found in this experiment that the peak of microwave emission occurred at the moment of the second peak in electron beam current, where the diode voltage V/sub d/ and diode current I/sub d/ are V/sub d/=290 kV and I/sub d/=21 kA, respectively. This observation might be strongly correlated to the inherent oscillations of the electron cloud in time and space originated from both the dynamics of the reflected electron beam due to virtual cathode and oscillations of the virtual cathode itself, from which the peak of high-power microwave can be generated in this experiment. The major operating output frequencies have been measured to be 6.68 GHz/spl les/f/spl les/7.19 GHz based on the waveguide dispersive delayed line method, which is in good agreement with that from "MAGIC" computational results for an electron beam voltage of 290 kV and a diode current of 21 kA. The maximum microwave output power from the virtual cathode oscillator has been measured to be 200 MW in this experiment. while its power conversion efficiency is approximately 3.3%. The dominant emission mode from the virtual cathode oscillator is shown to be the TM/sub 01/ mode based on the emission pattern of microwave induced air breakdown in this experiment.

Growth characteristics of a Raman free-electron laser in a dense plasma background are investigated. The background plasma is beneficial in the sense that it allows a beam current over the vacuum space-charge limit. It is found, however, that an excessive plasma density reduces the linear growth rate significantly if the radiation mode is not confined well inside the electron beam. The growth rate reduction accompanies a large positive frequency shift of the radiation field. This indicates that the background plasma causes the optical beam to expand outward farther into the plasma.

The pulsewidth and rising time of a mildly relativistic electron beam (300 kV, 1-3 kA) passing through a gas-filled diode region are investigated experimentally under various gas pressures P. The pulsewidth and rising time of a relativistic electron beam (REB) were controlled by adjusting the gas pressures P of the diode region. The pulsewidth and rising time of the relativistic electron beam are experimentally found to scale as P/sup -0.807/spl plusmn/0.054/ and P/sup -0.770/spl plusmn/0.058/, respectively. The REB pulsewidth and rising time are shown to have the same scaling law, within the experimental error range as a function of pressure. In particular, the empirical scaling law of the REB pulsewidth is in remarkably good agreement with the numerical scaling law P/sup -0.809/spl plusmn/0.059/ of the full-space charge neutralization time t/sub n/ at which the ion density n/sub i/ is just equal to the electron beam density n/sub b/ at the diode region under a given gas pressure P. It also is found that ion density n/sub i/ at the full space-charge neutralization time t/sub n/ has quite a similar profile in terms of pressure P to that of the REB peak current detected by a Faraday cup.

The interaction dynamics for a cyclotron autoresonance maser oscillator is studied in its nonlinear saturation regime. In a highly relativistic limit of electron energy, such that the longitudinal beam velocity approaches to the speed of light, the dynamics has a sensitive dependence on the longitudinal phase velocity of the radiation field. We investigate the dynamic variations and their consequences on the saturation efficiency, in a range that the phase velocity is slightly greater than the free space value. It is observed that, for an appropriate value of the phase velocity, the potential bucket in the phase space expands to a maximal area. Consequential enhancement of the nonlinear saturation efficiency is estimated. >

In the above titled paper (ibid., vol. 37, no. 1, pp. 184-189, Jan 09), the superscript 6 in (3) is incorrect. The correct equation is presented here.