Effects of external magnetic filter on behaviors of fast primary electrons in a volume negative ion source.
The relationship between enhancement of hydrogen negative ion H(-), volume production, and behaviors of fast primary electrons is studied theoretically. Trajectories of fast primary electrons are calculated, including collisional effects with hydrogen molecules. Active region of fast electrons depends strongly on the field intensity of the magnetic filter (MF), B(MF), and filament position. Therefore, spatial distributions of ionization and vibrational excitation collision points are also affected with changing B(MF). Although number of vibrational excitation collision points is much smaller than that of ionization collision points due to different mean free path, the relative intensity is varied by changing energy of fast electrons. According to the results and discussions including transport of produced vibrationally excited molecules, for enhancement of H(-) production, it is desired that fast electrons should approach the MF position and move into the downstream region as deep as possible without destruction of H(-) ions. To realize these conditions, good combination between filament position and the intensity of the MF should be required.
Improvement of charged particles transport across a transverse magnetic filter field by electrostatic trapping of magnetized electrons
A study on the transport of charged particles across a magnetic filter field has been carried out in a double plasma device (DPD) and presented in this manuscript. The DPD is virtually divided into two parts viz. source and target regions by a transverse magnetic field (TMF) which is constructed by inserting strontium ferrite magnets into two stainless steel rectangular tubes. Plasma electrons are magnetized but ions are unmagnetized inside the TMF region. Negative voltages are applied to the TMF tubes in order to reduce the loss of electrons towards them. Plasma is produced in the source region by filament discharge method and allowed to flow towards the target region through this negatively biased TMF. It is observed that in the target region, plasma density can be increased and electron temperature decreased with the help of negatively biased TMF. This observation is beneficial for negative ion source development. Plasma diffusion across the negatively biased TMF follows Bohm or anomalous diffusion process when negative bias voltage is very less. At higher negative bias, diffusion coefficient starts deviating from the Bohm diffusion value, associated with enhanced plasma flow in the target region.
The REX-ISOLDE project
The Radioactive Beam Experiment REX-ISOLDE [1–3] is a pilot experiment at ISOLDE (CERN) testing the new concept of post acceleration of radioactive ion beams by using charge breeding of the ions in a high charge state ion source and the efficient acceleration of the highly charged ions in a short LINAC using modern ion accelerator structures. In order to prepare the ions for the experiments singly charged radioactive ions from the on-line mass separator ISOLDE will be cooled and bunched in a Penning trap, charge bred in an electron beam ion source (EBIS) and finally accelerated in the LINAC. The LINAC consists of a radiofrequency quadrupole (RFQ) accelerator, which accelerates the ions up to 0.3 MeV/u, an interdigital H-type (IH) structure with a final energy between 1.1 and 1.2 MeV/u and three seven gap resonators, which allow the variation of the final energy. With an energy of the radioactive beams between 0.8 MeV/u and 2.2 MeV/u a wide range of experiments in the field of nuclear spectroscopy, astrophysics and solid state physics will be addressed by REX-ISOLDE.