Extending the low-recycling, flat temperature profile regime in the lithium tokamak experiment-β (LTX-β) with ohmic and neutral beam heating

Abstract

Recent experiments in the Lithium Tokamak Experiment-β (LTX-β) have extended the duration, performance, operating conditions, and diagnosis of the flat-temperature profile, low-recycling regime first observed in LTX. As expected, Li retains hydrogen and suppresses edge neutral cooling, allowing increased edge electron temperature, roughly equal to the core Te. Flat temperature profiles had been obtained transiently in LTX, as the plasma density decayed following the cessation of edge gas puffing. Careful control over the fueling in LTX-β has now been shown to sustain the flat Te profile and hot edge unique to the low-recycling regime for multiple confinement times in high performance discharges with decaying or steady density. With low density, the flat Te profile is also seen to extend into the scrape-off layer. Neutral beam heating is observed in target discharges with relatively flat electron temperature profiles (Tedge ~ Tcore/2), though beam heating is stronger in discharges with higher fueling, higher density, and depressed edge Te. Beam heating produces additional peaking of the Te profile, without degradation of the energy confinement time. Neutral beam heating of target discharges with relatively flat electron temperature profiles similarly results in broad beam heated temperature profiles. Energy confinement in LTX-β generally compares favorably to ohmic and H-mode scalings, frequently exceeding them by factors of 2-4. New and improved diagnostics in LTX-β enable better characterization of this unique regime, including measurements of ion temperature and high field side Thomson scattering profiles. As an initial step toward characterizing turbulence with no Te gradient and roughly equal density and pressure gradient, spectra have been measured in peaked Te discharges using far-forward scattering and fluctuation reflectometry.

NBI optimization on SMART and implications for scenario development

“…To reduce uncertainties in the predictive runs, electron density and plasma rotation are prescribed, as their predictions are presently unreliable when multiple transport channels are simultaneously activated in TRANSP predictions [41]. Density and rotation values are based on available results from other compact (or spherical) tokamaks with parameters similar to SMART, such as LTX [42,43] and Globus-M [44].…”

Investigating the role of edge neutrals in exciting tearing mode activity and achieving flat temperature profiles in LTX-β

“…It has a toroidal field of 3 kG on the axis. The maximum plasma current achieved so far is 135 kA with the help of an expanded Ohmic power supply [3]. A shell, constructed in four segments of a 1.5 mm stainless steel liner explosively bonded to 1 cm thick copper, serves as the first wall to the plasma.…”

“…This density threshold is further detailed in section 3. Plasma profiles, obtained from the TS diagnostic [3], in the density flat-top phase, sufficiently away from the gas puff are shown in figure 2. Note that the n e profile is always peaked with not much change in the peaking factor, which is the ratio of the maximum density in the radial profile to the line averaged density.…”

“…It has been reported that flat electron temperature (T e (r)) profiles were obtained in Lithium Tokamak Experiment (LTX) [1] and later in the upgraded Lithium Tokamak Experiment-Beta (LTX-β) tokamak [2,3], with a lithium coated first wall. Due to strong hydrogen retention and lower recycling of lithium, edge cooling can be minimized, and the plasma is thermally decoupled from the wall, permitting the edge temperature to approach the core temperature [4].…”

Analysis and design of fast flow liquid Li divertor for fusion nuclear science facility (FNSF) using coupled plasma boundary and LM MHD/heat transfer codes *

“…The use of liquid metal (LM) lithium (Li) offers significant benefits for PFCs owing to its low atomic number (Z = 3), low viscosity, high thermal conductivity, self-healing capabilities through liquid flow, and potential benefits to confinement under controlled Li concentration in the upstream via reducing recycling flux (D sticks on LM surface). Reduced recycling increases plasma temperature, resulting in improvements in core performance, as demonstrated in the [11][12][13][14][15][16][17]. Liquid Li can be used in various ways as a PFCs; it may be injected as a fast-flowing layer [4], employed as a gravity-driven system [18], utilized as a JXB driven [16] or Thermoelectric MHD flow system [19], or applied as an evaporative system known as a 'vapor box' [20,21].…”