Magnetic switch coupling to synchronize magnetic modulators

Apparatus for synchronizing the output pulses from a pair of magnetic switches. An electrically conductive loop is provided between the pair of switches with the loop having windlings about the core of each of the magnetic switches. The magnetic coupling created by the loop removes voltage and timing variations between the outputs of the two magnetic switches caused by any of a variety of factors. The only remaining variation is a very small fixed timing offset caused by the geometry and length of the loop itself.

BACKGROUND OF THE INVENTION 
This invention relates to magnetic switches. More particularly, this 
invention relates to an apparatus for causing multiple magnetic switches 
to switch in a synchronized manner. 
Numerous magnetic modulator applications including particle beam 
accelerators, high power linear induction voltage adders and laser drivers 
require the timing of the output pulses be synchronized both to the 
trigger pulse and between multiple machines. The timing drift on the 
output of a magnetic modulator can be separated into two categories. Slow 
changes such as core heating and prime power supply drift cause slow 
steady timing drifts that can be easily compensated by monitoring the 
output timing and slowly adjusting the trigger timing to compensate. 
Random shot to shot variations due to main power supply ripple, primary 
switch jitter and reflections that effect the bias state of the switches 
from shot to shot and therefore the timing, require a compensation 
mechanism that is fast enough to act between shots. 
Techniques have been developed to compensate for both of these types of 
time shifts. The most generalized of these utilize a combination of 
precision charge voltage control and trigger timing compensation based on 
measurement of the charge voltage and bias currents just before a shot. 
While this type of jitter control has been used with great success, it 
involves the complexity of both microprocessors and analog circuitry in 
the feedback loop, and it requires calibrations to be established and 
maintained that correlate the required timing offsets to the measured bias 
and charge voltage offsets. 
BRIEF SUMMARY OF THE INVENTION 
Magnetic coupling offers a simple, passive maintenance-free method to 
synchronize the outputs of two or more magnetic modulators that avoids the 
complexity of computerized and electronic feedback schemes. When two 
magnetic switches are properly coupled together, they are forced to switch 
at the same time to within the fixed delay in the coupling connections 
that is determined by the geometry. Thus, magnetic coupling compensates 
for timing variations at the outputs of the coupled modulators caused by 
slow parameter and charge voltage drifts as well as fast random variations 
due to power supply ripple, primary switch jitter and reflections. 
Magnetic coupling provides the additional benefit of helping to equalize 
the charge voltages at corresponding stages of the coupled modulators.

DETAILED DESCRIPTION OF THE INVENTION 
The magnetic coupling scheme is shown in FIG. 1, in which equal coupling 
windings 10 are put on the switches (switch A and switch B) to be coupled 
and then interconnected symmetrically so that no current flows in the 
coupling windings 10 if the currents in the main electrical windings 14 of 
switch A have the same input-to-output sense as the currents in the main 
electrical windings 14 of switch B. The main electrical windings 14 are 
those connected to the energy storage capacitors in the magnetic modulator 
circuit. A positive input-to-output sense is taken as flowing from the 
capacitor that is on the side of the magnetic switch that is nearest 
electrically to the input end of the magnetic modulator through the main 
magnetic switch windings to the capacitor that is nearest electrically to 
the load end of the magnetic modulator. In practice, the same 
input-to-output sense will occur in the main windings of switch MS1A and 
switch MS1B if the capacitors C1A and C1B preceding the switches, MS1A and 
MS1B respectively, charge together and if the capacitors C2A and C2B are 
both initially discharged. Similarly, the same input-to-output sense will 
occur in the main windings of switch MS2A and switch MS2B if the 
capacitors C2A and C2B preceding the switches, MS2A and MS2B respectively, 
charge together and if the capacitors C3A and C3B are both initially 
discharged. With corresponding magnetic switches magnetically coupled in 
this fashion, if the capacitor on the input side of one switch (input 
being closest electrically to the input of the magnetic modulator) starts 
to charge before the capacitor on the input side of the corresponding 
switch in the other modulator, then the transformer action of the coupling 
winding 10 will cause the capacitor on the input side of the other switch 
to begin to charge as well. If manufacturing differences, temperature 
differences, bias differences, charge voltage differences, charge time 
differences, trigger timing differences, reflections, or and other circuit 
parameters cause one magnetic switch to start to transition from the high 
inductance state to the low inductance state (to close) before the other, 
the coupling winding 10 acts as a shorted secondary on the slow switch and 
forces it into a low inductance or switched (closed) state. 
The two magnetic coupling windings on the coupled magnetic switches must be 
connected in such a relative polarity so that no current is induced in the 
coupling winding by simultaneous equal currents flowing in the main 
windings from the input toward the output of both magnetically coupled 
magnetic switches. The main electrical windings are those connected to the 
energy storage capacitors in the magnetic modulator circuit. A positive 
input-to-output sense is taken as flowing from the capacitor that is on 
the side of the magnetic switch that is nearest electrically to the input 
end of the magnetic modulator through the main magnetic switch windings to 
the capacitor that is nearest electrically to the load end of the magnetic 
modulator. 
A typical pair of Melville magnetic modulators is shown in FIG. 2 with 
magnetic couplings 10 and 20 added between the corresponding magnetic 
switches MS1A, MS1B and between MS2A, MS2B respectively. The effect on 
switch timing caused by different charge voltages on the two modulators is 
shown in the computer simulation of FIG. 3. Although this detailed 
description portrays the magnetic coupling between the various stages of a 
separate pair of magnetic modulators, the invention can be extended to 
provide magnetic coupling between three or more parallel magnetic 
modulators. 
In FIG. 3, the voltages on the capacitors in the two modulators are 
displayed so that the voltages on corresponding capacitors in the two 
machines can be compared. The time at which a given magnetic switch closes 
is marked by the inflection in the voltage on the capacitor that precedes 
the magnetic switch from an upward derivative to a downward derivative The 
A modulator has an initial charge of 150 kV on COA and the B modulator has 
an initial charge of 140 kV on COB. The results of a PSPICE simulation of 
this modulator pair without and with coupling are shown in FIGS. 3 (a) and 
(b), respectively. Without coupling, FIG. 3(a), the different charge 
voltages cause the corresponding switches to switch at different times, 
and most importantly the charge waveforms on the last capacitor do not 
occur at the same time. This asynchronism would prevent these output 
waveforms or waveforms derived from them from being combined in a linear 
induction voltage adder or particle accelerator. With magnetic coupling, 
however, FIG. 3(b), all of the magnetic switches switch at the same time, 
and most importantly the voltage waveforms on the output capacitors rise 
at the same time allowing them or waveforms derived from them to be 
efficiently combined in a linear induction voltage adder or particle 
accelerator. Also note that the magnetic coupling tends to pull the 
amplitudes of the capacitor charging waveforms at the peak closer 
together. The effect on switch timing of triggering the A-modulator 1 
.mu.s after the B-modulator is shown in FIGS. 4A and 4B. As would be 
expected, without magnetic coupling, FIG. 4(a), each of the A-modulator 
switches switch 1 .mu.s after their corresponding B-modulator 
counterparts. With magnetic coupling, however, FIG. 4(b), the A-modulator 
switches switch at the same time as the B-modulator counterparts. 
Differences in the corresponding switch parameters, such as magnetic core 
packing factor, FIGS. 5A and 5B, is another cause for differences in the 
switching times in the two modulators. Without magnetic coupling, FIG. 
5(a), a 2% difference in the packing factors in the magnetic cores of 
corresponding switches in the two modulators causes a significant 
difference in the switching times. With magnetic coupling between 
corresponding magnetic switches in the A and B modulators, FIG. 5(b), the 
corresponding A and B modulator stages charge and switch together. 
The effectiveness of this type of coupling arrangement has been 
experimentally demonstrated on the Dos Lineas magnetic modulator pair 
located at Sandia National Laboratories. The technique was tested with 
.+-.1.mu.s offsets in the triggers delivered to the primary SCR switches 
on the two machines, and it was tested with forced differences of 320 V 
out of 5 kV in the initial charge voltages supplied to the inputs of the 
two machines. For comparison to the PSPICE simulation of FIGS. 3A and 3B, 
this initial charge voltage difference corresponds to a 9 kV difference in 
the charge voltages on the magnetic switches. These tests yielded several 
important results. 
The peak voltages on the output voltages of the A and B machines were 
measured to be very nearly the same, as shown in FIG. 6a, demonstrating 
that the magnetic coupling pulled the amplitudes of the corresponding A 
and B capacitor charge voltages together, even in the presence of 
different initial charge voltages on the two machines. The measured 
results in FIG. 6a also. Furthermore, the fixed 70 ns offset that was 
measured between the outputs of the A and B modulators shows that the 
outputs are synchronized to within a fixed 70 ns timing difference both in 
the presence of a 320 volt difference in the input voltage to the 
modulators, FIG. 6(a), and in the presence of the prime switch SCR trigger 
timing offsets to the two machines, FIG. 6(b). The 70 ns offset between 
the two outputs is a constant for all shots and calculations indicate that 
it is caused by the constant propagation delay in the coupling windings. 
Each magnetic switch has a coupling winding that consists of ten turns of 
wire equally distributed around the core on a mandrill outside the main 
electrical windings. Corresponding switches (MS1A corresponds to MS1B and 
MS2A corresponds to MS2B) are magnetically coupled by connecting the free 
ends of the coupling winding on one switch to the corresponding free ends 
of the coupling windings on the other switch, in this case, via a coaxial 
cable between the switches in such a way that no current will flow in the 
coupling windings if the currents in the main switch windings have the 
same input to output sense. The two coupling windings on corresponding 
switches and the intervening coaxial cable form a continuous electrical 
path that closes upon itself to form a continuous electrical loop. The 
coaxial cable is used to interconnect the coupling windings of 
corresponding switches in order to minimize the uncoupled series 
inductance in the coupling loop. Equal numbers of windings are utilized 
for coupling essentially identical magnetic switches. If the switches or 
the other components in the two coupled modulators are dissimilar in 
impedance, it may be necessary to use different numbers of coupling 
windings on the different switches to compensate for the impedance 
mismatch. 
It has been experimentally demonstrated that magnetic coupling synchronized 
the switching times of corresponding magnetic switches in the two 
magnetically coupled modulators in the presence of forced trigger offsets, 
forced initial charge voltage offsets and intrinsic component differences. 
Magnetic coupling synchronized the outputs of the two modulators in the 
presence of forced trigger offsets, forced initial charge voltage offsets 
and intrinsic component differences. 
Magnetic coupling reduced the random timing jitter between the outputs of 
the two modulators due to reflections 10 fold compared to the uncoupled 
value, as shown in FIG. 7. 
Magnetic coupling yielded a more than 30 fold desensitization in the timing 
offsets at the outputs of the two magnetic modulators due to voltage 
variations on the CO capacitors at the start-up of the machines, FIG. 7. 
Even though the corresponding switches are switching simultaneously, the 
algorithm used to compare the A and B output pulses produces an apparent 
transient timing offset due to the change in the output inductance's 
caused by the voltage transient on the CO capacitors. 
Two further tests were conducted to demonstrate the effectiveness of the 
magnetic coupling. FIG. 8 shows the effect of a 1 .mu.s time shift that 
was introduced in the triggers to the two machines. The shift is clearly 
visible without coupling but eliminated with coupling. The inset in this 
figure shows that magnetic coupling reduces steady-state time-shift ripple 
from 1 .delta.=0.72 ns to 1 .delta.=0.07 ns. 
FIG. 9 shows similar reductions in time shift for a situation where 
modulator A was charged 324 volts higher than modulator B. The inset again 
shows the marked reduction in time-shift ripple. 
A significant number of industrial applications for continuously operating 
pulse powered X-ray and electron beam generators require power levels 
approaching 1 MW. Food irradiation to eliminate pathogens, waste water 
treatment applications, and hazardous waste treatment require accelerating 
potentials in the 5 MV to 10 MV range to maximize efficiency and treatment 
depth as well as high power levels to provide high throughput. Linear 
induction voltage addition allows the output voltage to be increased by 
simply adding more stages; however, the power required for this increased 
voltage requires multiple, parallel pulse forming and modular networks. 
The Repetitive High Energy Pulsed Power (RHEPP) program at Sandia National 
Laboratories has produced a 2.5 MV accelerator with an average output 
power of 350 kW from a single magnetic compressor and pulse forming line. 
Using the RHEPP technology to produce a 5 MV accelerator would require 700 
kW and 1.4 MW for a 10 MV accelerator. Impedance constraints coupled with 
physical size limitations, pulse risetime requirements, and component 
cooling requirements prohibit achieving these power levels from a single 
module. This technique solves the problem of synchronizing parallel, 
magnetically switched driver modules for application to high average power 
adders and accelerators.