Study On Microwave Coupling Of High Charge State ECR Ion Sources

High charge state ECR?Electron Cyclotron Resonance?ion source which is driven by microwave power is the best choice of providing very intense highly-charged heavy ion beams.In order to meet the needs of intense heavy ion beams for the development of modern heavy ion accelerators,the performance of ion source must be further improved.The microwave coupling and ECR heating determines the buildup of high temperature high density plasma,which is crucial to the performance of an ECR ion source.This paper has systematically investigated the relevant mechanism of microwave coupling to high charge state ECR ion sources,and demonstrated the relative key techniques and methods for ion source performance enhancement.In the 24-28 GHz ECR ion sources,microwave power is generally launched directly to heat ECR plasma using TE₀₁ mode via over-moded circular waveguides with the diameter of 32 mm.The commissioning results indicate ECR plasma heating through this traditional microwave coupling scheme is not promising.The produced ion beam intensities are still far from theoretical expectation,typically when high charge state ion beams intensities saturates at high microwave power level,further improvement of the ion source performance is impeded.Therefore,breakthroughs should be made with regard to microwave coupling and heating to ECR plasma,which is also critical for the development of the next generation ECR ion sources.In this research,we starting from the characteristics of ECR plasma stability and microwave absorption efficiency at high microwave power level,the electric field power distribution in the ECR ion source chamber under different microwave coupling modes and waveguide calibers were studied for the first time from the angle of traveling wave.This provides a new idea to interpret microwave coupling and ECR heating in a high charge state ECR ion source.We experimentally studied the microwave coupling effects to the yield of high charge state ion beams with a high performance ECR ion source via different microwave coupling schemes.Through experimental research and simulation analysis,this paper presents a new coupling scheme with small waveguide caliber in TE₀₁ mode.By using this coupling scheme,ECR heating efficiency has been improved and consequently intensity of highly-charged ion beams has been increased by a factor>30%.A number of highly-charged heavy ion beams intensity world records have been created through this technology with the superconducting sources SECRAL and SECRAL-II at Institute of Modern Physics,such as 1.42 emA Ar¹²⁺,1.04 emA Ar¹⁴⁺,1.1 emA Xe²⁶⁺,0.92 emA Xe²⁷⁺and etc.This new coupling scheme has been adopted and validated with 28 GHz VENUS ion source at LBNL in the United States.Apparently,breakthrough on microwave coupling and ECR heating scheme has made significant impact to the performance of 3rd generation ECR ion sources.In order to meet the beam needs for HIAF?High Intensity heavy ion Accelerator Facility?project in China,the Institute of Modern Physics is pioneering the research and development work for a fourth generation ECR ion source,called FECR.A successful 4^th generation ECR ion source has many challenges in both physics and technical issues.Among them,the high efficiency coupling and heating of the ECR plasma with 45 GHz@20 kW microwave power is one of the key issues.In our research,we propose a solution of microwave transmission and coupling scheme by using quasi optics combined with over-moded waveguide technique based on research with the third generation equipment.As a result,high efficiency transmission and launching has be realized.For the first time,a stable 45 GHz ECR plasma has been produced and subsequently intense high charge state ion beams have been extracted.Additionally,we have investigated the possibilities of double frequency heating with 28+45 GHz and three frequency heating with 28+45+18 GHz for the first time.In this attempt,we have made intense ultra-high charge state heavy ion beams through the effective improvement of ECR plasma stability,such as 53e?A Xe³⁸⁺,17 e?A Xe⁴²⁺and etc.Based on the systematic studies on intense highly charged ion beam production through microwave coupling and heating improvement with high performance ECR ion sources,this paper eventually provides the technical proposal on effective coupling and heating of ECR plasma with 45 GHz microwave for the next generation ECR ion sources.