Interaction of the electron beam load with the KALIF-HELIA induction cavities
The energy delivered by the 6 induction cavities of the pulsed power generator KALIF-HELIA is added by a magnetically insulated voltage adder (MIVA). This adder consists of an inner conductor (negative polarity) which is inserted into the cavity bores (ground)1. The electron beam load is connected to the MIVA by a short coaxial magnetically insulated transmission line (MITL). After the acceleration gap of each induction cavity the surface current on the stepwise tapered cathode stalk and the cylindrical anode bore with constant diameter is measured by 4 azimuthally distributed button shunt monitors2.
The signals of these current monitors show the propagation of the pulse down the MIVA/MITL to the electron beam load and the reflection of the pulse back to the cells due to the rapid turn on of the diode to a final value of approximately 8W. This load impedance change has 2 effects:
1. It strongly reduces the electron vacuum current, i.e. after diode turn-on the cathode current approaches the anode current and
2. Anodes as well as cathode current strongly increase within about 10ns. The voltages induced by this high dI/dt values act against the feed voltage of each induction cells.
Transmission line calculations confirmed these observations. Hence the losses between the pulse forming lines and the load are not due to insulation problems of the induction cores2 but related to the interaction between the fast load impedance change and the induction cavities of the pulse generator.
1. J. Fockler et al., Proc. 9th IEEE Int. Pulsed Power Conf., Albuquerque, vol. 1 (1993) 431
2. P. Hoppé et al., Proc. 13th IEEE Int. Pulsed Power Conf., Las Vegas NV, (2001) 591