| Terahertz radiation has broad application prospects in many frontier fields.In the development of terahertz science and technology,the research of terahertz source is always the core content.However,according to the research status of terahertz sources at home and abroad,it can be found that there is still a lack of effective and convenient methods to achieve high-power terahertz radiation,which leads to a great limitation on the applications of the related technologies.Therefore,it is urgent to explore a high-power terahertz source with new physical structures and new working mechanisms.Electron beam-plasma system is a new terahertz source,which has unique advantages of high power,high efficiency,compact design and low cost.In this paper,a series of studies are carried out on the mechanisms of terahertz radiation excited by electron beam-plasma system.In the theoretical part,the mechanism of electromagnetic instability excited by the interaction of electron beam,plasma and wave is studied by linear theory,and the formation condition of virtual cathode of electron beam-plasma system is obtained by Gauss’s law,Ampere’s law and energy conservation equation;in the simulation part,the particle in cell（PIC）method is used to study the electron beam-plasma system through nonlinearity.In the experimental part,a compact experimental device is designed and constructed.On this basis,the mechanism of electromagnetic instability excited by the system is verified,which provides experimental support for the theory.The main contents and innovations are as follows:1.Based on the results of PIC simulation,the theoretical model of the inhomogeneous electron beam-ion channel system is established.The dispersion relation of the electromagnetic instability of the system is derived by using the linear theory,and the interaction between various modes in the system and the physical mechanism of terahertz radiation excited by the system are analyzed.There are two kinds of instability in the system:the first appears near the center of the system,which is caused by the coupling of fast space charge wave and forward electromagnetic wave;the second appears in the region with larger radius,which is caused by the coupling of slow space charge wave and electron beam mode.In this paper,a new viewpoint is proposed for the mechanism of electromagnetic radiation excited by the electron beam-ion channel system:the electrons near the boundary of the electron beam first obtain the longitudinal oscillation energy through the beam instability.When these electrons reach the central region along with the self-focusing movement,they will yield the energy through the electromagnetic instability,thus stimulating the electromagnetic radiation.Further research shows that in the electron beam-ion channel system with the increase of plasma density,the frequency and bandwidth of the system excited electromagnetic instability will increase.When the plasma density reaches 1×1022/m3,the radiation frequency will be close to more than 1THz,which is also supported by PIC simulation.2.A new terahertz radiation mechanism is found in the electron beam-plasma system by PIC simulation.The generation of this mechanism must meet the following conditions:（1）the electron beam density is far lower than the plasma density;（2）the electron beam diameter is less than the plasma wavelength;（3）the pulse width is far greater than the plasma oscillation period.Under this condition,the electron beam will drive the plasma wake field in the plasma,and the plasma wakefield will react on the electron beam.Finally,the long electron beam will be modulated into a series of short bunches with intervals equal the plasma wavelength.This process is called"self-modulation".When the electron beam develops to the stage of self-modulation nonlinearity,it can drive the high-frequency electromagnetic radiation with the fundamental frequency near the plasma frequency and a series of high-order harmonics.When the electron beam density is 1×1018/m3and the plasma density is 1×1022/m3,the radiation frequency of the system has reached the terahertz band,and the power density can reach more than 1GW/m2.Through theoretical analysis,it is found that the terahertz radiation mechanism of the system is essentially the resonance scattering of the high frequency electron beam mode on the oscillating electrons in plasma wakefield.3.A terahertz radiation source based on self-modulated electron beam is proposed.In this scheme,the electron beam and plasma are injected into a medium loaded slow wave structure at the same time,so that the electron beam interacts with the higher eigenmodes of a specific frequency of the system through self-modulation.PIC simulation results show that the self-modulated electron beam can selectively excite specific high-order eigenmodes in the slow wave structure,while the low-order eigenmodes with frequencies lower than the plasma frequency cannot exist in the plasma beacause they will be suppressed.It is also found that the growth of the modes in the system is in good agreement with the dispersion relation.Under the current parameters,the peak power of the radiation can reach up to 7.9MW,and the corresponding energy conversion efficiency can be 8.3%.4.The mechanism of virtual cathode based on electron beam-plasma system is studied.The threshold equation of virtual cathode under the mechanism of ion focusing is established,and the parameter region of virtual cathode formation is obtained by solving the equation.If the initial parameters of the system are in this region,a virtual cathode will be formed.In this study,a large number of sampling points are selected inside and outside the virtual cathode parameter area and near the threshold.Through PIC simulation,it is found that the theoretical results are highly consistent with the simulation results.In this series of simulation,there is no virtual cathode to excite electromagnetic radiation.That is because the current simulation does not consider the actual electron beam cathode,so the reflected current of virtual cathode will not further form the oscillating current,and excite the radiation.However,a mechanism of spontaneous oscillation emission of virtual cathode is found in this study when the electron beam density is lower than the plasma density.In this case,the ion channel will become a closed cavity,and the reflected current of the virtual cathode will be bound in the channel to form the oscillating current,thus stimulating the broadband radiation.Under the current parameter conditions,the radiation spectrum has covered the THz band.5.An experimental scheme of high density electron beam-ion channel system driven by high voltage pulse power is proposed.According to this scheme,a set of"beam-plasma system（PBS）"experimental device is designed and established.The radiation mechanism of electron beam ion channel system is verified on PBS.The plasma density is 1017/m31018/m3,the pulse voltage is 20kV,and the pulse width is20μs.In the experiment,when the oscilloscope detects the rising edge signal of the voltage pulse,the high frequency signal in the frequency range of the plasma appears in the radiation spectrum of the system,and the peak value of the spectrum appears at about 8GHz.It can be found through comparative studies that the high frequency signal is neither the plasma noise nor the intrinsic mode excited by the voltage pulse in the vacuum chamber,so only the interaction between the voltage pulse and the plasma is possible.As the experimental radiation frequency is basically the same as the theoretical radiation frequency,the theory proposed in this paper is supported by the experimental results.Theoretically,as long as the plasma density is further increased,the radiation with higher frequency or even THz band can be obtained.Through the present research,various terahertz radiation mechanisms of electron beam-plasma system under different parameters are revealed.Some new mechanisms has been discussed in this paper,which are the mode coupling mechanism in electron beam-ion channel system,resonance radiation mechanism induced by self-modulated beam and plasma wakefield and virtual cathode mechanism.This paper also provides a new idea and theoretical basis for the research and development of high-power terahertz source. |
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