Velocity Map Imaging Spectrometer To Study The Space Charge Effects Of Charged Particle Beams

Space charge effect(SCE)is a collective effect resulted from Coulomb interaction between the charged particles,which drives the Coulomb potential energy of particles rapidly converting to the kinetic energy.It is therefore one of the main reasons for the heating and de-coherence of the charged particle beam.The development of the ultracold electron/ion source and beam technologies requires the further understanding of SCE.However,it is so far limited by the experiments due to the complexity of the multi-body interaction process,the low accuracy,and the disturbance from the thermal expansion.In this work,we have performed the ion velocity map imaging(VMI)experiments on multiphoton ionization of fullerene molecules and the single photon ionization of cold atoms trapped in the magneto-optical trap(MOT).The critical charged particle density of SCE is reduced by one order of magnitude,and the expansion dynamics of low density ultracold ion beams was quantitatively described.The results provide an important reference for the development of ultracold charged particle beam and the study of expansion dynamics of ultracold neutral plasmas.For the multiphoton ionization process of fullerene molecules,we have carried out experiment of⁺₆₀ and⁺₇₀ ions with different delayed times using time-sliced VMI,and measured the energy broadening and thermionic emission behavior of the fullerene molecular ions.As a result,the SCE was observed under the density threshold of 3×10⁷ cm^-3.It was one order of magnitude lower than the value of 3×10⁸cm^-3which was observed in time-of-flight mass spectrometry(TOF-MS)experiment,and the new threshold provided a benchmark for precise measurement in multiphoton ionization experiments.Meanwhile,the species and the origin of detected ions were distinguished in experiment,as well as the contamination from fragment ions was eliminated.So,the dependence of contamination ratio with delay time was presented,and it was of significance for improving determination precisions of some basic decay parameters of neutral fullerene molecules.On the other hand,we have performed the COMSOL software simulation and the experimental study on SCE in rarefied ultra-cold ion beams based on the magneto-optical trap with ion velocity imaging(MOT-VMI)platform.The experiment was comprehensively simulated using COMSOL software,and the reason for the distortion in the image of MOT-VIM was demonstrated resulting from the asymmetric fringe field effect.Then,the MOT-VMI system was calibrated using the two-color two-photon resonant ionization of Rb atoms,and the result was consistent with the COMSOL simulations.Finally,the experiment of velocity imaging of ion beams with an initial density as low as 3×10⁷ cm^-3was carried out,and the expansion process of the ion beam driven by SCE was observed.A theoretical model was provided to calculate the expansion of single-component charged particle beam driven by SCE.The combination of theoretical calculation and experimental observation quantitatively describes the expansion dynamics of low-density ultra-cold ion beams.In brief,we measured the SCE in the low-density fullerene ion beam and the ultra-cold rubidium ion beam using VMI technology,and extended the understanding of SCE to the new charge density threshold.The results laid a foundation for the development of ultracold ion beams and the research of expansion dynamics of ultracold neutral plasmas.