The Transverse Oscillation of Coaxial Multiple Beams Propagating along a Magnetic Field in a Vacuum Tube

A fluid Maxwell theory has been derived to study a system of multibeams propagating parallel to an applied axial magnetic field in an evacuated conducting drift tube. The stability analysis is performed for a rigid-rotor and cold-laminar flow equilibria. It is assumed that the particle beams are tenuous and the guiding field is very strong. As a result, the perturbation theory is derived under the condition that the plasma frequency is much smaller than the cyclotron frequency for each beam particle. A dispersion relation is obtained for a special case of sharp-boundary density profiles. The stability properties of infinitely long beams are illustrated in detail for different geometries and various beam parameters. The results agree with those obtained by Uhm [8] in a special case where a solid electron beam propagates through an annular electron beam. The finite geometry effect of the accelerator is discussed briefly. It might have a substantial influence on the behavior of a real device.     

Numerical Simulation of Autoresonant Ion Acceleration

Computational and analytic studies of the Autoresonant Acceleration proposal for collective ion acceleration are presented. Linear theory is reviewed, the electrostatic well depth is estimated nonlinearly, and an electron beam envelope equation is derived and solved. Two-dimensional numerical simulation results are given. Together, these calcualtions demonstrate unneutralized electron beam equilibrium in a diverging magnetic guide field, the behaviour of large amplitude slow cyclotron waves in the beam, and the acceleration of test ions over short distances in the wave troughs. In addition, the computer simulations point up the need for improved understanding of the linear theory of radially inhomogeneous noneutral beams and for methods of suppressing radial modulation at the diode-waveguide interface.     

ELECTROSTATIC WAVE EXCITATION BY LITHIUM ION BEAM IN UPSTREAM RECION OF AMPTE LITHIUM RELEASES

It is shown that the electron cyclotron harmonic waves and the ion acoustic broad band obsefved in the transition region and the upstream region of the two AMPTE lithium releases can be explained ,by the interaction of the lithium ion beams and the solar wind plasmas.The cycloidal motion of the freshly produced lithium ion in the solaz wind magnetic and electric fields is essential for these wave excitations.Two simplified models in ion velocity distribution are used in the dispersion relation analysis,one is an orientating ion beam, the other is an ion beam ring (the hollow beam).It is shown that the electron cyclotron. harmonics can be effectively excited by both of these beams if they are very cold. Satisfactory consistence of the theory with the observed results is obtained for the harmonic excitations.The strong Iow frequency (much less than the electron cyclotron frequency) noices might be multioriginal.It is also proposed that the interaction of the lithium ion beam and the solar wind protons provides a suitable mechanism for exciting these broad bands.     

Resistive electromagnetic ion cyclotron instability in counterstreaming plasmas

Obliquely propagating electromagnetic ion cyclotron waves are found to be unstable in a collisional, fully-ionized plasma consisting of two electron beams counterstreaming along an external magnetic field in a background of stationary ions.     

Instability of Electromagnetic Ion Cyclotron Harmonic Waves in Plasmas

The stability of electromagnetic ion cyclotron harmonic waves propagating normal to an external magnetic field is studied for plasmas whose ions possess loss cone type velocity distributions. It is found that, if the ions are stationary, no instability develops except in the extreme case when the ratio of parallel to perpendicular "temperatures" of the ions is of the order mi/me, where mi and me are the ion and electron masses respectively. However, for the case of two counterstreaming ion beams in a neutralizing background of electrons, instability at zero frequency and near the first several ion cyclotron harmonics can occur if the streaming velocity is of the order of the electron thermal speed.     

Theory of second-order cyclotron resonance as related to the origin of discrete VLF emissions in the magnetosphere

Recent analytical and numerical results concerning the role of the second-order cyclotron resonance effects in formation of discrete emissions in the magnetosphere are reviewed. Peculiarities of whistler cyclotron interactions with energetic particles having sharp (step-like or beam-like) distribution functions evolving in space and time are studied. Formation of such distributions is considered, and an analytical self-consistent theory of the second-order cyclotron resonance effects is developed. In particular, characteristics of electron beams produced by the interaction of a VLF wave packet from a ground-based transmitter are studied. It is shown that spatial and temporal gradients of the parallel velocity of the beams formed can be opposite to the case of a pure adiabatic motion of a single particle. Such a behavior can be significant for the generation of secondary emissions. It is proven that the optimal conditions for the instability occur for a nonstationary quasi-monochromatic wavelets whose frequency changes in time. The theory developed permits one to estimate the wave amplification and spatio-temporal characteristics of these wavelets. Numerical results on beam formation and generation of secondary emissions are presented. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 42, No. 8, pp. 713–727, August 1999.     

On the amplification mechanism of the ion-channel laser

The amplification mechanism of the ion-channel laser (ICL) in the low-gain regime is studied. In this concept, a relativistic electron beam is injected into a plasma whose density is comparable to or lower than the beam's density. The head of the electron beam pushes out the plasma electrons, leaving an ion channel. The ion-focusing force causes the electrons to oscillate (betatron oscillations) about the axis and plays a role similar to the magnetic field in a cyclotron autoresonance maser (CARM). Radiation can be produced with wave frequencies from microwaves to X-rays depending on the beam energy and plasma density: ω~2γ^3/2ω_pe, where γ is the Lorentz factor of the beam and ω_pe is the plasma frequency. Transverse (relativistic) bunching and axial (conventional) bunching are the amplification mechanisms in ICLs; only the latter effect operates in free-electron lasers. The competition of these two bunching mechanisms depends on beam velocity ν_0z; their dependences on ν_0z cancel for the cyclotron autoresonance masers. A linear theory is developed to study the physical mechanisms, and a PIC (particle-in-cell) simulation code is used to verify the theory. The mechanism is examined as a possible explanation for experimentally observed millimeter radiation from relativistic electron beams interacting with plasmas     

Effect of background plasma on slow cyclotron waves of electron beam

The effect of plasma loading in the field structure and dispersive characteristics of slow cyclotron waves propagated in electron beams is studied. A requirement is established for the maximum permissible plasma concentration at which a slow cyclotron wave can be used for collective ion acceleration. The degree of vacuum required for operation of an autoresonant accelerator is estimated.Moscow Radio-Engineering Institute, Academy of Sciences of the USSR. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 34, No. 8, pp. 897–907, August, 1991.     

Free electron laser operation in the whistler mode

The introduction of a strongly magnetized plasma in the inner region of a free electron laser opens up the possibility of generating coherent radiation in the slow whistler mode using mildly relativistic electron beams. The frequency of emission, however, is limited to below the electron cyclotron frequency. The efficiency of the device can be enhanced by tapering the guide magnetic field     

Design of a 1-MW, CW coaxial gyrotron with two gaussian beam outputs

The design of a 170 GHz, 1 MW-CW gyrotron for electron cyclotron heating of nuclear fusion plasmas is presented. The designed gyrotron incorporates a coaxial cavity to reduce mode competition, and a coaxial electron gun to support the cavity inner conductor. A new mode converter splits the generated wave into two beams and radiates them in different directions. The radiated beams are transmitted to two output windows through two mirror systems, being transformed into Gaussian-like beams. A single-stage depressed collector improves the overall efficiency of the gyrotron and reduces the heat flux to the collector surface.     

Study of runaway electron behaviour during electron cyclotron resonance heating in the HL-2A Tokamak

During the current flat-top phase of electron cyclotron resonance heating discharges in the HL-2A Tokamak, the behaviour of runaway electrons has been studied by means of hard x-ray detectors and neutron diagnostics. During electron cyclotron resonance heating, it can be found that both hard x-ray radiation intensity and neutron emission flux fall rapidly to a very low level, which suggests that runaway electrons have been suppressed by electron cyclotron resonance heating. From the set of discharges studied in the present experiments, it has also been observed that the efficiency of runaway suppression by electron cyclotron resonance heating was apparently affected by two factors: electron cyclotron resonance heating power and duration. These results have been analysed by using a test particle model. The decrease of the toroidal electric field due to electron cyclotron resonance heating results in a rapid fall in the runaway electron energy that may lead to a suppression of runaway electrons. During electron cyclotron resonance heating with different powers and durations, the runaway electrons will experience different slowing down processes. These different decay processes are the major cause for influencing the efficiency of runaway suppression. This result is related to the safe operation of the Tokamak and may bring an effective control of runaway electrons.     

Collective and single-electron interactions of electron beams with electromagnetic waves,and free-electron lasers

The field of radiation emission from electron beams is reviewed with special reference to work related to free-electron lasers. Different schemes of interaction in periodic structures, electromagnetic slow-wave structures, and in transverse confining force are distinguished. Various effects and devices such as traveling wave amplifiers, Smith-Purcell radiators, Cerenkov and bremsstrahlung-free electron lasers, cyclotron resonance masers, coherent bremsstrahlung and channeling radiation are discussed and the differences and relations among them are explained. A simple comprehensive model is developed to describe electron-beam interaction with an electromagnetic wave in periodic electromagnetic structures. The model is general enough to describe both collective and single-electron modes of interaction and quantum mechanical, classical and Fermi degenerate regimes. Simplified expressions are developed for the gain by stimulated emission of radiation and for gain conditions of the Smith-Purcell-Cerenkov type free-electron lasers under conditions of very thin electron beams and infinite interaction length. This research is supported by the Air Force Office of Scientific Research under contract AFOSR-76-2933     

A facility upgrade at Texas A&;M University for accelerated radioactive beams

The Cyclotron Institute at Texas A&M University is carrying out an upgrade project which will lead to accelerated radioactive ion beams at intermediate energies. The project involves recommissioning a K150 cyclotron for acceleration of stable beams which will be used to produce radioactive ions. Both light-ion and heavy-ion guides will be used to stop and transport the radioactive ions to a charge breeding electron cyclotron resonance ion source. Following charge breeding, highly-charged ions will be injected into the K500 cyclotron, accelerated and then transported to existing equipment to carry out experiments.     

On the possibility of harmonic operation of cyclotron waveparametric amplifiers

Cyclotron wave amplifiers at the harmonics of the electron cyclotron frequency are investigated. Since the waves on the beam are electrostatic, harmonics are strongly excited in nonrelativistic beams if they are rotating rather than filamentary. These modes at the harmonics can couple to input Cuccia couplers, and pump fields which drive parametric amplification, in very much the same way as they do on filamentary beams at the cyclotron frequency. Harmonic cyclotron wave amplifiers have the possibility of giving rise to a new class of devices at millimeter wave frequencies     

Large-signal theory of gyro traveling wave tubes at cyclotronharmonics

A nonlinear theory of gyrotron traveling wave tubes (gyro-TWTs) at cyclotron harmonics has been developed taking into account the electron velocity spread and the possibility of operating with significant Doppler frequency up-shift (CARM operation). It is shown that the orbital efficiency of the relativistic gyro-TWT operating at the second cyclotron harmonic with large frequency up-conversion may exceed 60%. It is also shown that the influence of the axial inhomogeneity of the wave field on the relation between amplitudes of electric and magnetic fields of the wave causes small changes in the efficiency of gyro-TWTs. The results obtained demonstrate the sensitivity of the harmonic gyro-TWT efficiency with respect to electron velocity spread at different axial wave numbers. The expressions for the gain are derived and discussed,     

Low-power RF plasma sources for technological applications: II. plasma sources under anomalous skin effect conditions

A theory of a planar disk-shaped RF plasma source under anomalous skin effect conditions is developed. In the absence of an external magnetic field, such conditions are satisfied for transverse electromagnetic waves with phase velocities below the electron thermal velocity, and, in the presence of this field, they are satisfied for electron cyclotron waves with frequencies corresponding to the resonant absorption line. For each of these cases, the RF field power deposited in a plasma with given parameters is determined and the equivalent plasma resistance is calculated.     

Excitation of ion cyclotron waves at the difference frequency of two microwave beams in a semiconductor

This paper presents an investigation of the resonant excitation of the electrostatic ion cyclotron wave at the difference frequency of two microwave beams propagating in a magnetoactive solid state plasma, viz. n InSb. The resonant excitation of the electrostatic ion cyclotron wave occurs when the difference frequency of the two microwave beams and the difference of their propagation vectors satisfy the dispersion relation corresponding to the electrostatic ion cyclotron wave. For typical plasma parameters of n InSb and microwave beams of power densities 1 MW cm^?2, the power density of the excited ion cyclotron wave is 0.40 kW cm^?2 when external magnetic field is $\text{1.46 kG (}\text{Ω}{}_{\mathrm{c}}\text{ω}\text{) = 0.1)}$. The power density of the excited ion cyclotron wave increases with the magnetic field. This study may provide new means for the characterisation and diagnostic of semiconductors.     

Experimental study of an FEM with a microwave system of a new type

Operability of the wide-band reflector based on the effect of microwave beams multiplication was tested in the “hot” experiment. A microwave system of the new type including such a reflector was used in a free electron maser (FEM)-oscillator with a guide magnetic field. The high-current accelerator “Sinus-6” forming a 500 keV/120 A electron beam was used as the FEM driver. The double-resonance regime, when both the undulator and the cyclotron resonance conditions were fulfilled simultaneously, provided maximum output power 7 MW and electron efficiency 12% at the designed frequency 45 GHz     

Stabilization of counter streaming electron beam instability

Instability at half the electron cyclotron frequency that arises in a system of symmetrical counter streaming electron beams is stabilized by the injection of third beam.