Simulation of the extraction from an electron cyclotron resonance ion source under the influence of space charge

Abstract

The typical extracted particle density from an ion source of the electron cyclotron resonance (ECR) type has been increased during the last few years by several improvements: stronger magnetic fields, higher radio frequency, biased probes, mixing gas, afterglow mode, and other means. The extraction system remained unchanged in most cases, causing problems in beam quality because of the stronger space charge. Simulation of the extraction is helpful in understanding the physics, but a correct simulation requires a three-dimensional model. Whereas the geometry and the solenoidal component of the magnetic field is cylinder symmetric, the hexapole field determines indirectly the spatial distribution of the ions by Coulomb interaction between electrons and ions. The area where ions are started for the simulation depends therefore on the hexapole field strength. If higher energy electrons within the plasma are present, they should be included to describe the actual plasma boundary more precise. The measured charge state distribution should be used to define the real composition of the plasma. Using all these ingredients, an accel-decel extraction system has been investigated which should be able to handle higher ion currents as were available from ECR sources so far. It could be shown that the emittance of the extracted beam strongly depends on a good matching of the particle density within the plasma to the extraction field strength. Any nonlinearity of the fields causes emittance growth. Such a nonlinearity is produced by an azimuthal change in the location of the plasma boundary. Therefore, it would be highly desirable to decrease the hexapole field close to the plasma side of the extraction system. A much better beam quality would be achievable.