Han S. Uhm

Theory of free electron laser instability in a relativistic annular electron beam

A self-consistent theory of the free electron laser instability is developed for a hollow electron beam propagating through an undulator (multiple mirror) magnetic field. The stability analysis is carried out within the framework of the linearized Vlasov-Maxwell equations. The dispersion relation describing the free electron laser instability in a hollow relativistic electron beam is obtained for an equilibrium distribution function in which all electrons have same value of transverse energy and the same value of canonical angular momentum, and a Lorentian distribution in axial momentum. It is shown that the influence of finite radial geometry plays a critical role in determining detailed stability behavior. Moreover, the growth rate and bandwidth of the instability can be expressed in terms of Budkers parameter upsilon, instead of the plasma frequency as in the case of a uniform density beam. Furthermore, it is found that free electron laser stability properties exhibit a sensitive dependence on axial momentum spread.

Theory of Gyrotron Amplifiers in a Disk or a Helix Loaded Waveguide

The gyrotron amplifiers in two slow wave structures, disc and helix-loaded waveguides, are investigated for wideband applications, For each structure, properties of the vacuum waveguide mode are examined in connection with the gyrotron application, and the gyrotron dispersion relation is obtained. For the disc-loaded waveguide, it is found that the group velocity of the waveguide mode can be easily varied by the disc parameters, and the gain and the bandwidth are at least comparable to those of the ordinary gyrotron. In the helix-loaded waveguide configuration, the gyrotron can be very broad in its bandwidth. Moreover, the hybrid mode operation, using the helix mode as a carrier, shows possibility of fast wave wideband amplification by contouring the waveguide.

Linear theory of cusptron microwave tubes

Stability properties of the negative‐mass instability in a rotating annular electron beam (E layer) are investigated, in connection with applications on the cusptron microwave tubes. The analysis is carried out for an infinitely long E layer propagating through a magnetron‐type conducting wall and propagating parallel to an applied axial magnetic field. It is assumed that the E layer is thin and very tenuous. A closed algebraic dispersion relation of the negative‐mass instability is obtained, including the important influence of conducting boundaries on the mode control in microwave generation and amplification. It is shown that for typical present experimental beam parameters, the gain of the cusptron normalized to the excitation frequency can be comparable to that of the gyrotron. Moreover, under the appropriate geometric configuration, perturbations with azimuthal mode number N are dominantly unstable. This optimizes the microwave power output of the radiation with frequency ω≂Nωc, where ωc is the elec...

Properties of the electromagnetic wave propagation in a helix‐loaded waveguide

Properties of the electromagnetic waves propagating through a helix‐loaded waveguide are investigated, including the important influence of the outer conducting wall on the dispersion properties. A closed algebraic dispersion relation for the eigenfrequency ω and the axial wave number k is obtained for arbitrary azimuthal harmonic number. It is shown that in the limiting case, where the outer conducting wall approaches close to the helix, this dispersion relation is reduced to three distinctive modes. These are the transverse electric mode, the transverse magnetic mode, and the helix mode, which can be further simplified to straight lines in the (ω, k) parameter space. Numerical investigation of the dispersion relation is also presented.

Theory of the resistive hose instability in relativistic electron beams

A Vlasov-Maxwell theory of the resistive hose instability is developed, for an infinitely long relativistic electron beam propagating parallel to an applied axial magnetic field. Complete space charge neutralization by the ambient plasma and paraxial flow (p(z)/sup 2/ >> p(r) /sup 2/ + p(theta)/sup 2/) are assumed. The analysis is performed for rigid-rotor and cold laminar flow equilibria. An integro-differential eigenvalue equation is obtained for the general case, and is reduced to an ordinary differential equation in either the cold laminar flow limit or the case of a square beam density profile. Using a variational technique, an approximate dispersion relation is found for arbitrary density profile, and evaluated in closed form for either the Bennett or square profile. Stability properties are illustrated and discussed in detail for a square profile, including the influence of the applied magnetic field (stabilizing), proximity to a conducting guide (stabilizing), and partial current neutralization (destabilizing).

Analysis of the wide band gyrotron amplifier in a dielectric loaded waveguide

The effect of broadening the bandwidth with a dielectric load in a cylindrical gyrotron is investigated for a hollow electron beam. The linear dispersion relation for the azimuthally symmetric, transverse electric (TE) modes is obtained by the method of the wave impedance matching. It is found that the TE perturbations exhibit three unstable modes characterized by their phase velocities vph : one fast wave, the long wavelength mode (LWM, vph ≳c), and two slow waves, the intermediate (IWM, c≳vph ≳ce−1/2) and the short (SWM, vph <ce−1/2) wavelength modes. The optimum conditions for the wide band operation are obtained individually for each mode. Although the bandwidth in excess of 40% is possible for the slow waves (IWM, SWM) at a small axial momentum spread, it decreases rapidly as the spread increases. On the other hand, the LWM yields approximately 10% of the bandwidth insensitive to the spread. It is also shown that for a small spread (<5%), the slow wave (IWM) is preferable for the wide band operation,...

Cellular and molecular responses of Neurospora crassa to non-thermal plasma at atmospheric pressure

Filamentous fungi have been rarely explored in terms of plasma treatments. This letter presents the cellular and molecular responses of the filamentous fungus Neurospora crassa to an argon plasma jet at atmospheric pressure. The viability and cell morphology of N. crassa spores exposed to plasma were both significantly reduced depending on the exposure time when treated in water. The intracellular genomic DNA content was dramatically reduced in fungal tissues after a plasma treatment and the transcription factor tah-3 was found to be required for fungal tolerance to a harsh plasma environment.

Influence of the gas-flow Reynolds number on a plasma column in a glass tube

Atmospheric-plasma generation inside a glass tube is influenced by gas stream behavior as described by the Reynolds number (Rn). In experiments with He, Ne, and Ar, the plasma column length increases with an increase in the gas flow rate under laminar flow characterized by Rn   4000, the length of the plasma column is short in front of the electrode, eventually leading to a shutdown.

Free electron laser instability for a relativistic solid electron beam in a helical wiggler field

A free electron laser instability is investigated for a relativistic annular electron beam propagating through a helical wiggler magnetic field. It is assumed that the beam is thin, with radial thickness (2a) much smaller than the beam radius (R0), and that ν/γb≪1, where ν is Budker’s parameter. The stability analysis is carried out within the framework of the linearized Vlasov–Maxwell equations for perturbations with general azimuthal harmonic number l and radial mode number s, including the important influence of (a) finite beam geometry in the radial direction, (b) positioning of the beam radius relative to the outer conducting wall (R0/Rc), and (c) finite wiggler amplitude (δB). All of these effects are shown to have an important influence on stability behavior. Moreover, the maximum coupling between the transverse and longitudinal modes increases substantially with increasing radial mode number s. It is also found that the transverse magnetic (TM) mode has slightly larger growth rate than the transve...

Influence of Wall Impedance on the Electron Cyclotron Maser Instability.

The influence of finite‐wall impedance on the cyclotron maser instability is investigated for a hollow electron beam. The stability analysis is carried out within the framework of the linearized Vlasov–Maxwell equations, assuming that the beam thickness is much less than the mean radius of the beam. The formal dispersion relation for azimuthally symmetric electromagnetic perturbations including the important influence of arbitrary wall impedance is obtained. One of the most important features of the analysis is that, for a purely resistive wall, the instability growth rate is substantially reduced by a very small wall resistivity. Moreover, the range of axial wavenumbers corresponding to instability increases rapidly as the wall resistivity is increased. Cyclotron maser stability properties for a dielectric loaded waveguide are also investigated. For an appropriate choice of the dielectric constant e and the thickness of the dielectric material, it is shown that the instability bandwidth can be more than ...