Expression of cusp loss width derived by Bosch and Merlino is applied to JAERI’s Kamaboko source. The width is related to the ambipolar diffusion coefficient across the cusp magnetic field. Electron–ion collision is found 1.2–7.4 times larger as compared with electron-neutral collision. Averaged cusp magnetic field in the diffusion coefficient is taken as a parameter in the simulation code for Kamaboko source. When the averaged magnetic field is 48 G, simulation results agree well with JAERI’s experiment in a wide range of pressure and arc power variation. The value of 48 G is reasonable from the consideration of confining the equation of ion source plasma. The obtained width is about 10 times the value evaluated by two times ion Larmor radius on the surface of cusp magnet.
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