Space Charge Mode Research Articles

Photoionization cross-section measurements from the 6s5d 1D2 excited state of atomic ytterbium

We present new experimental measurements on the photoionization cross-sections from the 6s5d1D2 excited state of ytterbium using a heat pipe-cum-linear thermionic diode ion detector, working in the space-charge limited mode to generate ytterbium vapor. A Nd:YAG laser pumped narrow bandwidth (~ 0.2 cm-1) Hanna-type dye laser, charged with LDS-698 dye and tuned at 722.6 nm, was used to excite atoms via the 6s2 1S0 → 6s5d1D2 two-photon transition from the ground state. The excited state population was then promoted to the ionization threshold at 439.2 nm ionizing laser wavelength and above the ionization threshold at 435 nm, 355 nm, and 266 nm laser wavelengths. The photoionization cross-sections at each of these wavelengths have been determined using the saturation technique. At the first ionization threshold, the photoionization cross-section from the 6s5d1D2 excited state is extracted as 40.5 ± 6.2 Mb, whereas at higher energies in the continuum, the cross-section decreases up to 6.4 Mb at 266 nm, which corresponds to 1.84 eV excess energy.

Experimental Study on Microwave Generation due to Merged Instability in F-Band Surface Wave Oscillator

A surface wave oscillator (SWO) is driven by an electron beam to generate intense microwaves. The electron beam possesses slow space-charge (SSC) and slow cyclotron (SC) modes that interact with the surface wave leading to microwave generation. The beam current and external magnetic field affect the relationship between SSC and SC modes. The SSC mode gradually approaches the SC mode when the beam current increases. Meanwhile the SC mode gradually approaches the SSC mode when the magnetic field decreases. The two modes merge in a low magnetic field and high beam current. In this work, we experimentally examine the operation of an F-band SWO in the low magnetic field region. The output power decreases with low beam current when magnetic field decreases. Meanwhile, the SWO maintains its power level with high beam current even though the magnetic field decreases to around 0.4 T. The merged instability enables a sustained power level in the low magnetic field region.

Full-wave Analysis of Plasma-Loaded Coaxial Cavity with Wedge-Shaped Corrugations on the Insert

High voltages and the existence of scant amount of gas inside gyrotron overmoded cavities leads to the creation of plasma. The presence of magnetic field leads to this plasma being magnetized. In plasma waveguides, space charge modes, cyclotron modes, and EH and HE modes can exist. Inside the gyrotron cavity, magnetized plasma affects the propagation of desired and competing modes. Therefore, for accurate prediction of gyrotron properties, plasma analysis is necessary. For high frequency megawatt class gyrotron, coaxial cavities provide advantages such as reduction in mode competition and reduction of voltage depression. In this paper, full-wave analysis is applied to plasma-filled waveguide with wedge-shaped corrugations on the insert.

Excitation of terahertz radiation by parametric mixing of four waves in magnetized plasma

THz radiation may be excited by nonlinear mixing of two counter-propagating laser beams, having frequency difference ω1∼ω2≥2ωp, in under dense plasma. Beating laser beams exert nonlinear ponderomotive force on plasma electrons and excite a nonlinear density perturbation. The nonlinear density perturbation drives a non-resonant space charge mode which may excite a pair of plasma wave and back scattered electromagnetic terahertz wave at resonance. The presence of transverse magnetic field further enhances the growth of THz wave. The transverse magnetic field also tunes the amplitude and frequency of THz wave. The maximum growth rate of excitation process Γ∼6.22×10−6 s−1 is achieved for applied magnetic field ∼175 kG.

Dispersion characteristics and excitation of space-charge and cyclotron modes in inhomogeneous warm plasma waveguide

In this paper, we are going to investigate the effects of plasma parameters on dispersion characteristics of electrostatic modes in a cylindrical inhomogeneous warm magnetized plasma waveguide. To this aim, the radial inhomogeneity for different characteristic length with strongly spatial dispersion are taken into account. Due to radial inhomogeneity of plasma, all essential equations for studying the properties of dispersion are calculated in Bessel–Fourier and differential Bessel–Fourier expansion. Moreover, two cases of strong and weak magnetic fields are considered for the analyses. We show that there was just an interval in which the space-charge and cyclotron modes are excited. We called it the Longitudinal Wavenumber Excitation Interval that is due to the thermal effect consideration. The effect of different radial inhomogeneity characteristic length on the dispersion properties is also studied in this work.

Modified Space Charge Waves in Magnetized Semiconductor Quantum Plasmas

A dispersion relation of space charge wave is derived using one dimensional quantum hydrodynamic model in semiconductor plasma subjected to d.c. electric and magneto-static fields. It is studied analytically for both classical and quantum plasma systems. Here the carrier drift due to d.c. electric field acts as source of free energy in the system and may be attributed as cause for unstable space charge mode. It is found that the quantum parameter-H and the orientation of applied magnetic field together not only play a key role in the dynamics of space charge wave but also induce four new channels of propagation. The phase speed and growth rate of all modes are found to be very sensitive to the orientation of the magnetic field.

Numerical Modeling of Plasma Devices by the Particle-In-Cell Method on Unstructured Grids

The paper considers methods and algorithms providing the basis for a computer program implementing an axial-symmetric electrostatic version of the particle-in-cell method on unstructured triangular grids. In the presented implementation, the Poisson equation is approximated using the finite volume method. A discrete analog of the Poisson equation is solved by the multigrid method. Charged particle trajectories are calculated using the Boris method. Methods for interpolating electrostatic fields on unstructured grids and obtaining the charge density in the computational domain are considered. Special attention is paid to the specifics of implementing these methods in axisymmetric geometry. The developed computer code is tested on the problem of a flat diode operating in the space charge mode.

Full Wave Analysis of Plasma Loaded Coaxial Gyrotron Cavity With Triangular Corrugations on the Insert

The presence of plasma inside the gyrotron interaction structure alters the mode field. The presence of a magnetic field changes the plasma to an anisotropic media. Field analysis of a plasma loaded coaxial gyrotron cavity with triangular corrugations on the insert is undertaken using full wave approach. Modes inside a plasma loaded interaction structure have all six nonzero field components and hence are hybrid modes. Plasma modes (space charge modes), cyclotron modes, and waveguide EH and HE modes are the three families of modes that can exist in plasma loaded waveguide. Inside the gyrotron interaction structure, the cyclotron mode and the desired mode (HE) couple and this coupling leads to change in the eigenvalue of the modes. In this paper, a full wave approach has been used to analyze the dispersion relation and calculate eigenvalue of the desired HE mode.

Импульсный высоковольтный модулятор с частичным разрядом емкостного накопителя

For stationary high-voltage pulsed devices, which do not necessarily have to comply with stringent requirements for their weight and dimensions, a modulator with partial discharge of the capacitive storage can be competitive to a modulator with complete discharge of the capacitor. The switch shall be rigidly controlled and have a low forward voltage drop on the open device; the thermionic cathode operating in the space charge mode shall ensure the specified pulse duration and repetition rate. With the new high-voltage modulator tubes, electron-beam valves featuring small power losses at the anode, and high-perveance electro-optical systems, for example, an EBV 50/100 system, used as a “rigid” switch, it becomes possible to design series or parallel modulator circuits with a capacity of more than 1 MW, which are quite competitive to the schemes synthesized based on a large number of series-connected relatively low-voltage semiconductor switches. If properly calculated using well-known techniques, such a modulator can generate pulses the parameters of which are almost independent on the load resistance varying in a wide range, and the power circuit themselves are insensitive to the effect of any external electromagnetic or X-ray radiation. Two basic modulator circuit arrangements are considered: a parallel one, which generates a negative polarity pulse, and a series one, generating pulses of both positive and negative polarity. The modulator’s load resistance shall be no less than 10 times the anode-cathode resistance of the open switch. For the possibility of determining, in a prompt and vivid manner, the limit values of the switch operating mode and the load resistance, and for optimizing the modulator’s nominal electrical parameters, the electric circuit is simulated on a computer using the EWB program.

Electromagnetic Waves Dispersion and Interaction of an Annular Beam-Ion Channel System in Plasma Waveguide

A linear theory for the electromagnetic properties and interactions of an annular beam-ion channel system in plasma waveguide is presented. The dispersion relations for two families of propagating modes, including the electrostatic and transverse magnetic modes, are derived. The dependencies of the dispersion behavior and interaction for different wave modes on the thickness of the annular beam and betatron oscillation frequency are studied in detail by numerical calculations. The results show that the inner and outer radii of the beam have different influences on propagation properties of the electrostatic and electromagnetic modes with different betatron oscillation parameters. In the weak ion channel situation, the two types of electrostatic waves, that is, space charge and betatron modes, have no interaction with the transverse magnetic modes. However, in the strong ion channel situation, the transverse magnetic modes will have two branches and a low frequency mode emerged as the new branch. In this case, compared with the solid beam case, the betatron modes not only can interact with the high frequency branch at small wavenumber but also can interact with the low frequency branch at large wavenumber.

Study on Operation of a Surface-Wave Oscillator Around the $\pi$ -Point Region

We numerically and experimentally studied the operation of a W-band (75–110 GHz) surface-wave oscillator driven by the electron beam energy of less than 100 keV. Two numerical methods of the real wavenumber analysis and the saddle point analysis are studied to examine the operation of the oscillator. We analyzed the Cherenkov instability for the interaction of the space charge modes and the structure modes EH01. We showed that the instabilities are of absolute instabilities when the oscillator operates around the $\pi $ -point region. In the experiment, radiations due to the absolute instability are detected around the $\pi $ -point region when the length of the slow-wave structure (SWS) is short enough. On the other hand, the absolute instability generates no meaningful radiation in the $\pi $ -point region for the long length of SWS.

Analysis of Plasma-Loaded Noncorrugated and Triangular Corrugated Coaxial Cavity

A field analysis of plasma-loaded noncorrugated coaxial cavity and plasma-loaded triangular corrugated cavities for megawatt-class gyrotrons has been undertaken. The presence of magnetic field in plasma changes its characteristics to an anisotropic media. In plasma waveguide, three families of modes can exist: plasma modes (space charge modes), cyclotron modes, and waveguide EH and HE modes. The modes present in the plasma are hybrid modes with all six nonzero field components. The presence of the low-density plasma in a gyrotron cavity leads to the coupling of these families of modes and causes a shift in the eigenvalue of the operating (desired) mode. In this paper, the dispersion relation for a plasma-loaded noncorrugated and triangular corrugated coaxial cavity has been derived. For the gyrotron case, where the cyclotron frequency of the plasma is approximately equal to the operating frequency and the operating frequency is near mode cutoff frequency in the cavity, the desired HE mode couples with the cyclotron mode. This coupling leads to shift in the eigenvalue of the HE mode.

A self-consistent approach to the analysis of thermionic devices

Research in thermionics has been reinvigorated recently by the advent of nanotechnology and nanomaterials. Thermionic energy convertors are commonly modelled using the Poisson-Vlasov system of equations under various limitations and approximations. With the ever-growing demands of emergent thermionic devices, more comprehensive approaches are needed in order to be able to treat a broader range of device configurations and operational parameters. Here, we propose a self-consistent approach that, by iterating between the Poisson and Vlasov equations, does not rely on the existence of an analytical solution to the latter. Specifically, we present a particle-tracing implementation of this method for solving the system numerically in an efficient manner. In the case where an analytical solution does exist, we present an asymptotic expansion of the ill-behaving functions that arise; this approach improves the effectiveness of the method in the deep space-charge mode. We also demonstrate the applicability of this approach in the presence of back-emission.

Simulation of transverse modes with their intrinsic Landau damping for bunched beams in the presence of space charge

Transverse dipole modes in bunches with space charge are simulated using the synergia accelerator modeling package and analyzed with dynamic mode decomposition. The properties of the first three space charge modes, including their shape, damping rates, and tune shifts are described over the entire range of space charge strength. The intrinsic Landau damping predicted and estimated in 2009 by one of the authors is confirmed with a reasonable scaling factor of $\ensuremath{\simeq}2.4$. For the KV distribution, very good agreement with PATRIC simulations performed by Kornilov and Boine-Frankenheim is obtained.

Multidimensional aspects of nonlinear electromagnetic solitary pulses

This work is devoted to the study of the interaction between pulses of electromagnetic radiation and space-charge waves in laser-plasmas settings. We analyse modes depending only on the co-moving coordinate of the beam frame, but make no a priori assumption on the longitudinal-to-transverse aspect ratio of the pulses. The model thus constructed allows us to investigate regimes where transverse and longitudinal length scales of the pulses are comparable. Resonant and nonresonant excitations of space-charge modes are analyzed and the endurance of trains of solitons against transverse effects is clarified.

Cylindrical Surface Waves and Slow-Wave Instabilities of Periodically Corrugated Cylinder

Cylindrical surface waves (CSWs) and slow-wave instabilities of a rectangularly corrugated cylinder are numerically examined. CSWs are slow waves with upper cutoffs at the π point. The upper cutoff frequency increases with increasing cylindrical radius R0. There are two types of higher-order CSWs: one is due to azimuthal standing waves and the other is due to radial standing waves in the corrugation. Both higher-order types of SWSs have lower cutoffs as well as upper cutoffs leading to pass and stop bands. Slow space charge and slow cyclotron modes of an annular beam exist, which excite the Cherenkov and slow cyclotron instabilities of CSWs, respectively. The growth rates of the higher-order CSWs are comparable to those of the fundamental SWSs.

Compton Mode Conversion of Laser Beat Wave-Driven Plasma Wave by an Electron Beam in a Plasma Channel

A large amplitude plasma wave, driven by beating two lasers in a depressed density plasma channel, is shown to effectively couple to a mildly relativistic electron beam to produce terahertz (THz) radiation. The plasma wave parametrically couples the THz electromagnetic mode to a negative energy beam space charge mode, and both decay waves grow at the expense of the beam and the plasma wave. In the low beam current Compton regime, the growth rate scales to one-third power of beam current and two-thirds power of plasma wave amplitude.

Electromagnetic Wave Propagation in an Annular Electron Beam With Ion Channel Guiding

The dispersion characteristics of electrostatic and electromagnetic modes of an electron tube, with ion channel guiding, in a cylindrical waveguide are presented. The analysis takes into account a relativistic hollow electron beam guided by an ion channel. Dispersion relations are derived and solved numerically to study the waveguide, betatron, and space charge modes. The effect of the annular electron beam thickness on frequencies of the electrostatic and electromagnetic waves is investigated. This paper reveals the strong dependence of the electromagnetic and electrostatic waves' frequencies on the electron beam thickness. The results of this paper are compared with previous works where an electron tube is guided with an axial magnetic field.

Stimulated Raman scattering of laser in heavily doped semiconductors

An intense short pulse laser propagating through a heavily doped degenerate semiconductor, parametrically excites a free carrier space charge mode and back scattered electromagnetic mode via free carrier nonlinearity. Landau damping and collisional damping of the decay waves determine the threshold for the instability. Above the threshold, the growth rate scales as γ∼(v0/2c)(εLωω0)1/2, where v0 is the free carrier oscillatory velocity, c is the velocity of light in vacuum, εL is the lattice permittivity, ω is the frequency of the space charge mode, and ω0 is the pump frequency.

Mode Coupling in a Partially Filled Plasma Waveguide With a Dielectric Liner

The stimulated Raman scattering of a perturbed TM mode in a dielectric loaded partially filled plasma waveguide is considered. The radiation is the result of the scattering of pump electromagnetic wave, which is an EH waveguide mode, off of a space-charge wave. A nonlinear wave equation for a three-wave interaction is used to investigate the coupling of the space-charge and waveguide modes. Dispersion relations for electromagnetic modes are solved numerically to study the frequency characteristics of the interaction of the EH waveguide modes with the space-charge modes. Formulas for the growth rates of the backscattered wave near the electron cyclotron frequency are derived, and the effect of a dielectric liner on the growth rates is investigated. Numerical studies show that the presence of the dielectric liner has an effect on the phase-matching condition and leads to rather larger growth rates close to electron cyclotron resonance.