Description of the Plasma Potential Control (PPC) System on the Tandem Mirror Experiment-Upgrade (TMX-U) Page: 4 of 5
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vessel. Each rack houses the switches and monitoring g
equipment for the 18 PPC plates on that end of the
machine. To handle the communication between these
two racks and the operator, a CAMAC (Computer
Automated Measurement And Control, IEEE Std 583-1975)
system was employed using a serial highway interface
system conforming to IEEE Std 595-1976 (see Fig. 4).
Since the electro-magnetic interference is such a
concern, no switching data is transmitted over the
long distance between the control room and the pit
racks during a TMX-U shot. The switching times are
stored in LeCroy 8201 memry modules located in each
CAMAC crate. This switching data is then clocked
through to a memory buffer module installed
Immediately next to each memory module during each 100
msec shot. The memory buffer module contains data bus
buffers, data latches, and the Field Effect
Transitors (FETS) that ultimately control each relay.
Each memory module and memory buffer combination
handles 6 plates or 12 relays, making three sets
necessary in each CAMAC crate to control the 18 plates
via the 36 relays at each end of the machine. The
memory module has 16K bits of memory available and can
be configured in either 8-bit or 16 bit words. PPC
uses a 16-bit word of which 12 bits are actually used
to switch the 12 relays. This allows 1K of memory per
relay and a total use of 12K for the switching data.
Because of the high potentials switched at each of
these racks, a fiber optic serial highway was
installed to provide the necessary isolation between
the operator and the PPC switching racks. This scheme
of control has proven very reliable with essentially
no collieion of the switching relays, which would
prove catastrophic, even in this very hostile
operating environment.
Shot Timing
Timing in the PPC system and therefore operation of
the relays has to be in synchronization with the whole
TMX-U Project during a shot. To accommodate this
precision, the "GO" signal to start the timing
sequence and consequently clock through the timing
data stored in each memory module, is generated at the
ThX-U master timing computer. This go signal is then
sent via a fiber optic link to each CAMAC crate at the
vessel (see Fig. 5). The "GO" signal is processed by
a Honeywell LED transmitter and receiver set that
operates at a very stable 2.5 MHZ during idle or "0"
mode and 5 MHZ at the "GO" signal or "1" mode. Since
this frequency is very reliable, the 5 MHZ is used to
generate the timirng to clock through the timing data
stored in each memory module to the memory buffermodule. The 5 MHZ is divided to achieve a 10 kHZ
clock by one of the memory buffer modules at each
rack. This clock rate is then sent to the other two
memory buffer modules and to their own respective
memory modules thereby clocking and latching the data
out of the memory module.
A 10 kHZ clock rate allows 10 data bits per
millisecond and a 100 msec resolution overall. During
the 100 millisecond shot, each 12-bit word (one bit
for each relay) is clocked from the memory module to
the memory buffer where it is buffered and latched to
either turn the relay driving FET "on" if a "1" is
present or "off" if a "0" is present. This method of
control provides the immunity to noise which would
otherwise cause false triggering and erroneous data to
be collected.
Diapnostic Data
Besides the ability to aid confinement in the TMX-U
vessel, the PPC plates provide an important diagnostic
tool to helW understand the plasma action. When the
plates are in the floating mode, they are tied t- a
resistive divider network which measures the elecL "r
potential at each plate. In the grounded mode, th
PPC system is capable of measuring electron current on
each plate through a Hall Effect transformer in series
with the lead from the plate. When the plates are
externally biased, the voltage and current data can
also be collected in the same mariner. This data is
then sent to the main data acquisition computer (an HP
1000 System) for ultimate storage and interpretation
of the entire PPC system. The PPC computer has also
been programmed to diagnose the response of the relays
and call the operator's attention to poor timing
tolerances. The graphics of the computer has provided
an excellent means for providing the switching pattern
of the relays both before a shot ant after the shot so
a comparison of the desired switch time with the
actual switch time can be made.
Biasing
The PPC plates have the ability of being biased by two
available voltage levels. Two 7200 pfd capacitor
banks are available for voltages up to 5 kV at each
rack. These capacitor banks are independently
controlled, remotely from the PPC operator's console
in the control room. The ability to make two
different potentials available to the plates gives the
PPC system even more flexibility.t n-
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Surrena, P.S. & Underwood, R.H. Description of the Plasma Potential Control (PPC) System on the Tandem Mirror Experiment-Upgrade (TMX-U), article, November 11, 1985; California. (https://digital.library.unt.edu/ark:/67531/metadc1205009/m1/4/: accessed June 1, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.