Quantum-kinetic calculation for net acceleration of high-energy electrons by a transverse undulating magnetic field and an electromagnetic wave

The average force acting on a highly relativistic electron beam traveling in a laser wave and a transverse undulating magnetic field (magnetic wiggler) is calculated using quantum kinetics. The quantum-kinetic calculation shows that the net dc force due to net inverse bremsstrahlung in the magnetic wiggler (“inverse free electron lasing”) increases linearly with the beam energy, and can be greater than the Lorentz force of the laser wave.     

A free-electron laser in a uniform magnetic field

The interaction of free electrons and free electromagnetic radiation, in the presence of a uniform magnetic field, can result in stimulated emission or absorption. We analyze the dynamics of single electrons by solving the classical, relativistic Lorentz force equations of motion in these combined fields. An electron may gain energy from, or lose energy to, the radiation field, depending crucially on the phase and oscillation frequency of the electron's helical motion within the superposed, circularly polarized light wave. To first order in the radiation field strength, electrons in a monoenergetic, uniformly distributed beam become spatially bunched, but there is no net energy change. To second order, however, the beam may experience a gain or loss of energy, corresponding to attenuation or amplification of radiation. We compare the bunching of this laser process to the bunching processes involved in 1) the Stanford free-electron laser and 2) the cyclotron maser, and find significant differences in each case. Our analytic results provide a clear, simple picture of the interaction process, and can be useful in exploring light amplification in astrophysical magnetic fields, the magnetosphere, or in laboratory devices. Supported in part by Army Contract No. DASG 60-77-C-0083 and NASA Grant NSG-7490.     

Electron acceleration by net inverse bremsstrahlung of a laser wave in a uniform magnetic field and longitudinal electric waves

The nonzero net dc (ponderomotive force) acting on high-energy beam electrons due to net inverse bremsstrahlung (the absorption by inverse bremsstrahlung minus the emission by stimulated bremsstrahlung) of the electromagnetic wave in a uniform magnetic field and longitudinal electric waves is calculated by using quantum kinetics in accordance with the correspondence principle. It is found that the ponderomotive force can be far stronger than the Lorentz force of the laser wave for an electron-energy range far beyond the free electron lasing regime.     

Resonant third harmonic generation of a short pulse laser in plasma by applying a wiggler magnetic field

We examine the effect of wiggler magnetic field on pulse slippage of short pulse laser-induced third harmonic generation in plasma. The process of third harmonic generation of an intense short pulse laser in plasma is resonantly enhanced by the application of a magnetic wiggler. The laser exerts a ponderomotive force at second harmonic driving density oscillations. The second harmonic oscillations coupled with electron velocity at the laser frequency, produces a non-linear current, driving the third harmonic. Third harmonic pulse generates in the fundamental pulse domain. However, the group velocity of the third harmonic wave is greater than the fundamental wave. Hence, the third harmonic pulse saturates strongly and moves forward from the fundamental pulse at shorter distance than the second harmonic pulse.     

Whistler modes with wave magnetic fields exceeding the ambient field

Whistler-mode wave packets with fields exceeding the ambient dc magnetic field have been excited in a large, high electron-beta plasma. The waves are induced with a loop antenna with dipole moment either along or opposite to the dc field. In the latter case the excited wave packets have the topology of a spheromak but are propagating in the whistler mode along and opposite to the dc magnetic field. Field-reversed configurations with net zero helicity have also been produced. The electron magnetohydrodynamics fields are force free, have wave energy density exceeding the particle energy density, and propagate stably at subelectron thermal velocities through a nearly uniform stationary ion density background.     

Beat excitation of terahertz radiation in a semiconductor slab in a magnetic field

Two infrared lasers of frequencies ω₁ and ω₂ propagating in the TM/TE mode along $\stackrel{?}{z}$ direction in a rippled density semiconductor waveguide are shown to resonantly excite terahertz radiation at the beat frequency when ripple wave number is suitably chosen to satisfy the phase matching. The wave vector of the density ripple is along the direction of laser propagation while a static magnetic field is applied transverse to it. The lasers exert a ponderomotive force on the electrons at the beat frequency. This force, in the presence of density ripple and transverse magnetic field, produces a nonlinear current at the terahertz frequency. The magnetic field enhances the amplitude of the terahertz wave. However terahertz yield is significantly higher in the TM mode laser beating than in the TE mode laser beating.     

Cyclotron damping of the space charge wave of a beam in a nonuniform magnetic field region

The space charge wave of an electron beam excited by the reactive medium instability in a uniform magnetic field region is observed to propagate into the region of increasing magnetic field. When the wave passes through the cyclotron resonance region, it suffers heavy cyclotron damping.     

Electrodynamic response of a nanosphere placed in a magnetic field

Absorption of electromagnetic radiation by 2D electrons at the surface of a quantum sphere placed in a weak magnetic field is studied. It is shown that at low temperatures, the absorption curve exhibits four resonance peaks observed in the Faraday geometry (photon wave vector parallel to the magnetic field) and six peaks in the Voigt geometry (photon wave vector perpendicular to the magnetic field). In the particular case of the Voigt geometry, where the photon polarization vector is parallel to the magnetic field, the absorption curve exhibits only two resonance peaks. The shape, position, and intensity of the peaks are examined. It is shown that at temperatures close to zero, steps of two types appear in the absorption curve. One type of steps is associated with crossing of the μ-?ω level by the electron energy levels, while steps of the other type arise when electron energy levels cross the chemical potential μ.     

磁场对螺旋波等离子体波和能量吸收影响的数值研究

本文考察在径向电子数密度呈抛物形分布的情况下,外加稳恒磁场,射频通过螺旋波天线在等离子体中激发电磁波的传播性质.采用线性扰动波假设,数值求解Maxwell方程组,得到80—800 G(1 G=10-4T)磁场条件下等离子体中径向电、磁场强度及能量沉积密度的分布情形.计算结果表明,磁场增大(80→800G)时,螺旋波受到的阻尼较小,可深入等离子体传播;Trivelpiece-Gould(TG)波受到的阻尼增大,在等离子体-真空边界处衰减增强;整体的能量吸收向边界集中.磁感应强度小于100 G时,TG波可深入主等离子体区传播,等离子体径向能量吸收相对均匀.     

Analysis of the uniform magnetic field free-electron laser with scalarized photons in the microwave-spectral region

The recently developed concept of scalarized photons (formally photons of any polarization) is used to analyze the spontaneous and stimulated emission in the uniform magnetic field free-electron laser in the microwave spectral region. In fact, this free-electron laser is the simplest of many other, wiggler and wiggler-free free-electron lasers whose analyses could be done with scalarized photons in the small signal regime and whose physical parameters can be conveniently chosen for radiation to be generated in the microwave spectral region. As to the uniform magnetic field free-electron laser, which is treated here in some detail, with the electron beam energy of up to 10 MeV and the uniform magnetic field of up to 4 Tesla, the radiation (occurring with the fundamental and higher harmonic frequencies) can cover easily a 10 to 10,000 GHz spectral region.     

Electron acceleration in the inverse free electron laser with a helical wiggler by axial magnetic field and ion-channel guiding

Electron acceleration in the inverse free electron laser (IFEL) with a helical wiggler in the presence of ion-channel guiding and axial magnetic field is investigated in this article. The effects of tapering wiggler amplitude and axial magnetic field are calculated for the electron acceleration. In free electron lasers, electron beams lose energy through radiation while in IFEL electron beams gain energy from the laser. The equation of electron motion and the equation of energy exchange between a single electron and electromagnetic waves are derived and then solved numerically using the fourth order Runge-Kutta method. The tapering effects of a wiggler magnetic field on electron acceleration are investigated and the results show that the electron acceleration increases in the case of a tapered wiggler magnetic field with a proper taper constant.     

Anisotropic momentum transfer in low-dimensional electron systems in a magnetic field

In low-dimensional systems with an asymmetric quantizing potential, an asymmetric electron energy spectrum ε(p)≠ε(−p), where p is the electron momentum, arises in the presence of a magnetic field. A consequence of such an energy spectrum is that momentum transfer to the electron system in mutually opposite directions in the presence of an external perturbation is different. Therefore, in the presence of a standing electromagnetic wave momentum is transferred from the wave to the electrons, which gives rise to a new type of electromotive force. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 8, 551–555 (25 October 1997)     

激光驱动的冲击波自生磁场以及外加磁场的冲击波放大研究

冲击波是天体物理观测中常见的现象, 其对粒子的加速被认为是高能宇宙射线的来源. 宇宙中冲击波周围往往存在很强的磁场, 但人们对于此类强磁场的产生放大过程的理解并不充分. 本文利用二维粒子模拟程序研究了激光与磁化或者非磁化等离子体相互作用产生的冲击波现象, 给出了冲击波波前处磁场的产生放大特性. 研究发现, 作用过程中的自生磁场可以储存能量, 从而进一步加速电子; 当存在外加磁场时, 由冲击波加速的电子和离子的能量都比同条件下非磁化等离子体的能量高; 而且外加磁场藉由冲击波放大倍数则与其值有极大关系. 与天文观测中推断的磁场与背景磁场相比放大千倍这一研究结果的比较可以看出, 天体冲击波周围磁场放大主要是由局域内生磁场导致的.     

Enhanced electron trapping by a static longitudinal magnetic field in laser wakefield acceleration

An externally applied longitudinal magnetic field was found to enhance the particle trapping in the laser wakefield acceleration. When a static magnetic field of a few tens of tesla is applied in parallel with the propagation direction of a driving laser pulse, it is shown from two-dimensional particle-in-cell simulations that total charge of the trapped beam and its maximum energy increase. The analysis of electron trajectories strongly suggests that the enhanced trapping originates from the suppression of the transverse motion by the magnetic field. The enhanced trapping by the magnetic field was observed consistently for various values of the plasma density, the amplitude of the laser pulse and pulse spot size.     

导引场下电磁泵浦自由电子激光的非线性理论

本文以Vlasov-Maxwell方程组为基础,用非线性动力学理论研究了存在导引场情况下电磁波泵浦的自由电子激光器的作用机制,导出了线性及非线性色散关系式,求出了非线性不稳定性增长率和自由电子激光的能量转换效率.结果表明,只要合理地选择参量条件,附加导引场可以大大提高自由电子激光的增长率和能量转换效率.     

Behavior of the fast cyclotron wave in a nonuniform magnetic field

The fast cyclotron wave becomes unstable and is excited in a spiral electron beam-plasma system when the perpendicular energy component of the beam is sufficiently large. When a nonuniform magnetic field is applied to the system, the cyclotron frequency as well as the parallel velocity component of the beam vary spatially. It is confirmed experimentally, that the variation affects the excited wave and results in a spatial variation of its wavenumber in the way predicted by the dispersion relation of the fast cyclotron wave.     

Some properties of an electron gas in a quantizing magnetic field

The electron gas neutralized by a rigid positive background and in a quantizing magnetic field, is studied in a ‘quasi-classical’ model previously proposed by the author and collaborators. The plasma oscillation of the electron gas shows an acoustic-type dispersion for small wave vector. The potential behind an ion moving along the direction of the magnetic field is calculated and is found to have a sinusoidal behaviour. Some consequences of this potential are pointed out. The energy loss by a moving charge in the electron gas is shown to exhibit some interesting properties. Other quantities such as light scattering and magneto-acoustic oscillations are also discussed. A derivation is given for a ‘quasi-classical’ linear response in the finite relaxation time approximation.     

Internal dynamics of a plasma propelled across a magnetic field

When a plasma is pushed across a magnetic field by some nonelectromagnetic force, ions and electrons get turned in opposite directions by the magnetic field. This creates an internal current as well as sheaths at the plasma surfaces and results in an electric field which allows the plasma to maintain some, or even most of its initial momentum in the form of E&oarr;×B&oarr; drift. An exact analysis of that process is presented for the internal region of the plasma. The energy provided by the initial push is used, in part, to create some gyrations inside the plasma. When the rest energy density of the plasma exceeds twice the magnetic energy density (or when the Alfven speed is less than c), there will be enough energy to spare for the plasma to continue across the magnetic field at half its initial momentum. Two cases are considered: an impulsive start and a gentle push such as provided by gravity. The amplitude of the resulting internal gyrations becomes small in the second case. The frequencies of the gyrations are those of extraordinary modes of very long spatial wavelength     

Weakly bound electrons in an external magnetic field

The electron behavior in a spherical symmetric potential well in the presence of an external magnetic field is studied. The transcendental equation for the energy spectrum is derived. The energy of the bound state is approximately calculated. The electron wave function and the probability current are found.     

New quantum electron transport phenomena in low-dimensional systems in a magnetic field

In low-dimensional electron systems with an asymmetric quantizing potential in a magnetic field, the electromotive force appears in the presence of a standing acoustic wave [O.V. Kibis, Phys. Lett. A 237 (1998) 292]. The consequence of this quantum macroscopic effect is that homogeneous heating of the electron system leads to the emergence of a phonon drag of electrons, which leads to a new class of electron transport phenomena.