Axial Field Profile Research Articles

Effect of cavity geometry on the performance of a gyrotron

The effect of interaction cavity geometry on the gyrotron performance in terms of cold cavity parameters (Q value and axial electric field profile), Ohmic wall loss, start oscillation current and interaction efficiency are analyzed in detail in this article. The measurement of Q value and axial electric field profile is also performed for the cavities with different geometries by using non-destructive and perturbation techniques, respectively. Scattering matrix code is used for the computation of Q value and axial electric field profile and results are compared with the experimental data. A Particle-in-Cell code and a specific beam–wave interaction computation code based on generalized non-linear theory are used in the efficiency calculations. For all numerical and experimental analyses, the case of 42GHz, 200kW gyrotron is considered here.

Cold characterization of cylindrical open resonator for gyrotron

This paper presents experimental results for cold testing a gyrotron open resonator. Experiments were accomplished to measure resonant frequency and their particular quality factor for TE mode at the operating frequency. The perturbation technique was used to determine the axial, radial and azimuthal electric field profile for identification of TE 031 mode at operating frequency 42 GHz. The good agreement between experimental and theoretical was found. The results were quite good and suitable for suppression the undesired modes in the specific gyrotron cavity.

High resolution simulation of beam dynamics in electron linacs for x-ray free electron lasers

In this paper we report on large scale multi-physics simulation of beam dynamics in electron linacs for next generation free electron lasers (FELs). We describe key features of a parallel macroparticle simulation code including three-dimensional (3D) space-charge effects, short-range structure wake fields, longitudinal coherent synchrotron radiation (CSR) wake fields, and treatment of radiofrequency (RF) accelerating cavities using maps obtained from axial field profiles. A macroparticle up-sampling scheme is described that reduces the shot noise from an initial distribution with a smaller number of macroparticles while maintaining the global properties of the original distribution. We present a study of the microbunching instability which is a critical issue for future FELs due to its impact on beam quality at the end of the linac. Using parameters of a planned FEL linac at Lawrence Berkeley National Laboratory (LBNL), we show that a large number of macroparticles (beyond 100 million) is needed to control numerical shot noise that drives the microbunching instability. We also explore the effect of the longitudinal grid on simulation results. We show that acceptable results are obtained with around 2048 longitudinal grid points, and we discuss this in view of the spectral growth rate predicted from linear theory. As anmore » application, we present results from simulations using one billion macroparticles of the FEL linac under design at LBNL. We show that the final uncorrelated energy spread of the beam depends not only on the initial uncorrelated energy spread but also depends strongly on the shape of the initial current profile. By using a parabolic initial current profile, 5 keV initial uncorrelated energy spread at 40 MeV injection energy, and improved linac design, those simulations demonstrate that a reasonable beam quality can be achieved at the end of the linac, with the final distribution having about 100 keV energy spread, 2.4 GeV energy, and 1.2 kA peak current.« less

Dynamics of Mode Competition in the Gyrotron Backward-Wave Oscillator

The axial modes of the gyrotron backward-wave oscillator (gyro-BWO) each exhibit a distinctive asymmetry in axial field profile. As a result, and in sharp contrast to the behavior of the familiar resonator-based gyrotron oscillator, particle simulations of the gyro-BWO reveal a radically different pattern of mode competition in which a fast-growing and well-established mode is subsequently suppressed by a later-starting mode with a more favorable field profile. This is verified in a Ka-band experiment and the interaction dynamics are elucidated with a time-frequency analysis.

Spatial growth rate and field profiles of symmetric mode in a rod dielectric Čerenkov maser with a magnetized plasma column

By numerically solving the exact dispersion equation, the dispersion relation of symmetrical TM waves propagating in a Čerenkov maser including a thin annular relativistic electron beam (TAREB), a strongly magnetized plasma column, and a dielectric rod is investigated. The effects of accelerating voltage, radii of TAREB and plasma column as well, on the frequency spectra and spatial growth-rate coefficient are presented. The axial electric field profiles during the wave amplification and the conditions under which the spatial growth rates are maximum are presented.

Internal plasma potential profiles in a laboratory-model Hall thruster

The Plasmadynamics and Electric Propulsion Laboratory High-speed Axial Reciprocating Probe system is used in conjunction with a floating emissive probe to measure plasma potential in the discharge chamber of the P5 Hall thruster. Plasma potential measurements are made at a constant voltage, 300 V, at two different discharge current conditions: 5.4 and 10 A. The plasma potential contours for the 5.4 A case indicate that the acceleration region begins several millimeters upstream of the exit plane, extends several centimeters downstream, and is uniform across the width of the discharge chamber. The 10 A case is similar to the 5.4 A case with the exception that the acceleration region is shifted downstream on centerline. Axial electric field profiles, computed from the measured potential, show a double peak structure in the 5.4 A case, indicating a zone of ion deceleration. Perturbations to the discharge current are shown to correspond spatially with the location of the peak electric field indicating that thruster perturbations may result from a disturbance to the Hall current, as opposed to ablation of probe material. This conclusion is supported by the lack of any observable material ablation.

Experimental Study of a Ka-Band Cylindrical Open Resonator

A study of the electrodynamical properties of a Ka-band gyrotron open resonator was experimentally conducted. Experiments were accomplished to measure resonant frequencies and their respective loaded quality factors for TE modes in the frequency range from 26 to 40 GHz. In particular, a perturbation technique was used to determine the axial, radial and azimuthal electric field profiles, as an identification method of the TE021 mode operating around 35 GHz. In any experimental event, good agreement with the values predicted by theory was found.

Beam-cavity interaction circuit at W-band

In this paper, we describe the design, fabrication, and bench study of a millimeter-wave cavity employed as a relativistic klystron output structure. The oxygen-free electronic-grade copper cavity was prepared by electro-discharge machining and diffusion bonding, and cleaned and tuned to 91.4 GHz. Measured cavity characteristics are presented and compared with theory, including quality factor Q, coupling parameter /spl beta/, scattering matrix S/sub 11/, and axial electric field profile E/sub z/. This paper provides the basis for an understanding of the cavity as a transfer structure.

5646531 High-frequency antenna system of a device for nuclear magnetic resonance

The HF antenna system of a nuclear resonance device includes a plurality of antenna elements arranged one behind another in the direction of a longitudinal axis of the system, that are mutually at least largely magnetically decoupled and each connected to a transmitting and/or receiving device. To improve the axial field profile, the antenna elements are each cage-shaped with two end-located conductor rings, between which a plurality of conductor pieces extend at least approximately parallel to the longitudinal axis. Preferably the antenna system is composed of three or five antenna elements.

Nonlinear theory of the long-wavelength external m=1 mode in screw pinches and tokamaks

The nonlinear evolution of the long-wavelength external m=1 (kink) mode of a general, straight screw pinch is investigated analytically by means of bifurcation theory. The investigation covers both the ordinary screw pinch ordering of parameters, Bz approximately Bphi , as well as the tokamak limit, Bz>>Bphi . Conducting wall effects are included in both cases. In the case of the ordinary screw pinch, general expressions, valid up to second order in the axial wavenumber, are derived for the coefficients D1 and D3 in the mode evolution equation eta n+D1 eta +D3 eta 3=0. Generally, the nonlinear stability near a linear stability boundary is found to depend on the current profile, but not on the axial field profile. Furthermore, peaked current profiles are found to be more stable than broad profiles, but for complete nonlinear stability a conducting wall is in general required. In the tokamak case, a general expression for D3, covering all previously calculated values of this coefficient (for particular profiles), is derived. Near the first marginal point qa=1/b2 (b is the wall radius normalized to the plasma radius), the shape of the current profile is found to be of decisive importance for the nonlinear evolution of the m=1 mode. In general, sufficiently peaked profiles lead to nonlinear stability, whereas broad profiles lead to explosive growth of the mode. Near the second marginal point qa=1, D3 is shown to have a singularity either of order (1-qa)-1, or of order (1-qa)-2, depending on whether Ja not=0 or Ja=0, respectively, where Ja is the current density at the plasma edge.

Synthesis of an arbitrary axial field profile by computer-generated holograms

Computer-generated holograms are employed to design any desired intensity distribution along the propagation axis for a finite specified distance.

Exact solutions of the dissipative screw-pinch problem

The authors study the full system of Braginskii one-fluid transport equations with a self-consistent account of transport and dissipative phenomena in the screw-pinch geometry and derive exact self-similar solutions. The local and global plasma parameters-magnetic field, temperature and density profiles, magnetic Reynolds number, beta and pinch radius-are determined by a minimal set of external parameters, namely properly scaled magnitude of the applied axial field and an index related to the current time-evolution, and are computed as eigenfunctions and eigenvalues of the corresponding boundary value problem. The paramagnetic and diamagnetic properties of the axial field profiles are discussed in detail. Calculation of the m=1 growth rates demonstrates a relatively weak decrease of the maximum growth rate as the axial field amplitude increases although the range of unstable wavenumbers becomes substantially narrower. The solutions obtained describe plasma dynamics and profile structure in 'stabilized' pinch systems: Z-pinches with externally applied axial field and in ULQ pinches provided self-relaxation processes are suppressed.

Indirect measurement of nbti wire temperature field response to local heat input

Indirect measurement of temperature field profile on the NbTi multifilament wire locally heated by steady or transient thermal disturbance is presented. Contrary to point like measurements by thermometers glued upon the wire surface it can be applied for continual mapping of the T(x) profile. Especially useful may be the method in the case of tiny conductors where influence of gauge thermal conductivity and capacity may result in distortion of axial temperature field profile as well as its dynamics. Based upon experimental T c(B) data it can be applied both for steady local heating in I c(B,T) measurements or in monitoring of T(x,t) dynamics during superconductor recovery after transient heat input.

A simple microwave plasma source for diamond deposition

We describe a simple microwave-produced plasma source that has been constructed for plasma-assisted chemical vapor deposition of diamond thin films. Microwave power from a 700 W domestic microwave oven magnetron is fed into a water-cooled cylindrical stainless steel vacuum vessel. A methane/hydrogen gas mixture introduced into the vessel is excited by the microwaves to produce a well-defined plasma ball which does not interact with the walls of the vessel. The position of the plasma ball in the vessel can be predicted by cold cavity calculations of the axial electric field profile. The vessel has several diagnostic ports which do not alter its resonant condition. Diamond has been deposited on various substrates, placed beneath the plasma ball on a graphite-capped quartz pedestal. Some of these results are presented and discussed.

Superconducting magnets for 60 to 140 GHz gyrotrons

Reliable superconducting magnets and cryostats have been designed and built for use with several different types of high-power gyrotron tube operating in the frequency range from 60 to 140 GHz. Methods for controlling and monitoring the axial field profiles are described. Ways of minimising the radial field variation and achieving geometrical alignment of the magnetic field and cryostat are discussed. Transverse ‘beam steering’ coils can be fitted. A full set of magnet power supplies and monitoring electronics has been developed. The system may be operated and monitored by computer. Efficient liquid nitrogen shielded, liquid helium cooled cryostats with long hold times have been developed. For installations involving a small number of gyrotrons, simple enthalpy cooled cryostats are often an economic solution. For multiple gyrotron installations, there is a requirement for reliable and economic long-hold cryostats which require little attention during use. To address this problem, the incorporation of a two-stage closed-cycle cooler is described. The closed-cycle cooler may be removed for routing maintenance without the need to warm the cryostat.

Cold tests of A 10-GHz gyrotron cavity

Experiments have been conducted to characterize a gyrotron cavity designed to operate in theTE021 mode at 10 GHz. Cavity excitation was accomplished via a coupling hole introduced into the cavity wall and mode detection was carried out by means of two experimental arrangements. In the first, electromagnetic energy is coupled into a receiving waveguide through a small second hole drilled in the opposite side of the cavity. The other scheme uses a horn antenna to receive the power reradiated by the open resonator. Both schemes are discussed regarding mode detection, and measured data includes resonant frequency, loadedQ factor, axial electric field profile and farfield radiation pattern. Evaluation of the loadedQ factor is based on bandwidth measurements whereas standing-wave electric field profile is determined by using perturbation techniques. For severalTE modes, close agreeent between theory and experiment is found.

Performance of 1.5 mu m DFB lasers with a narrow stripe region

The performance of distributed feedback (DFB) lasers with a narrow region in the middle of the waveguiding strip and antireflective(AR)-coated facet ends is presented. The narrow region is designed to act as a three-quarter wavelength phase shift. Single longitudinal mode (SLM) capability is discussed both theoretically and experimentally. Calculated results predict the shift region is effective in relaxing axial hole burning because of a dip at the center of the axial field profile and a smaller optical confinement factor in the region. Experimentally, stable SLM operation up to more than 50 mW, a sidemode suppression ratio of more than 40 dB, and 3 dB modulation bandwidth over 10 GHz are achieved. >

The circular orbit linear theory of the axial injection orbitron maser (AXIOM) oscillator

The linear theory of axial injection orbitron maser (AXIOM) oscillators is formulated analytically for circular electron orbits using the relativistic Vlasov equation and Maxwell’s equations. The theory treats transverse electric (TE) or transverse magnetic (TM) modes with sinusoidal axial field profiles. Owing to the simplicity of modeling circular electron orbits the theory is fully relativistic. The power transferred from the electron beam to the cavity fields is calculated in terms of a general distribution function and the cavity parameters. A cold beam distribution function is used to evaluate the theory. The results of the theory are presented in terms of the threshold value of the cavity Q times the electron beam current required for self-oscillation. The results show that TE modes with small azimuthal mode numbers have the strongest interaction and that modes with large azimuthal mode numbers resonate at lower electrostatic fields but require larger electron beam currents for self-oscillation.

Optimization of RF field profiles for enhanced gyrotron efficiency

Physically realizable RF field profiles are investigated with the objective of maximizing gyrotron efficiency. For this purpose, a study is made of the resonant properties of continuous profile open resonators and truncated cones cavities, the former allowing greater flexibility in the shaping of asymmetric axial field profiles. High perpendicular efficiencies (η ⊥ ⋍ 0.86) have been calculated for both type of cavity, for interaction at the fundamental cyclotron harmonic and for uniform external magnetic fields. A maximum-efficiency scaling parameter has been introduced that is applicable to a variety of field profiles.

Analytical treatment of linearized self-consistent theory of a gyromonotron with a non-fixed structure

The linearized self-consistent theory of gyrotron is analytically treated for a self-excited oscillator with non-fixed structure. The self-consistent RF axial field profile consists of four waves, namely the forward and backward travelling waves of the waveguide, and two electron cyclotron waves. According to the magnetic field value (or the detuning parameter), the electron cyclotron wave couples strongly with the forward wave, or weakly with backward wave of the waveguide mode. Also, the starting current and the axial field profile are significantly different for the forward versus the backward interaction.