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1.
A general theory is developed for self-consistent calculations of mode competition in a gyrotron with nonfixed axial structure of the RF field for arbitrary cyclotron harmonics. The theory is applied to the gyrotron operating at the Kernforschungszentrum Karlsruhe with a frequency of 150 GHz. The formalism presented allows a self-consistent calculation of mode competition for the operating and parasitic modes at the cyclotron resonance at arbitrary harmonics. Specific calculations are carried out for the cases n0=1, n1 =2 and n0=2, n1=1. It is emphasized that the formalism considered applies only to the case in which the azimuthal orthogonality condition is satisfied: n0m1n1 m0. There are circumstances when this condition is not satisfied  相似文献   

2.
The relativistic harmonic content of large-amplitude electromagnetic waves propagating in underdense plasmas is investigated. The steady-state harmonic content of nonlinear linearly polarized waves is calculated for both the very underdense (wp/w 0)≪1 and critical density (wp/w0)≃1 limits. For weak nonlinearities, eE0/mcω0<1, the nonlinear source term for the third harmonic is derived for arbitrary wp/w0. Arguments are given for extending these results for arbitrary wave amplitudes. It is also shown that the use of the variable x-ct and the quasi-static approximation leads to errors in both magnitude and sign when calculating the third harmonic. In the absence of damping or density gradients the third harmonic's amplitude is found to oscillate between zero and twice the steady-state value. Preliminary PIC simulation results are presented. The simulation results are in basic agreement with the uniform plasma predictions for the third-harmonic amplitude. However, the higher harmonics are orders of magnitude larger than expected and the presence of density ramps significantly modifies the results  相似文献   

3.
A three-wave free-electron laser (FEL) was operated with a powerful 8.4-GHz electromagnetic pump wave replacing the usual magnetostatic wiggler. The presence of a uniform axial magnetic field B0 produced cyclotron-harmonic idler waves. Peaks in the emission spectrum corresponding to cyclotron harmonics were observed covering a frequency range from 16.5 to 130 GHz. The frequency spectrum of this novel FEL mechanism was tuned continuously by the variation of B0  相似文献   

4.
A thin, annular plasma sheet in a waveguide slows down the phase velocity of electromagnetic modes in a plasma density window for frequencies below the electron cyclotron frequency via Cerenkov interaction. At lower densities, the plasma is not effective enough to reduce the phase velocity of the EM wave below c. At higher densities the plasma expels the radiation field, leading to enhancement of phase velocity. The lowest mode having azimuthal number l=0 is most unstable. The radial mode having amplitude maximum at r=a, viz the plasma boundary, has the largest growth rate  相似文献   

5.
A simple calculation of a free-electron laser in the Compton regime that uses a dielectric-lined waveguide is presented. The introduction of a dielectric lining in a free-electron laser considerably reduces the requirements on beam voltage for generating a given frequency ω1=k0c/(1 - vb η/c), where k0 is the wiggler wave period η is the effective index of refraction (1<η<√ϵ) and ϵ is the permittivity. The system supports electromagnetic waves whose Poynting flux is largely concentrated in the dielectric; hence the electron beam is required to propagate close to the dielectric lining. The mode structure and dispersion behavior of the guiding system without the beam are discussed. a thin annular beam is introduced, and a perturbation theory is used to obtain the frequency and growth rate of radiation  相似文献   

6.
A partially dielectric-loaded waveguide supports modes with phase velocity νph>c/∈1/2, where ∈ is the dielectric constant of the lining. The coupling of a fast mode (with νph>c) with a slow mode (with c /∈1/2ph<c) via a negative-energy beam mode gives rise to the explosive growth of all the three waves at the expense of the energy of the beam. The slow mode of lower frequency can be launched from an external source and can become an efficient wiggler for the generation of the fast free-electron laser mode  相似文献   

7.
A 30-A, 3-mm-diameter, wall-stabilized argon arc with 1% hydrogen is examined spectrometrically at pressures of 0.1-10 bar. Values of Tg, and Texa diverge as pressure decreases below 5 bar (ne⩽1×1017 cm-3) at r=0. Texβ is 20-40% larger than the other temperatures. The results are dependent on the transition probability scale used  相似文献   

8.
A self-consistent solution for the dynamics of a high voltage, capacitive radio frequency (RF) sheath driven by a sinusoidal current source is obtained under the assumptions of time-independent, collisionless ion motion and inertialess electrons. Expressions are obtained for the time-average ion and electron densities, electric field and potential within the sheath. The nonlinear oscillation motion of the electron sheath boundary and the nonlinear oscillating sheath voltage are also obtained. The effective sheath capacitance and conductance are also determined. It was found that: (1) the ion-sheath thickness S m is √50/27 larger than a Child's law sheath for the DC voltage and ion current density; (2) the sheath capacitance per unit area for the fundamental voltage harmonic is 2.452 ϵ0 /Sm, where ϵ0 is the free space permittivity; (3) the ratio of the DC to peak value of the oscillating voltage is 54/125; (4) the second and third voltage harmonics are, respectively, 12.3 and 4.2% of the fundamental; and (5) the conductance per unit area for stochastic heating by the oscillating sheath is 2.98 (λD/Sm)2/3 (e 2n0/mue), where n 0 is the ion density, λD is the Debye length at the plasma-sheath edge, and ue is the mean electron speed  相似文献   

9.
Millimeter-wave emission from the rippled-field magnetron (cross-field free electron laser (FEL)) is investigated experimentally and theoretically. In this device, electrons move in quasi-circular orbits under the combined action of a radial electric field, a uniform axial magnetic field, and a radial azimuthally periodic wiggler magnetic field. In excess of 300 kW of RF power is observed in two narrow spectral lines whose frequency can be tuned continuously from ~25 to ~50 GHz by variation of the axial magnetic field. The observations are interpreted as a FEL type of instability, associated with a resonance in the particle motion of a layer of electrons embedded in the dense spacecharge cloud. The resonance is shown to occur when 2kw?0 ? (?>0/?0) ?1 -(?p/?0)2, where kw is the wiggler wavenumber, ?0 is the azimuthal electron velocity, ?0 is the relativistic cyclotron frequency in the axial magnetic field, wp is the relativistic plasma frequency, and ?0 = [1 - (?0/c)2]-1/2 of the resonant electron layer.  相似文献   

10.
Experiments are described which show that the reflection coefficient for ion acoustic waves (IAW) from the sheath at a grid is affected by an HF electric field with a frequency fHF≲5fpi(fpi =ion plasma frequency). For peak-to-peak amplitudes of the HF voltage drop across the sheath Φ0kB Te/e and fHF>f pi, the energy distribution of the ions passing through the grid develops a hot tail and the reflected wave suffers enhanced Landau damping. If Φ0kBTe/e and fHF<fpi, a large-amplitude IAW is excited at the grid; a well-defined ion beam is formed; and local growth of the reflected wave is observed. Test waves launched from the grid show the same propagation characteristics as the reflected waves  相似文献   

11.
The general dispersion relation is derived for the fundamental transverse magnetic modes driven by a cold relativistic electron beam in a dielectric-loaded cylindrical waveguide using the fluid Maxwell equations. It is then reduced to the algebraic equation for the space charge and cyclotron modes using a tenuous beam approximation. Solutions of the resulting equation are obtained by varying several parameters, such as the external magnetic field the dielectric constant and the thickness of the dielectric material. It is shown that the growth rate of the slow cyclotron instability is greatly increased for the region of Bo≲1000 G to the extent that it becomes comparable to the growth rate of a slow space-charge instability. In this region the magnetic-field effect on the slow space-charge mode is shown to increase the growth rate by up to 10%. In the limit of the critical external magnetic field defined as the field below which no beam equilibrium exists, it is found that two slow modes of cyclotron and space-charge modes become degenerate with a finite value of growth rate  相似文献   

12.
Allowance for finite V of an annular electron beam propagating through a dielectric-loaded waveguide immersed in an axial magnetic field opens up the possibility of excitation of TE modes. The interaction is observed using Cerenkov and cyclotron resonances. On approximating the field distribution at the electron orbits to some suitable form, an analytical solution to the Vlasov equation is obtained, leading to a comprehensive dispersion relation for azimuthally symmetric TE0n modes. In the special case where all the guiding centres lie on the axis, a fluid treatment for the arbitrary azimuthal-mode number is applicable. In these cases the growth rate increases with V  相似文献   

13.
Validity conditions for complete and partial local thermodynamic equilibrium (CLTE and PLTE) of homogeneous, time-dependent, and optically thin plasmas are derived. For Cu I, electron densities of ne⩾(5×1022-5×1023 ) m-3 are required for the establishment of CLTE. For Cu I and Cu II, ne⩾(5×1021-5×1021 -5×1022) m-3 is necessary for PLTE (for electron temperatures of 1-2 eV). Application to low-current copper vapor arcs in vacuum shows that CLTE can be expected for r<200-600 μm (r=distance from the cathode spot). A further limitation follows for temperatures of 2 eV or higher if diffusion effects are taken into consideration. Consequently, the use of the LTE formulas in plasma spectroscopy of low-current vacuum arcs is very limited  相似文献   

14.
The development of a scalable hydrogenic ionization model is described. The model utilizes correct energy and level structure data for each ionization stage and can be coupled self-consistently to a radiation transport calculation of the full nonhydrogenic X-ray spectrum. Thus it can be used to calculate accurately the effects of opacity on radiation emission rates that are of relevance to plasmas designed to produce recombination pumped population inversions due to rapid plasma cooling. The model is applied to a selenium plasma. It is found that, at ion densities where X-ray lasing has been observed, line emission from Δn≠0 transitions is a more important contributor to selenium's cooling rate than line emission from the Δn=0 transitions. Plasma opacity can also play an important role  相似文献   

15.
Observations of the cathodic copper plasma expansion at low pressures of He, Ar, and SF6 showed that, for background gas mass densities of ρg=1 to 4×10-4 kg/m 3 and higher, the plasma and gas are separated into two volumes. A shock wave acts as a boundary between the two volumes. The boundary attains a stationary position once its expansion velocity decreases to the velocity of sound in the background gas. This position corresponds to a distance Rc to the cathode that agrees with a snowplow expansion model, giving Rc βf=Er, where f is a function of the arc current and background gas characteristics, E r is the erosion rate of the cathode, and β varies between 2.1 and 2.5. The interaction model is based on kinetic energy exchanges between two gas-like volumes without other energy losses. A maximum pressure limit for vacuum arc deposition is set for ρg /I=2 to 9×10-6 kg/m3 A  相似文献   

16.
The authors have designed and tested an electromagnet planar microwiggler for use in free electron lasers (FELs), constructed of current conductors wound on ferromagnetic cores. A prototype with a period of 1 cm and a gap of 0.5 cm produced a peak field on axis in excess of 4.6 kG, with a linear B/H characteristic to about 3.2 kG. The field of each half-period of the wiggler is independently tunable by adjusting the current delivered to each, thus allowing for precision tuning and/or wiggler tapering. General scaling laws are employed to predict the performance of a geometrically similar design with a period of 5 mm  相似文献   

17.
S.C. Wilks et al. (1988) showed that when an infinite expanse of gas, carrying a linearly polarized electromagnetic wave, is instantly ionized, the initial wave is frequency upshifted. This phenomenon of frequency upconversion through flash ionization gives rise to steady-state transmitted and reflected electromagnetic waves and to a time-independent magnetic field. The case in which the final state of ionization is achieved not instantly but in a finite turn-on time, 0⩽tt 0, which is followed by the steady state, is studied. It is shown that the electric field is obtained from the one-dimensional Helmholtz equation, d2F(t)/dt2 02g(t)F( t)=0, if electrons are born at rest when they are created during ionization. As a result, the instantaneous frequency of the upshifted radiation is ω(t)=√g(t). The electric field can be solved exactly for specific choices of g(t). It is solved using WKB approximations for arbitrary g(t). The magnetic field is then found by integrating Faraday's law. It is found that the steady-state electric field amplitude depends on the steady-state value o f g(t) but does not depend on the ionization time t0. Conversely, the static magnetic field amplitude decreases with increasing turn-on time  相似文献   

18.
A parallel mode launch in the electron cyclotron range of frequencies is performed on axis in the east end cell of the Phaedrus-B tandem mirror. Propagation of the energy launched toward a magnetic beach in a radially hollow density profile was studied experimentally. A strong refractive dispersal of the launched energy was observed and shown to be in good agreement with a WKB-type ray tracing model using a cold plasma Appleton-Hartree dispersion relation. Measurements of plasma density and electron temperature indicate that the use of this dispersion relation is well justified. These results are compared with additional ray tracing studies for radially hollow and nonhollow, high ( flaunch<fp) and low (f launch>fp) density models. For the experimental conditions examined, a reversal of refractive behavior is shown to exist as the energy propagates from a low density (flaunch>fp) region to a high density (flaunch<fp) region  相似文献   

19.
Particle simulation in a one-dimensional bounded system is used to examine the formation of acoustic double layers in the presence of two ion species. Double-layer formation depends critically on the details of the distribution functions of the supporting ion populations, and their relative drifts with respect to the electrons. The effect of having two ion components, an H+ and an O+ beam, on double-layer evolution from ion acoustic turbulence driven by an electron drift relative to the H+ beam of ≈0.5u e, where uue is the electron thermal speed, is examined. The ratio of ion drifts is taken to be consistent with acceleration by a quasi-static auroral potential drop (i.e. V H/VO=√MO/ MH=4.0). Acoustic double layers form in either ion species on the time scale τ≈100ωps-1, where ωps is the ion plasma frequency for species `s' and s=H or O, and for drifts relative to the electrons lower than that required for double layer formation in simulations of single ion component plasma  相似文献   

20.
An `ion-focused' relativistic electron beam traversing a magnetic wiggler is subject to a transverse two-steam or `in-hose' instability, resulting from the coupling of transverse displacements of the beam centroid to the `slosh' motion of the (beam-focused) nonneutral ion plasma, and driven by the `V×B' deflection in the wiggler field. The equations of motion are resolved into an inhomogeneous `beam breakup' equation, and asymptotic growth is computed in the limit of linear focusing. The effect of nonlinearities is assessed numerically with a `distributed-mass' model. As examples, ion-hose growth is considered in a UV FEL and a microwave FEL two-beam accelerator  相似文献   

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