Expected secondary electron yields of a conversion surface are in the range of 01 electrons per incident particle in the energy range under investigation (101000eV per atom). Since these electrons might interfere with the measurement of negative ions a deflection magnet was built which would cause a magnetic field perpendicular to the incoming neutral particle beam and parallel to the surface. It was decided that the gyro radius of a low energy electron (E 2eV) next to the surface should be of the order of 1mm whereas oxygen atoms with an energy of 200eV should not be deflected at all. The necessary strength of the field is calculated by
where denotes the electron mass, the energy of the electron, the elementary charge, the gyro radius, and the magnetic flux density.
The necessary flux density computes to 3.3 mT. At this flux density protons and oxygen atoms are only marginally deflected. For the dimensioning of the magnet a simple approach giving a field of the right order of magnitude was chosen. For an electro-magnet with an iron core, and a small air gap with a moderate and approximately homogeneous field in the air gap the magnetic resistivity is defined by
where is the total magnetic flux, the current in the coil, and the number of turns on the coil. The magnetic resistivity may be written as the sum of the resistivity of the iron core and the resistivity of the air gap:
With the mechanical dimensions of the magnet a magnetomotive force
= 67A was calculated. A copper wire with 0.3 mm diameter
was used for the coil yielding four layers of wire within the available
space and a total of 300 turns. The total resistance is calculated
DC power for the magnet was provided using a circuit shown in Figure 6.4.to a few Ohm. This low impedance was needed to drive the high voltage insulation transformer inside the chamber. This transformer was built using a ferrite ring suitable for the 25 kHz. The primary and secondary coils were made using high voltage cables providing the 25 kV insulation needed. As a drawback only a few turns could be put on the ferrite ring because of the limited space available and the bulky cables. This was compensated by the rather high operating frequency of 25 kHz. On the high voltage side of the transformer a simple rectifier circuit converted the AC back to DC used to power the magnet.
A complete shielding around rectifier, capacitor, and coil of the magnet should minimize the danger of discharges to the high voltage side of the electronics (Figure 6.5).
March 2001 - Martin Wieser, Physikalisches Institut, University of Berne, Switzerland