Studies On The Process And Mechanism Of Plasma Cleaning For Superconducting Half Wave Resonator

By nearly half a century development,superconducting radio frequency（SRF）cavities have become the essential components in SRF particle accelerators,which provide energy for the charged particles.Compared to the normal conducting cavities,SRF cavities have the advantages of much higher accelerating gradient and much lower power dissipation.However,the performance of the state-of-the-art SRF cavities is mainly limited by the field emission effect inside the cavities,which is typically caused by the inner surface contaminations of the cavities,such as foreign metallic particles and dust particulates etc.,as has been observed over the years from experimental data,these contaminants can be removed by high pressure rinsing.However,the foreign chemical components inclusion such as hydrocarbons and absorbed residual gas are hardly removed by the standard cleaning methods.Many efforts have been made by the SRF community for the development of the in-situ techniques to improve the accelerating gradient of SRF cavities in cryomodules.Nowadays,the high power pulse conditioning technique and helium processing technique are routinely used,whose functions are attributed to the modification of the surface morphology of the cavity.However,the contaminants covering on the inner surface of SRF cavity cannot be removed completely,and therefore,the improvements of the performance of SRF cavities processed by these techniques are still limited.Recently,the in-situ plasma cleaning technique was developed by using the chemical reactive oxygen plasma to remove the hydrocarbon contaminants in the outmost layer of the inner surface of the multi-cell elliptical cavity,which is effective to romove chimiacal contaninants and rercover the performance of SRF cavities.The Chinese-Accelerator Driven System（C-ADS）has been developing for the treatment of the nuclear wastes at Institute of Modern Physics（IMP）,Chinese Academy of Sciences.As the first stage,a 25 MeV superconducting proton linac injector has been constructed and commissioned successfully for the continuous wave proton beams with the current of 170μA.However,the operation stability of this linac was mainly limited by the trips of the HWR cavities caused by field emission.In order to mitigate the field emission issues in the HWR cavities,the in-situ plasma cleaning technique was proposed at IMP.Firstly,for the purpose of cleaning the HWR cavity in the cryomodule by plasma,an offline apparatus for the plasma discharge study was set up,which duplicates the assembly of the cavity inside the cryomodule.Plasma ignition procedures were investigated.The plasma parameters such as free electron number density,electron temperature and dissociated oxygen atom intensity were diagnosed by optical emission spectra to optimize the operation conditions for plasma in-situ cleaning of the HWR cavity.Secondly,the sample studies were carried out,where the samples were cut out from the niobium sheets and prepared by the standard surface treatment of SRF cavities.The depth profile of chemical components and the corresponding work function of the sample sputtered by ions were identified from the XPS and UPS tests.The results of depth profile analysis reveal that the value of work function increases with the decrease of carbon contaminants,but increased as the increase of niobium oxide.Furthermore,the quantitative analysis indicated that the work function value is degraded linearly with the concentration of carbon contaminant,where the outmost layer of niobium oxide is Nb₂O₅.Thirdly,the characteristics of the work function variation of the samples processed by the in-situ plasma cleaning technique were investigated on the platform that integrated with an ICP plasma source and Kelven probe.It validates that the chemical activity of the oxygen is significant to remove the carbon contaminants from the niobium surface,therefore,increases the work function value and relieve the field emission effect.From the exponential dependence of work function on cleaning time,a model of the cleaning rate coefficient of contamination was established,which can explain the mechanism of plasma interaction with hydrocarbons.The model analyzed quantitatively for the contribution of ion flux and oxygen content in the Ar/O2 plasma to the cleaning rate coefficient..And the mechanism of the oxygen active plasma interaction with contaminant was identified as ion synergistic chemical sputtering.Fourthly,the experiments were performed on the vertical test stand to evaluate the impact of the carbon contaminants on the performance of SRF cavity,.According to the difference in the bonding energy between contaminants and niobium surface,the contamination mechanism were classified into two types,i.e.,physical adsorption with weak strength and chemical deposition with strong strength.For the weak strength condition,the methane was injected into the SRF cavity during vertical test to make a cryogenic adsorption layer on the inner surface of the cavity.The results revealed that the performance of SRF cavity degraded by methane physical adsorption,which enhanced the field emission effect inside the cavity,but the performance can be recovered by thermal cycle the cavity to 300K and pump methane out.For the strong strength condition,the chemical deposited dirty layer of carbon contamination was produced by using of Ar/CH₄ mixed plasma enhanced chemical vapor deposition method,the SRF cavity performance was deteriorated by the severe field emission,which cannot be recovered by the traditional RF conditioning technique.Finally,the Ar/O₂ in-situ plasma cleaning technique was implemented on the carbon deposited cavity above.The RGA data showed that the hydrocarbon contaminants on the inner surface of the cavity were removed,and vertical test results indicated that the maximal available accelerating gradient of the cavity was improved by 29%due to the mitigation of field emission effect inside the cavity after plasma cleaning.Moreover,in order to check the efficiency of plasma cleaning to the real online contamination of SRF cavities,the air was feed into the cavity directly,which led to the collapse of the cavity performance.Thus,the plasma cleaning was used again for the cavity,and its performance was certain recovered.