Studies On Ion Beam Extraction From High-intensity Highly Charged ECR Ion Sources And The Space Charge Effect

Electron cyclotron resonance（ECR）ion sources are the most frequently used devices for delivering high-intensity highly-charged ion beams to modern heavy ion accelerator installations.As the accelerator’s demand for ion beam intensity gradually increases,the space charge effect of the ion beam will have a more serious impact on the transmission of the beams in the low energy range and the beam quality.In addition,the complex magnetic fields,electric fields,ion distribution and other factors in the extraction region of the high-intensity high-charge-state ECR ion sources will affect the beam quality of the extracted ion beams as well.In this paper,systematic experimental studies on the space charge effect and space charge compensation degree（SCCD）of ion beams are carried out,and detailed theoretical studies with preliminary experimental verifications of the extraction of high-intensity highly-charged ion beams are made.The space charge effect of the beams will lead to the increases of the beam envelope and the projected emittance of the beams,which is an important factor affecting the beam transmission and beam quality,especially for low-energy high-intensity ion beams.The ion beams react with the remaining gas in the tube,producing secondary electrons that will partially compensate for the space charge effect of the beams.In this paper,using a three-grid energy analyzer,a fluorescent target,a beam profile monitor,and other devices,for He⁺,Ar⁺,O⁶⁺and other single charge state ion beams and mixed charge states oxygen ion beams,the secondary ion energy distributions of the beams were measured respectively.For the first time,a beam space charge model is established based on the two-dimensional current distribution information of the beam profile,to obtain a more accurate beam space charge compensation degree than the uniform distribution model commonly used in other studies.The experimentally measured SCCDs of 5.4 em A He⁺beams,1.08em A Ar⁺beams,0.53 em A O⁶⁺beams,and 1.02 em A O⁶⁺beams are about 81%,74%,46%and 62%,respectively,while the SCCDs of 1～7 em A hybrid oxygen beams are about 70%.These results demonstrate that the degree of space charge self-compensation for high-current ion beams（especially for hybrid beams）is higher than previously measured and predicted numbers.These results demonstrate that the degree of space charge self-compensation for high-current ion beams（especially for hybrid beams）obtained by the calculation of the actual current distribution of the beam current is higher and more accurate than the uniform beam current calculation usually used in previous studies.In addition,these results are also closer to the 70%to 90%space charge compensation level commonly estimated in the ion source industry before.There are many factors that need to be considered in the simulation of beam extraction from the high-intensity high-charge-state ECR ion sources,including the initial beam distribution,the hexapole and axial magnetic fields for the confinement of the ion source plasma,the electric fields formed by the extraction electrode structure,the space charge fields of the beams,the magnetic fields of the focusing solenoids,etc.In this paper,the 3D extraction model of the SECRAL（Superconducting ECR ion source with Advanced design in Lanzhou）ion source is established for the first time using the IBSimu program,and the influences of the main physical parameters of the extraction system of the ion source on the quality of the extracted beams are analyzed in detail through simulation calculation.Combined with the simulation results,we experimentally verified the influences of the distances between the extraction electrodes and extraction voltages of the ion source on the beam quality.In addition,we also designed and optimized the 50 k V four-electrode extraction system of the FECR（the First Fourth generation ECR ion source）ion source through simulation studies,which will provide a solution for the extraction of hybrid beams with a total current intensity of 20～50 em A of the FECR ion source in the future.