Ponderomotive effects in intense pumping wave action on electron and plasma bunches

Ponderomotive effects that arise when an intense plane pumping wave acts on low-concentration electron and plasma bunches are theoretically studied within the framework of a one-dimensional model. Using the Lagrange variables, an electron (plasma) bunch under the action of a pumping field can be represented as a gas comprising macroparticles with ponderomotive and Coulomb interactions. The ponderomotive force at small interparticle distances is attractive, that is, directed oppositely to the Coulomb force; it cannot, however, completely balance the latter. The constructed model is used to study superradiance, which arises when an intense pumping wave acts on an extended electron bunch. Radiation is then scattered in the form of narrow pulses whose amplitude is proportional to the total number of particles in the bunch. In addition, we describe acceleration of a neutral plasma layer, narrow on the wavelength scale, in the field of an intense wave and radiation field-induced partial contraction of an electron bunch with an incompletely compensated charge.     

Detection of spin-polarized electrons injected into a two-dimensional electron gas

The spin-dependent mean-free path of electrons in a high-mobility InAs two-dimensional electron gas (2DEG) is measured. Ferromagnetic metal/insulator/2DEG junctions are fabricated on a common channel in a nonlocal geometry and used as spin injectors and detectors. For electrons in spin-orbit eigenstates at 4.5 K, lower bounds for the spin mean-free path and relaxation time are Lambda(S) > or = 4.6 microm and tau(s) > or = 3.8 ps, respectively. The temperature dependence is weak over the range 4.5相似文献   

Dynamically stable electron bunches in beam interaction with an electromagnetic wave packet

Nonlinear interaction of electron beam with a whistler wave packet that effectively dissipates through collisions or wave leakage is studied. Independently of the dissipation type and nature of waves, self-organization of the beam structure leads to the formation of bunches continuously decelerated by waves. Strong dissipation prevents phase mixing required for the quasilinear theory and keeps wave phases in the packet correlated. Thus, dynamically stable bunches are present together with a plateau in the velocity distribution; asymptotically, wave emission by bunches is the main process.     

Laser-driven collisions of electron bunches

A novel scheme allowing for relativistic collisions of laser-accelerated electrons is introduced. Two spatially separated electron bunches are driven in opposite directions by two counterpropagating laser pulses until they reach the point of collision which lies within the laser fields. This method can be employed to accelerate electrons to the maximum kinetic energy which can be transferred to charged particles by plane propagating laser fields. Due to the symmetric setup, the center of momentum is at rest with respect to the laser propagation direction such that virtually the whole kinetic energy is available for particle reactions.     

Phase motion of particles upon formation of subpicosecond electron bunches in a traveling wave under heavy beam loading

The possibility of efficient bunch compression due to the phase motion of particles in the field of a traveling wave is demonstrated. The calculated bunch length is 120 μm, which makes it possible to expect spontaneous coherent radiation from the undulator at a wavelength of 240 μm.     

Acceleration and compression of charged particle bunches usingcounterpropagating laser beams

The nonlinear interaction between counterpropagating laser beams in a plasma results in the generation of large (enhanced) plasma wakes. The two beams need to be slightly detuned in frequency, and one of them has to be ultrashort (shorter than a plasma period). Thus produced wakes have a phase velocity close to the speed of light and can be used for acceleration and compression of charged bunches, The physical mechanism responsible for the enhanced wake generation is qualitatively described and compared with the conventional laser wakefield mechanism. We also demonstrate that depending on the sign of the frequency difference between the lasers, the enhanced wake can be used as a “snow-plow” to accelerate and compress either positively or negatively charged bunches. This ability can be used in an electron-positron injector     

Emission of relativistic electron bunches in a circular diaphragmatic waveguide

The longitudinally uniform circular diaphragmatic waveguide (CDW) was considered. Expressions for determining the loaded Q-factor of lossy (Q _L1) and lossless (Q _L1⁰) cells and the loaded Q-factor Q _L of a section of length l were derived. An expression for determining the electrical amplitude of the radiation field produced by a relativistic bunch with charge q, moving along the axis of CDW with series resistance R _s was derived. The electron beam energy, radiation power, and electronic efficiency were calculated.     

Plasma-density-gradient injection of low absolute-momentum-spread electron bunches

Plasma density gradients in a gas jet were used to control the wake phase velocity and trapping threshold in a laser wakefield accelerator, producing stable electron bunches with longitudinal and transverse momentum spreads more than 10 times lower than in previous experiments (0.17 and 0.02 MeV/c FWHM, respectively) and with central momenta of 0.76+/-0.02 MeV/c. Transition radiation measurements combined with simulations indicated that the bunches can be used as a wakefield accelerator injector to produce stable beams with 0.2 MeV/c-class momentum spread at high energies.     

Collective properties of a relativistic electron beam injected into a high intensity optical lattice

Behaviour of a relativistic electron bunch, injected and trapped in a high intensity optical lattice resulting from the interference of two laser beams is studied. The optical lattice modifies the phase space distribution of the electron bunch due to the trapping and compression of the electrons by a ponderomotive force. High-frequency longitudinal beam eigenmodes of the trapped electron bunch are described in the framework of fluid and kinetic models. Such beam oscillations are expected to play a pivotal role in a stimulated Raman scattering of laser beams on the electrons.     

Laser wakefield acceleration of short electron bunches

The theory of electron acceleration in a plasma wake wave is developed, and the dependence of the main characteristics of accelerated electron bunches on the wakefield parameters is investigated, It is shown that using a prebunching stage, under proper conditions, the final electron density of a compressed and accelerated bunch can exceed the initial electron beam density by orders of magnitude and that longitudinal bunch compression provides quasi-monoenergetic acceleration to high energies, It is demonstrated that, for an initial electron beam radius smaller than the optimal one for efficient beam trapping, the energy spread of the compressed and accelerated electron bunch and its length can be evaluated by using the simple analytical predictions of a one-dimensional (1-D) theory. The obtained analytical results are confirmed by three-dimensional (3-D) numerical modeling     

Compression of subrelativistic space-charge-dominated electron bunches for single-shot femtosecond electron diffraction

We demonstrate the compression of 95 keV, space-charge-dominated electron bunches to sub-100 fs durations. These bunches have sufficient charge (200 fC) and are of sufficient quality to capture a diffraction pattern with a single shot, which we demonstrate by a diffraction experiment on a polycrystalline gold foil. Compression is realized by means of velocity bunching by inverting the positive space-charge-induced velocity chirp. This inversion is induced by the oscillatory longitudinal electric field of a 3 GHz radio-frequency cavity. The arrival time jitter is measured to be 80 fs.     

Benchmarking of electro-optic monitors for femtosecond electron bunches

The longitudinal profiles of ultrashort relativistic electron bunches at the soft x-ray free-electron laser FLASH have been investigated using two single-shot detection schemes: an electro-optic (EO) detector measuring the Coulomb field of the bunch and a radio-frequency structure transforming the charge distribution into a transverse streak. A comparison permits an absolute calibration of the EO technique. EO signals as short as 60 fs (rms) have been observed, which is a new record in the EO detection of single electron bunches and close to the limit given by the EO material properties.     

Excitation of upper-hybrid plasma wake wave by a low-frequency extraordinary electromagnetic wave

The excitation of upper-hybrid wake electrostatic wave by interaction of an extraordinary Gaussian wave propagating perpendicular to the external magnetic field in a cold homogeneous plasma is investigated using magnetohydrodynamics theory. The plasma oscillations can be excited due to the charge separation appeared by the ponderomotive force of the electromagnetic wave whose frequency is considered in the lower pass band. By obtaining the equation governing the plasma wake, the dependency of the wake amplitude on the physical parameters is studied. It is observed that larger wake oscillation takes place when the pulse length is approximately close to 3λ_p/π and the X-wave frequency is greater than ω_p, which means that the phase velocity is less than the speed of light in vacuum (v_p < c).     

Generation of powerful subnanosecond microwave pulses by intense electron bunches moving in a periodic backward wave structure in the superradiative regime

Experimental results of the observation of coherent stimulated radiation from subnanosecond electron bunches moving through a periodic waveguide and interacting with a backward propagating wave are presented. The subnanosecond microwave pulses in Ka and W bands were generated with repetition frequencies of up to 25 Hz. The mechanism of microwave pulse generation was associated with self-bunching, and the mutual influence of different parts of the electron pulse due to slippage of the wave with respect to the electrons; this can be interpreted as superradiance. The illumination of a panel of neon bulbs resulted in a finely structured pattern corresponding to the excitation of the TM01 mode. Observation of rf breakdown of ambient air, as well as direct measurements by hot-carrier germanium detectors, leads to an estimate of the absolute peak power as high as 60 MW for the 300-ps pulses at 38 GHz. These results are compared with numerical simulations. The initial observation of 75-GHz, 10-15-MW radiation pulses with a duration of less than 150 ps is also reported.     

Optimal energy-phase correlation for superradiance of intense electron bunches

Investigated is the optimal energy-phase correlation for superradiance of an intense electron bunch in an undulator field. It is noted that the superradiant emission saturates due to the bunch self-field at an energy proportional to the total charge in the bunch.     

Transition radiation of nonrelativistic electron bunches passing through diaphragms

Experiments on generating electromagnetic pulses (EMPs) by using nonrelativistic electron bunches passing through diaphragms of various radii are described. Results obtained agree with the predictions of the transition radiation theory as applied to nonrelativistic charged particles passing through apertured conducting screens. They may also be used in designing devices for generating EMPs with tunable characteristics, as well as in developing a transition radiation theory for electrodynamic structures of complex geometries.     

Trapping, compression, and acceleration of an electron beam by the laser wake wave

The scheme of laser wake-field acceleration in plasma is proposed and considered for the case where a relatively rare nonrelativistic or weakly relativistic electron beam is initially situated ahead of the intense laser pulse. It is shown that an electron beam is trapped in the region of the first accelerating wake maximum; then it is strongly compressed and accelerated to ultrarelativistic energies.     

Transition radiation of surface electromagnetic waves by electron bunches in a cylindrical waveguide

Transition surface electromagnetic radiation from electron bunches that cross a metal-insulator-semiconductor structure in a perfectly conducting semi-infinite cylindrical waveguide is studied. It is shown that, using a periodic sequence (train) of electron bunches, a particular surface waveguide eigenmode can be amplified by bringing its frequency to resonance with the bunch repetition frequency in the train. Those eigenmodes are amplified most efficiently whose frequency falls into the range occupied by the first maximum of the geometric factor of one bunch.     

Generation of quasimonoenergetic electron bunches with 80-fs laser pulses

Highly collimated, quasimonoenergetic multi-MeV electron bunches were generated by the interaction of tightly focused, 80-fs laser pulses in a high-pressure gas jet. These monoenergetic bunches are characteristic of wakefield acceleration in the highly nonlinear wave breaking regime, which was previously thought to be accessible only by much shorter laser pulses in thinner plasmas. In our experiment, the initially long laser pulse was modified in underdense plasma to match the necessary conditions. This picture is confirmed by semianalytical scaling laws and 3D particle-in-cell simulations. Our results show that laser-plasma interaction can drive itself towards this type of laser wakefield acceleration even if the initial laser and plasma parameters are outside the required regime.