High-Q plasmas in the TFTR tokamak Page: 6 of 29
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This paper summarizes characteristics of the supershot regime, and discuss
recent enhancement of supershot performance. We then project the parameters
achieved in D plasmas to D-T operation, and show that experiments on alpha-driven
collective effects are possible with Qpyp in the range 0.3 to 0.5.
n. NEUTRON DETECTOR SYSTEM
The neutron rate in TFTR is measured principally with a system of fission
chambers that cover the range from lO^O n/s in ohmic D plasmas to 10*9 n/s expected
in beam-injected D-T plasmas.
Maintaining the neutron detector calibration is an ongoing routine operation. 3
Every 12 to 18 months, we perform an in situ absolute calibration of the most sensitive
detectors using a Cf-252 source or D-D generator at many positions inside the vacuum
vessel, to simulate the plasma source. About once per month we monitor the detector
drift, if any, with a Pu-Be or Cf-252 source placed adjacent to the detector. On each
day of plasma operation, we cross-calibrate the detectors against each other, using the
plasma neutron source.
The absolute uncertainty in the quoted neutron rate for a single shot is ±14%. The
injected beam power is known to ±13%, so that the uncertainty in QdD ‘s about ±19%.
m. DISTINCTIVE FEATURES OF SUPERSHOT PLASMAS
The basic technique for achieving supershots is to inject high-power neutral beams
into a low-density target plasma with a graphite first wall/limiter that has been
conditioned to achieve a low recycling coefficient RCy for hydrogenic species.2,4 a
second requirement is that central fueling and most beating be provided by
neutral-beam injection alone. Other requirements are that the beam power density be
high (central value > 1 MW/m3), and that the beams be injected with roughly balanced
co- and counter-injected power (with respect to the plasma current).
3
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Jassby, D. L.; Bell, M. G.; Bitter, M.; Boivin, R.; Bretz, N. L.; Budny, R. V. et al. High-Q plasmas in the TFTR tokamak, article, May 1, 1991; New Jersey. (https://digital.library.unt.edu/ark:/67531/metadc1100875/m1/6/: accessed July 17, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.