| High energy electrons and short-pulsed X-rays play an important role in particle physic-s,nuclear physics,material physics and other fields.The traditional accelerators can provide energetic,monochromatic and low-emittance high quality electron beams.The synchrotron ra-diation sources and free electron lasers,which are based on these high-energy electron beams,can also produce short-pulsed X-rays that satisfy the requirements of many applications.Un-fortunately,they are all limited by the volume and cost of high-energy accelerator and radiation device.The longing for a new compact,low-cost,high-energy electron accelerator and high quality short-pulsed X-ray source has been a long time.After the appearance of ultra short and ultra intense laser pulses,the mechanisms of electron acceleration and the generation of short-pulsed X-ray sources have sprung up like bamboo shoots after the rain.But the most outstanding one,which can simultaneously be an electron accelerator and a high-quality short-pulsed X-ray source,is laser wakefield acceleration(LWFA).It takes advantage of the bubble produced by ponderomotive force in the tail of the laser,when the ultra short ultra intense laser pulse transmit the plasma.The longitudinal electric field in the bubble can accelerate electrons under the acceleration gradient of～100 GV/m,while the transverse focusing force can make the electron reciprocate and radiate the X-rays.During the research of laser-plasma interaction,the vacuum direct laser acceleration is also investigated.Although the electron can not obtain net energy gain from the ideal plane wave,the space-time symmetry of the plane electromagnetic wave can be broken through extra field,and make the electron be accelerated.As early as 2004,Liu found that electrons can gain energy in laser field and external axial magnetic field in the numerical simulation,but didn’t analyze this effect carefully.In this paper,the motion of electron in laser field and external axial magnetic field is an-alyzed in detail.It is found that when the laser frequency that electron feels in the laboratory coordinate system consists with the relativistic electron cyclotron frequency in the external mag-netic field,there appears a special phenomenon:resonance.At that time,the electron will be continuously accelerated.This is called laser magnetic resonant acceleration(LMRA).In the process of resonant acceleration,the electron will not lose phase,but adaptively accelerates with the highest acceleration rate,which can reach up to～100 GV/cm.The average accelera-tion gradient is proportional to the laser intensity a02/3 and the acceleration length L-1/3.When the deviation from resonance occurs,the electron will dephase.In spite of this,the dephasing length is so long that the deviation of magnetic field within 20%and the injection angle puts off within 10° will not pull the electron into dephasing in l m.Owing to the independence between resonance condition and laser intensity,the resonance acceleration process can still occur under the condition of actual focused laser pulse.And the electron beam obtained by LMRA has a good collimation and quasi monochromaticity.All these characteristics indicate that this accel-eration mechanism can produce high quality electron beams.What’s more,short-pulsed X-rays of the class of synchrotron radiation will be released due to the rotation of small transverse ra-dius and high-speed longitudinal drift during the acceleration process.The coherence in time of the radiation mainly depends on the quality of the injected electron beam.For the electron beam of Maxwell distribution,the radiated X-rays is basically incoherent.In addition,the μm scale small-focal-spot and short-pulsed X-ray source has a very special feature:a hollow ring focal spot.This kind of X-rays can play an important role in imaging contains objects to be protected,inertial confinement fusion,and so on.Laser magnetic resonance accelerator may also be a good desktop synchrotron source. |
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