SIMULATION OF H - ION EXTRACTION IN CESIATED AND NON- CESIATED ION SOURCES V.K. Senecha # , R.M. Vadjikar and Ajeet Kumar Raja Ramanna Centre for Advanced Technology, Indore – 452 013, INDIA Abstract The two mechanisms of H - ions formation are through volume production and surface production processes. The design of a high current H - ion source can utilize both volume and surface production processes. A controlled Cs insertion helps in enhancing the H - ion current. Simulation results for H - ion extraction are presented, from plasma region to RFQ entrance using the software nIGUN, which consider the role of sheath formation in the plasma chamber near the chamber wall close to the extraction aperture. Presence of various ion species like H + ,H 2 + ,H 3 + , H - ,e and Cs + etc. have been considered for modelling of sheath and effect of space charge in the extraction region. Further, it has been shown that proper biasing of plasma electrode in the non-Cesiated ion source helps in building up the vibrationally excited H 2 molecules resulting in higher H - ion extraction INTRODUCTION The high intensity high current H - injector linac development for spallation neutron source (SNS) [1] requires a high brightness H - ion source. In order to design an H - ion source of 50 keV energy with 35 mA current,0.5 ms pulse duration 25 Hz pulse repetition rate the ion beam extraction and transport is of vital importance. nIGUN simulation code has been specifically developed for this purpose[2-3]. Presently attempts are being made to exploit the hybrid design of the ion source that can exploit both volume and surface production of negative ions. The role of Cs monolayer helps in enhancing the H - ion current through surface production. The plasma electrode and collar used near the extraction aperture are generally used for surface production through controlled Cs vapour deposition on their surfaces. H + Ions and H atoms impact these surfaces with a low work function to generate H - ions through cesium. The dissociative electron attachment cross section of highly excited molecular hydrogen is extremely large. Due to this thermal electrons can be easily attached to vibrationally or rotationally excited hydrogen molecules for negative hydrogen ion production. Volume produced H - ions are produced in a two step process. In the first stage vibrationally excited hydrogen molecules are produced by the interaction of hydrogen molecules with energetic electrons. In the second stage H - ions are formed from the interaction of vibrationally excited hydrogen molecules with slow electrons. The destruction of H - ions can also occur, but the cross section for this process is smaller at low electron energies. The extraction of H - ions occur in chamber with low electron temperature. This chamber is separated from a high temperature plasma discharge chamber by a magnetic filter field. The magnetic filter restricts the energetic electrons and allows the passage of slow electrons. The separation of energetic electrons from the slow electron by the magnetic filter reduces the destruction of H - ions. The magnetic filter field penetrates the extraction chamber. The design can be done with or without a collar. A collar design is generally preferred. The electrons can be dumped in the collar, in the gap between the plasma aperture and the extractor or behind the extractor. Various extraction geometries for the cesiated and non-cesiated ion sources have been simulated with the help of nIGUN 2D software capable to consider the space charge effect as well as inverted sheath formation near the extraction region to try and optimize the ion beam parameters. NIGUN-2D SIMULATION The program nIGUN utilizes a new theory of multi- particle plasma sheath for H - ion extraction[3] which has been used to modify the positive ion extraction program [4,5]. The stability of convergence is obtained for the nonlinear sheath region by directly inserting the analytical space charge according to the Poisson equation for each mesh point above the wall potential. The field strength in the extraction gap is used to determine the densities of all the species by the program. nIGUN has built in boundary processing to define internal electrodes and Neumann and dielectric boundaries. Close to extraction region the positive ions, protons and Cs ions from the plasma source are reflected by the extraction field resulting in a creation of virtual cathode. It is assumed in the program that the space charge of H - ions counteracts the extraction field without creating a saddle point of potentials. It uses the finite difference method (FDM) for the discretization of Poisson equation on a quadratic mesh either in cylindrical or XY Cartesian coordinates. The line successive over relaxation (LSOR) solver is used in radial direction allowing a fast convergence of problem with high space charge in the beams. nISHEATH program is first used for setting up the initial input variables to enable the inverted sheath near extraction region for H - ion and electron extraction. If a virtual cathode is developed in the extraction region due to initial set of particle energy and densities used along with a set of potential function considered for various electrode shape and geometry then program execution _____ ____________ # senecha@rrcat.gov.in