The edge plasma and divertor in TIBER Page: 2 of 5
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THE EDGE PLASMA AND DIVERTOR IN TIBER
UCRL—96677
W. L. Barr
Lawrence Livermore National Laboratory DE88 001661
Livermore, CA
Sin—rv
An open divertor configuration has been adopted
for TIBER. Host recent designs, including DI11-D [1],
NET [2] and CIT [3J use open configurations and rely
on a dense edge plasma to shield the plasma from the
gas produced at the neutraliser plate Fig. 1 a hows
Che DllI'D and TIBER-II configuration* on roughly the
sarte scale to allow a comparison of dimension*.
Experiments on ASDEX, POX, D-lll, and recently on
DII1-D have shown that a dense edge plasms can be
produced by re-ionizing most of the gas produced at
the plate. This high recycling mode allows a large
flux of particles to carry the heat to the plate, ao
that the mean energy per particle can be low.
Erosion of the plate can be greatly reduced if the
average Impact energy of the tons at the plate can be
reduced to near or below the threshold for sputtering
of the Mata material. The present configuration
allows part of the flux of edge plasma ions to he
neutralized at the entrance to the pumping duct so
chat helium is pumped ss well as hydrogen. This
configuration is shown in Fig. 2 and has the following
features:
* Peak heat loads of about 3 MW/m2, and less thsn 6.5
fW/a2 even if the plasma shifts position by as much
as 0.I m in any direction.
* High gas recycling to reduce sputtering and the
erosion of the plates and the production of
impurity ions.
* Smell vacuum ducts made possible by the high
pressure in the ducts that results from the high
recycling. Pumping speed is 50 n*/s and pressure
In the vacuum ducts is 40 mTorr.
* At least 0.48m of neutron shielding between plasma
and the superconducting magnets.
DIII-D TIBER
Fig. 1. DIII-D and TIBER shown on the same scale for
cotDorison of the divertors.
MASTER
The Divertor Confl gtjpation
The divertor configuration ahovn In Fig. 2 Is
intended to help TIBER meet its design goal of
ignition in a minimum-size device, and to maintain
the required spacing (about 0.5 m) for neutron
shielding everywhere between the plasma and the
superconducting magnets. The minimum distance that can
be allowed between Che neutralizer plates and the
closed field lines Is determined by the need to
prevent gas and impurities from reaching the confined
plasma. With high recycling at the plates, the plasma
tempatoture end density D**r the plates will be about
10 eV and 1020 m'J, respectively. In such a plasmr,
the mean free paths (mfp) for Ionization of the
recycled neutrals are short, with the longest being
about 50 am for the fast neutrals that result from
charge exchange (CX). Because the cross section for
CX of D° and 1° Ls target than that for ionization,
multiple CX uvents can allow deeper penetration than
the mfp would suggest. In TIBER'S divertor
configuration the minimum apace between a plate and
the confined plasma is 15l» ». This is much greater
thsn any of the relevant mfp* when the recycle rate,
and therefore the density, la high.
Identical top and bottom divertors are aach
divided into 16 modules so that aach module can be
removed between the 16 toroidal field (Tr) colls. A
slab-shaped vacuus duct under each lover plate
connect* to a circular vacuum pipe, which leads to sn
external pumping manifold.
TIBER’S first wall ls located on magnetic flux
surfaces to minimize the particle and heat load to the
wall. The separation of tho first wall from the
confined plasma is several radial decay lengths for
both particles and energy. The decay length for
:;■ = -I :
Fig. 2. The TIBER divertor configuration showing
uagnetic flux surfaces in the edge plasna and one of
16 vacuum pipes.
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Barr, W.L. The edge plasma and divertor in TIBER, article, October 16, 1987; [Livermore,] California. (https://digital.library.unt.edu/ark:/67531/metadc1100980/m1/2/: accessed June 3, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.