摘要:

The spot size and focal length of a one‐eighth betatron wavelength final focusing cell with a nonuniform current density distribution are predicted. The final focusing cell is modeled with an azimuthal magnetic field distribution that varies asrN. A Lie transform method is used to determine the behavior of the ion beam as it passes through the focusing cell. The analysis indicates that a final focusing cell with a current density distribution that is strongly concentrated at the channel edge focuses the beam much less efficiently than a channel with a uniform current density distribution, and provides a scaling relation for the focused beam radius:rfoc∝rc(Ic/If)1/2N, wherercis the unfocused beam radius,Ifis the discharge current in the focusing cell, andIcis the discharge current in the transport channel.

ISSN:0899-8221    

DOI:10.1063/1.859198

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

Strong beam–plasma interactions occur when beam densitynbapproaches plasma density,nb/np>0.01. Energy flow from resonant waves to short wavelengths is modeled with existing theory, using instability rates from linear and nonlinear (Zakharov) dispersion relations. Langmuir waves lose energy by wave convection, through observed electromagnetic emission, and to heating by induced return currents acting on anomalous resistivity. Inputs to the model equations are current, voltage, and other parameters of a 600 keV, 5 kA electron beam. The beam propagates in 10 mT helium preionized to 2×1012cm−3, emitting radiation of peak power 100 kW at an efficiency of 3×10−5. Turbulent electric fields reach 27 kV/cm [Phys. Fluids B1, 2488 (1989)]. Comparison between experiment and model calculations shows satisfactory agreement.

ISSN:0899-8221    

DOI:10.1063/1.859182

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

Optical diagnostics using laser fluorescence techniques are used to measure the rms electric field in a superstrongly turbulent, relativistic beam–plasma system. This yields the mapping of ⟨E2⟩ as a function of radial locationrand timet. This ⟨E2(r,t)⟩ allows studies of growth and evolution of turbulent fields, their diffusion, and decay. Fluctuating electric fields occur when a 700 keV, 4 kA, 2 &mgr;sec electron beam propagates into a 20 cm diam, 1.5 m long drift tube filled with 10 mTorr of helium plasma. Stark effect shifts appear in suitable forbidden and allowed transitions, originating from the same upper energy level for the measurement: Hei6632 A˚ and Hei5015.7 A˚. The spectral bandwidth includes the forbidden line and its satellites. Using the ratio of the intensity of the forbidden plus satellite lines, to the allowed line intensity, yields the rms field as the combined field of oscillation near the plasma frequency. Fields up to 28 kV/cm result. These results can be explained by an analytical model of production of strong electric fields by beam–plasma instability, including modulational transfer inkspace, plasma heating, radiation, and wave convection. Comparison between experiment and the numerically integrated model shows good agreement.

ISSN:0899-8221    

DOI:10.1063/1.859183

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

A relativistic fluid theory is constructed to analyze the inverse Compton scattering of an unmagnetized counterpropagating plasma wave by a cold relativistic electron beam. A Langmuir wave and electromagnetic plasma wave wigglers are considered, and the properties of the two types of wigglers are comparatively discussed. The growth rates are comparable to those of the ac free‐electron laser and the free‐electron lasers with magnetostatic wigglers in the Raman regime.

ISSN:0899-8221    

DOI:10.1063/1.859184

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

In this paper a comprehensive kinetic, gyrokinetics theory is presented to provide a unified description of transverse‐magnetic‐mode gyrotron, gyropeniotron, cyclotron autoresonance maser, and nonwiggler free‐electron laser amplifiers. By introducing gyrokinetic variables, a unified dispersion equation is derived in which the effect of the guiding‐center shift is taken into account. Similar to the transverse‐electric‐mode cases, it is found that at the zero point of theJm−l(R¯) instability is still effective, whereJm−lis the Bessel function of order (m−l),mandlare, respectively, the azimuthal index of the mode and the index of the synchronous harmonic, andR¯ is the electron beam radius normalized by the cutoff wavenumber.

ISSN:0899-8221    

DOI:10.1063/1.859185

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

A theoretical formulation that provides a basis for generating approximate solutions of the Boltzmann equation for the study of high‐powered microwave pulses interacting with a background atmosphere is given. A numerical scheme is implemented using realistic cross sections for electron–nitrogen collisions, including excitation and ionization. It is found that the numerical results agree well with previous experimental and theoretical values when &agr; is less than about 0.1 kV/cm Torr, where &agr; is the effective electric field divided by the pressure (&agr;=Ee/P). Results are presented providing a simple fit for the time averaged momentum transfer rate and the ionization rate as a function of the microwave electric fieldE, wavelength &lgr;, and gas pressureP, for 0.1≤&agr;≤1.0 kV/cm Torr. This fit is compared with recent experimental data for breakdown times.

ISSN:0899-8221    

DOI:10.1063/1.859186

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

A two‐dimensional electrostatic particle‐in‐cell code is used to simulate the convection of a finite stream of initially neutral plasma across a uniform magnetic field. The simulations show that the stream loses momentum with distance as a result of two erosion mechanisms that have greater effects for denser plasmas: (1) erosion of the charge layers at the sides of the stream as a result of velocity shear and (2) erosion of the head of the stream as a result of charge separation where ions travel ahead of the electrons. The electron charge layer exhibits a velocity shear that excites the diocotron instability. This instability occurs earlier for denser plasmas but it does not appear when the length of the stream is shorter than four wavelengths. The charge separation at the head of the stream causes the eroded plasma to drift with a sheared velocity. A flutelike instability develops at the head of the plasma stream for sufficiently dense plasmas. The simulations show that the plasma is eroded faster by the head erosion mechanism. Electric field fringe effects cause the plasma head to broaden and the tail of the plasma to narrow. The simulations show that although the plasma configuration is changed a great deal by erosion and fringe effects, the stream is convected across the magnetic field with a constant velocity for sufficiently dense plasmas and with a velocity that approaches the injection for denser plasmas. The simulations also show that the convection velocity for a partially and for a completely eroded plasma is the same.

ISSN:0899-8221    

DOI:10.1063/1.859187

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

The electron thermal balance equation from the Braginskii equations with the finite Larmor radius heat flux is analyzed for the space‐time‐average power balance in the presence of electromagnetic fluctuations. Formulas for the anomalous thermal flux associated with theE×Bmotion and the magnetic &dgr;B⊥fluctuations are derived and evaluated for thec/&ohgr;pescale electromagnetic turbulence typical of the ∇Te‐driven short wavelength drift modes. The result is compared with several Ohkawa‐type formulas for the anomalous electron thermal energy transport.

ISSN:0899-8221    

DOI:10.1063/1.859188

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

The results of a series of particle‐in‐cell simulations of stochastic wave–particle interaction are presented. The threshold for stochasticity was confirmed. The simulations demonstrate that in a strong magnetic field plasma waves with quiver velocities much less than the speed of light but above a certain threshold can stochastically accelerate electrons to energies far greater than 1 MeV. Moreover, self‐consistency effects drive return currents and produce energetic runaway electrons that violate an invariant of motion.

ISSN:0899-8221    

DOI:10.1063/1.859189

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

Ohkawa has proposed a tokamak current‐drive scheme [Comm. Plasma Phys. Controlled Fusion12, 165 (1989)], which relies on the injection of circularly polarized magnetohydrodynamic waves. It is shown here that the favorable current‐drive efficiency predicted by Ohkawa is not attained because excessive power is dissipated by the fluctuating fields. The ratio of power absorbed by the plasma to the dc Ohmic power required to drive the same current isPabsorbed/POhmic≊4(Ba/b)2, whereBais the static toroidal field strength andbis the strength of the fluctuating field.

ISSN:0899-8221    

DOI:10.1063/1.859190

出版商:AIP    

年代:1989

数据来源: AIP