Long Pulse Fusion Physics Experiments without Superconducting Electromagnets Page: 1 of 5
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Long Pulse Fusion Physics Experiments
Without Superconducting Electromagnets
Robert D. Woolley
Princeton Plasma Physics Laboratory*
P.O. Box 451, Princeton, New Jersey 08543
*Supported by U.S.Department of Energy Contract No. DE-AC02-76CH03073.
Long pulse fusion physics experiments can be performed
economically via resistive electromagnets designed for
thermally steady-state operation. Possible fusion
experiments using resistive electromagnets include long
pulse ignition with DT fuel.1,2,,4 Long pulse resistive
electromagnets are alternatives to today's delicate and costly
superconductors.' At any rate, superconducting technology
is now evolving independent of fusion, so near-term
superconducting experience may not ultimately be useful.
High magnetic field copper coils can be operated for long
pulses if actively cooled by subcooled liquid nitrogen,
thermally designed for steady state operation. (Optimum
cooling parameters are characterized herein.) This cooling
scheme uses the thermal mass of an external liquid nitrogen
reservoir to absorb the long pulse resistive magnet heating.
Pulse length is thus independent of device size and is easily
extended. This scheme is most effective if the conductor
material is OFHC copper, whose resistivity at liquid
nitrogen temperature is small. Active LN2 cooling also
allows slow TF ramp-up and avoids high resistance during
current flattop; these factors reduce power system cost
relative to short pulse adiabatic designs.
The main design issue for long pulse resistive
electromagnets is heat removal (within stress limitations).
Successful designs can employ thermally steady cooling via
forced convection of a liquid coolant, pressurized to prevent
film boiling. True steady-state operation would require the
warmed coolant fluid to be continuously recooled, but for
intermittent long pulse operation the warmed fluid can be
stored in a reservoir and recooled later between pulses.
Although choosing water as the coolant has some practical
advantages (e.g., economy, high heat capacity, and the
ability to exhaust steady magnet heat directly into the
environment without any refrigeration system), the
consequent "room temperature" magnet operation can
require a costly electric power system capability. But since
resistive power is proportional to conductor resistivity (see
Figure 1) the alternative use of a cryogen as combined
coolant and thermal mass for intermittent long pulse
operation can greatly reduce resistive electric power
consumption, which eases the heat removal task while also
reducing capital cost requirements for the electrical power
system. Liquid nitrogen is a natural choice for the cryogen.
A byproduct of industrial oxygen production, it is cheaply
available by the truckload in many locations, and so does
not necessarily require the purchase of a cryogenic
refrigerator. Large insulated dewar tanks to store it are
available and inexpensive. It is nontoxic and can be
released safely into the environment. It is not chemically
explosive as are some cryogens. It is almost inert
chemically and will not attack any materials used in magnet
construction. It is also an excellent electrical insulator.
Ebctrkba1Resstrty Vs. Tem peatum
farEeCu (rishCondutrtyC1751O) and
forOxygen FMe Hih Conductirty (OFHC)
- -14 0-00
100 150 200
Figure 1:Copper Alloy and Copper Resistivity
A particularly simple design concept is a "once-through"
system employing two identical large storage dewars with
interconnected gas spaces, as depicted in Figure 2. One
dewar is initially filled with liquid nitrogen at atmospheric
pressure but subcooled to nitrogen's 63.2 K triple point
temperature, while the second dewar is initially empty.
During the long pulse, liquid nitrogen from the first dewar
is pumped through cooling passages in the electromagnets
sufficiently fast that its exit temperature does not exceed
liquid nitrogen's 77.4 K normal boiling point. Exiting
liquid nitrogen flows into the second dewar at atmospheric
pressure. During time intervals between pulses the nitrogen
is returned to the first dewar and recooled to the triple point.
Useful features of this long pulse scheme include:
-No real-time coolant release to the atmosphere is involved.
(This safety feature is important if coolant nitrogen
becomes temporarily activated by DT neutrons.)
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Woolley, R.D. Long Pulse Fusion Physics Experiments without Superconducting Electromagnets, report, August 19, 1998; Princeton, New Jersey. (https://digital.library.unt.edu/ark:/67531/metadc665819/m1/1/: accessed April 24, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.