H.S. Uhm

Diocotron instability for relativistic non-neutral electron flow in planar magnetron geometry

Diocotron stability properties of relativistic non‐neutral electron flow in a planar magnetron are investigated within the framework of the cold‐fluid‐Maxwell equations. The eigenvalue equation for the extraordinary‐mode waves in a relativistic velocity‐sheared electron layer is obtained, and is solved in the massless, guiding‐center approximation. Approximating the electromagnetic field in the anode resonator by the lowest‐order mode, the dispersion relation for the diocotron instability is obtained. Although the tenuous beam approximation is assumed, the eigenvalue equation and corresponding dispersion relation are both fully electromagnetic, and valid for relativistic electron flow. The dispersion relation is numerically investigated for a broad range of system parameters. From numerical calculations of the dispersion relation, it is shown that the typical growth rate of the diocotron instability indicates a strong instability. The early evolution of the diocotron instability as an important precursor to the evolution of the full magnetron oscillation is discussed.

Microwave generation from a cusptron oscillator with a six-vane circuit

Microwave radiation at high harmonics of the electron cyclotron frequency is generated from a cusptron device. An axis-rotating beam of 30kV, 3.5 A, 4μs, and 60pps interacts with modes in a six-vane circuit by the negative mass instability. Radiation power is more than l0kW with approximately 10% electronic efficiency at 6·0GHz which corresponds to the sixth harmonic of the electron cyclotron frequency. With the same circuit and a 28xa0kV, 1·5 A beam, we also obtained approximately 40kW radiation with 9·5% efficiency at the fourth harmonic frequency of 3·9xa0GHz

Simulation studies of the relativistic magnetron

Summary form only given. Two-dimensional particle-in-cell (PIC) simulations of the high-power magnetron using the codes MAGIC and MASK have been performed. The simulations have been specialized to the A6 magnetron geometry, or slight variations thereof. Several scenarios with both MAGIC and MASK have been run to saturation. With MAGIC the following was observed: (1) mode competition between the π and 2π modes, and, alone, the l=5 mode (5.0 GHz), all in agreement with Buneman-Hartree theory; (2) the density profiles, which depart markedly from those of Brillouin flow, are rounded with a negative density gradient with radius; (3) the RF fields at saturation are comparable in magnitude to the applied fields; and (4) saturation of the RF is accompanied by energetic bombardment of the anode. In simulations using MASK, the π mode and the 2π mode have been observed separately. Also either the π mode or the 2π mode is seen to dominate at saturation, depending on the cathode radius

Dispersion characteristics of cusptron vacuum waveguide modes

With successful demonstration of the ‘proof-of-principle’ experiment of the cusptron, the configuration of the cusptron waveguide gains importance. Although the dispersion relation for the cusptron rf circuit has been derived and experimentally verified before, the detailed properties of its dispersion characteristics are not well understood. In this paper, we will examine various aspects of the dispersion properties in conjunction with the design process of the cusptron tubes. In particular, two unique properties of the cusptron rf circuit, the mixture and the specific grouping of the azimuthal mode numbers are closely examined and demonstrated with simple physics. Special emphases are on the identification and designation of the modes, the profiles of various field components, the spectral distribution with respect to azimuthal mode numbers, and their relation to the cusptron microwave tube design

Phase-locking simulation of dual magnetrons

Summary form only given. The analysis of phase-locking of dual magnetrons is underway by means of direct particle simulation of two coupled magnetrons. The computer code allows two magnetrons to run side by side without any crosstalk other than being coupled through a transmission line. The phase evolution can be simulated with two magnetrons connected by a transmission line with various lengths and impedances. The Physics International magnetron geometry configuration was used. and the preliminary results demonstrate that a half-integral wavelength connector produced 180° out-of-phase operation. The phase-locking time can be observed easily from the phase evolution of two magnetrons

Preliminary study of cusptron amplifier

The cusptron microwave tube concept is under development at NSWC as a compact high power device using the harmonic frequency generating scheme by an axis-rotating beam and a multivane circuit. Sixth and fourth harmonic frequencies have been independently generated with electronic efficiencies of approximately 10%. With this successful oscillator mode of operation, the cusptron is now being converted to an amplifier. The input and output couplers are one-turn loops located in different vanes. A TWT operating 4.0-8.0 GHz will be used as an RF driver. In this paper, we will present theoretical and experimental studies on the cusptron amplifier with a 30-50 keV, axis-rotating electron beam and a six-vane circuit.

A theory of the plasma torch for waste-treatment

Summary form only given. Arc-plasma technology has broad applications to waste treatment processing including the safe disposal of hazardous and low-level radioactive wastes. The plasma torch could be useful to the development of an efficient, compact, lightweight, clean burning incinerator for industrial and municipal waste disposal in an environmentally beneficial way. We therefore develop a simple theoretical model describing physics of the plasma torch plume in connection with its applications to the arc-plasma waste-treatment system. The theoretical analysis is carried out by making use of Bernoullis pressure-balance equation, which provides a stable equilibrium solution of the gas density in the plume ejected from the torch into a high-pressure reactor chamber with 4/spl epsi/<1. The pressure depression parameter E is proportional to the gas temperature and inversely proportional to the square of the chamber pressure.

A nonlinear theory of premodulated electron beam propagating through a helix-loaded waveguide

Summary form only given, as follows. One of the most important issues in the charged particle beam physics is evolution of prebunched beam during propagation. The prebunched charged particle beams have various applications, including accelerator physics, relativistic klystron amplifiers, and high-power traveling wave tubes. Particularly, the prebunched beam train may provide a compact devices for these applications. It is therefore important to investigate evolution of beam profile during propagation in downstream. In this presentation, we develop a self-consistent nonlinear theory of prebunched electron beam propagating through a helix-loaded waveguide. The prebunched beam particles move downstream with traveling waves of electric potential caused by non-uniform charge distribution along the propagation direction. A closed integrodifferential equation for the beam current is obtained in terms of time and propagation distance. The equation consists of the space-charge force caused by the self-electric field and of the helix effects. Properties of the current and energy modulations are investigated from this equation for a broad range of system parameters. Linearization of the integrodifferential equation leads to the mode evolution of individual mode during propagation. It is observed from the linear theory that the mode grows exponentially when the helix effects dominate. On the other hand, for an intense beam, the space charge oscillation occurs at the beginning of the propagation. However, all the modes grow exponentially in a long range propagation regardless of the beam intensity.

An analytical theory of electron-cyclotron-resonance plasmas

Summary form only given, as follows. An analytical theory of the electron-cyclotron-resonance (ECR) plasmas is developed. In the steady-state, plasma generation is in balance with plasma loss due to diffusion. Based on the ambipolar diffusion and Poisson equation caused by charge separation, coupled differential equations are obtained for electron density and electric field, which are described interns of the position x, the normalized ionization rate /spl alpha/, the current density /spl zeta/ and the maximum plasma density n/sub 0/. Making use of these coupled differential equations, we obtain simple analytical expressions for electron and ion densities, and therefore the electric field and potential. Particularly, the conditions for ion sheath development, and a simple scaling law of the ion sheath strength and associated electric field are found. Several points are noteworthy from the analytical study. First, normalized strength of the ion sheath at the beginning is inversely proportional to the one-third power of the plasma density n/sub 0/. Electric field at the beginning of the sheath is proportional to the one-third power of the plasma density, thereby showing that the field strength increases as the plasma density increases. Second, the cathode ion sheath is stronger than the anode ion sheath. The anode ion sheath disappears when the current density /spl zeta/ is larger than the square root of the ionization rate /spl alpha/. Third, the electric potential for the current density satisfying 0</spl zeta/</spl alpha//sup 1/2/ increases, reaches its peak and decreases, as the position moves from the anode to cathode. Otherwise, the potential decreases monotonically.

Energy distribution properties of focused ion beams from liquid metal ion sources

Summary form only given, as follows. The field-emitted electron and ion beams in a high-voltage diode are found to be sources of very high brightness. For example, liquid metal ion sources (LMIS) produce ion beams of 5 /spl mu/A current from a source size estimated to 10/sup -7/ cm. This is why the focused ion beams are often used in the direct doping of semiconductors, in lithography, in semiconductor mask repair, and in micromachining. The width of energy distribution in high-brightness beams is often several tens of electron volts, even if thermal agitation at the emitter is a fraction of an electron volt. Typical diode voltage of the field emission source is 10 kV, which is much larger than several tens of electron volts. The energy distribution may not look important. However, a 10-eV energy spread causes a tremendous problem if the ion beams mentioned above must be focused to a spot the size of a fraction of a micron (/spl mu/m) to be useful in semiconductor mask repair. A theoretical model for energy spread and shift of peak energy distribution in a charged particle beam is developed. The beam may expand radially and axially as it propagates downstream. Charged particles placed randomly in the beam possess different potentials, which are converted to different kinetic energies as the beam propagates and expands. This randomness introduces energy spread in the beam. It is also shown that the shift of peak energy distribution in beam particles is proportional to the one-third power of the beam current, which is the manifestation of random particle locations.