NEW BEVATRON POLE-FACE WINDINGS POWER SYSTEM. Page: 4 of 7
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This report was prepared as an account of work
sponsored by the United States Government. Neither
the United States nor the United States Atomic Energy
Commission, nor any of their employees, nor any of
their contractors, subcontractors, or their employees,
makes any warranty, express or implied, or assumes any
legal liability or responsibility for the accuracy, com-
pleteness or usefulness of any information, apparatus,
product or process disclosed, or represents that its use
wo it infringe privately owned rights.
NEW BEVATRON POLE-FACE WINDINGS POWER SYSTEM*
L. T. Jackson
Lawrence Radiation Laboratory, University of California
The new power systems which individually drive
each of the Bevatron pole-face windings must handle a
variety of operating parameters as well as satisfying
the stringent specification of dI/dt<14A/sec during
flattop. The 19 pole-face windings, which run
circumferencially around the machine, each constitutes
an active load for their power system, with B voltages
increasing with radius up to'190V during rectification
on the outermost winding. During flattop there is
approximately 10V pp of predominately 720 and 144011z
signals across the winding. The 0.552 windings require
up to +30A during injection and ilOOA during flattop.
To satisfy the above requirements, a transistor
actuator was chosen utilizing 2i113902 high-voltage
transistors. A dc supply with multiple taps on the
transformer provides output voltages from 12V to 270V
to allow matching to each pole-face winding with a
voltage higher than the B voltage of that winding so
the difference appearing across the actuator is correct
for transistor operation. On the positive actuator,
the difference voltage must be great enough to drive
the 30A peak required through the winding during
injection. Characteristic waveshapes for PFW13 positive
and negative actuators are shown in Fig. 1.
The 2N3902 has a maximum power rating of 100W,
but above 80V is second breakdown limited. It was
necessary to add 0.2552 resistors in series with windings
11, 12, and 13 to keep the load line within the tran-
sistor's safe operating area with B+ = 115V. Pole-face
windings 14, 15, and 16 require only negative current
during flattop, so rather than install prohibitively
large series resistors to keep within safe limits on
positive currents due to the increased B+, the computer
terminal does an ExI product with an update time of
lmsec. Upon sensing a beyond-limit power level the
reference word at the computer terminal is zeroed and
the supply contactor opened. The zeroing catches any
non-allowed reference signals before harm can be done.
The contactor clearing time of 32msec will prevent
transistor failures for most of the other possible
Twenty-four output and one driver stage 2N3902's
are mounted on each of the two water-cooled slabs in
an actuator. The 213902's operate at 100W dissipation
with no more than a 55*C case temperature. The actuator
is photographed in Fig. 2, and the system schematic is
shown in Fig. 3. The low-powered electronics on printed-
circuit boards are nested in between the two water-cooled
slabs. The three boards contain (1) the regulator and
monitor amplifiers and the Digital-Analogue Converter,
* (2) the pre-drivers and current limiters, and (3) the
pulse-auration-modulation receiver (serial data). The
regulator and monitoring shunts are also mounted
between the slabs on a copper bar connected to the
back slab at the right side of Fig. 2. Each shunt is
composed of two paralleled 0.02D 25W Daleohm resistors
arranged in a quasi-coaxial configuration with the
incoming and outgoing cables. By leading in the cable
between the two r-sistors and tnereoy having the return
currents through the resistors above and below the
cable, the frequency response is enhanced and pickup
minimized. The close-proximity of all the active
elements in the closed loop system also substantially
reduces pickup ani maximizes the frequency response.
* This work performed unaer the auspices of the
Atomic Energy Commission.
The electronic power supplies and metering can be
seen in an adjacent chassis to the left of the actuator.
All the connecting leads are buffered at the actuator.
Every attempt has been made to design a reliable
system because the operation of the Bevatron is directly
dependent on it. But in the event of malfunction,
rapid replacement of any part of the system is required.
To facilitate maintenance, all the major and minor parts
are completely interchangeable between each system.
All the boards are interchangeable; the pre-driver
boards made so by having both the + and - actuator pre-
drivers mounted on all boards and then choosing the
appropriate one by the cable connection. In the event
that many of the output transistors are blown (each
one individually disconnects by having 0.32 fuseable
emitter resistors), the complete actuator can be
quickly slid out by disconnecting three lugs, the
electronic cables and two quick-disconnect water fit-
Because of having an active voltage source in the
load, the actuators are driven in the common-emitter
connection. This configuration stacks the common of I
and - power supplies away from ground by the load volt-
age and gives the actuator voltages shown in Fig. 1.
By minimizing the capacity to ground of the power sup-
plies and having a transformer capacity of O.Oljfd. tie
tank circuit formed by this effective C and the L-s400th
of the windings is kept well beyond the active frequency
range of the closed-loop. The positive actuator then
acts as a current source driving the pole-face windinr
through the power supply. The effective collector
resistance of the fifty 2N3902's at lA is 1552, which
appears in shunt with the idealized current source, aid
with the 400pH load $nd determines the transfer response
between the amplifier output voltage and the shunt
voltage. The same actuator becomes the negative
actuator by driving the PNP 2N5416 pre-driver rather
than the 2N3439 for the positive side. The effective
collector resistance for this side is 1052 for the
There are two transistor protection systems buil,
into the actuators. The first involves diode clamps
across each actuator with related 10,500Pfd capacitor
banks across the power supplies. When an actuator is
turned-off at the end of flattop, the current flowing
in the load commuttes from this actuator to the diod!
across the other actuator through the associated
capacitor bank. The energy transferred to the bank
raises its voltage 5V; the 15002 across the capacitors
returns the voltage to normal before the next pulse.
On positive-actuator-only systems, a lA supply, capa-
citor bank, ana diode are used to provide the same
function. The supply is adjusted to bias the capacitors
to a high enough voltage that the diode isn't turned
on during inversion.
The second protection system utilizes FET transis-
tors (21N4391) as current limiters in the pre-drivers
stages of the negative actuator. The 2N5416's span
the same voltage range as the 2N3902's, and if a turn-
on signal comes along just after a turn-off and while
the loaa is commutating, the transistor would be over-
powered without the current-limiting feature. The
limiters are also Ilacea in the positive pre-drivers
to limit the drain on the +15V supply.
The I specification placed on the current trans-
lates to Ipkt = 6mA at 360hz and proportionately lower
lates to~ TIpk = 6m at~ ~i 360S and 1 1~ pr p ri n t l ( lowe)
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Jackson, L.T. NEW BEVATRON POLE-FACE WINDINGS POWER SYSTEM., report, January 1, 1971; [Berkeley, California]. (digital.library.unt.edu/ark:/67531/metadc865613/m1/4/: accessed October 19, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.