CURRENT MONITOR FOR THE ISIS SYNCHROTRON RF CAVITY BIAS
A. Daly, C.W. Appelbee, D.L. Bayley
STFC/RAL/ISIS, Chilton, Didcot, Oxon, UK
The ISIS facility at the Rutherford Appleton Laboratory
in the UK is currently the world’s most intense pulsed
neutron source. The accelerator consists of a 70 MeV H-
linac and an 800 MeV, 50 Hz, proton synchrotron. The
synchrotron beam is accelerated using six, fundamental
harmonic, ferrite loaded, RF cavities each having its own
high voltage RF drive amplifier and bias system. Each of
these RF cavities is driven as a high Q tuned RF circuit;
the resonant frequency being controlled by passing a
current through a bias winding. This current comes from
the cavity’s own Bias Regulator system which consists in
part of eight banks of 10 transistors.
The machine is currently being upgraded by adding four
second harmonic cavities, each having its own drive
amplifier and bias system. The second harmonic bias
regulators have eight banks of 40 transistors (i.e. four
transistors in parallel to replace each of the 10 single ones
used in the original system).
This paper describes the design of a modular system
which will use digital techniques to monitor and display
the current of each of the 80 transistors in the
Fundamental Bias Regulator system, or 320 transistors in
the Second Harmonic System.
The layout of a fundamental bias system is shown in
Figure 1 below. The Bias Regulator consists of 8 banks of
10 transistors, giving a total of 80 transistors per system
that need to be individually monitored (this rises to 320
for the second harmonic system).
The function of the bias system is to drive a precisely
controlled current waveform through the bias winding of
an RF cavity. During the 10ms acceleration period this
biasing current is swept from 300 to 2300A causing the
resonant frequency to vary from 1.34 MHz to 3.09 MHz
(2.68 MHz to 6.18 MHz for the second harmonic system).
A further 10ms is taken to drive the biasing current back
down to 300A ready for the next acceleration pulse. The
Bias Regulator waveform is shown in Figure 2.
The main complexity of the current monitoring system
is that the ground lines of the transistors are connected to
the emitter busbar and not to earth. Therefore the emitter
Q1 Q2 Q10
Common Base Driver
Current Monitoring Connector
8 OF THESE TRANSISTOR BANKS FOR EACH
FUNDAMENTAL BIAS REGULATOR SYSTEM
Figure 1: Fundamental Bias Regulator System.
Proceedings of PAC07, Albuquerque, New Mexico, USAMOPAN104
06 Instrumentation, Controls, Feedback & Operational Aspects
1-4244-0917-9/07/$25.00 c ?2007 IEEE
T21 Reliability, Operability