Page 7 - CHARACTERIZATION OF THE C-2W PLASMA GUNS

Nucl. Fusion 57 (2017) 116021 H. Gota et al
Figure 6. Top: Normalized excluded- ux radius evolutions in C-2 and C-2U experiments. Bottom: time evolutions of FRC-core electron temperatures under C-2 HPF14 (squares) and C-2U with 10 MW NBI (circles) regimes; Te is averaged inside the separatrix as well as shot averaged for each data set. Note that NBI durations are longer than FRC lifetimes of shots 34913 and 42411, and NBs on shot 43833 are terminated at 10 ms.
Figure 7. Correlation between NB pulse duration and plasma parameters such as excluded- ux radius, density (measured by CO2/FIR interferometer) and total temperature (estimated from pressure balance). Neutral beams are terminated at 1, 2, 3, 4 and 5 ms in 5 plasma discharges under the same operating condition except for the NB duration, and all quantities are normalized.
in equilibrium and transport parameters, and the previously
reported C-2 scaling of electron energy con nement time
(see gure 27 in [4]) persists at the higher electron temper-
ature range accessible in C-2U, as shown in gure 8. Given
uncertainties in the measurements and assuming the power-
law model, regression analysis shows that the electron energy
con nement time τ is proportional to T1.8 when tting for E,e e
the entire ensemble of the C-2/C-2U data set. This positive con nement scaling is very attractive, and similar features of temperature dependence have also been observed in other high-beta device such as NSTX, whereby the energy con ne- ment time scales nearly inversely with collisionality [26, 27]. The emergence of this attractive scaling result, paired with the considerable accomplishment of plasma sustainment obtained in C-2U, is very encouraging for the FRC and innovative con- nement concepts communities, and may lead to intriguing possibilities for possible future FRC-based fusion reactors.
Moving on to the next step, in order to further improve the FRC performance such as plasma lifetime and temperature, the C-2U device is being replaced by C-2W [8] featuring higher NBI power (>13 MW in phase I and over 21 MW in
Figure 8. Global energy con nement time of electrons as a function of electron temperature for ensembles of C-2 (triangles) and C-2U (circles) discharges.
phase II) and longer NB pulse duration ( at-top up to 30 ms) as well as enhanced edge-biasing systems and substantially upgraded divertors. The goal of this device will be to test and validate the emerging energy con nement at higher plasma temperature and elevated system energies; the device will operate at about an order of magnitude higher stored energy than C-2U. A key to increasing the electron temperature of the FRC core and SOL is to reduce the energy loss via the
7