Energy confinement in the spherical tokamak Globus-M2 with a toroidal magnetic field reaching 0.8 T

Abstract

The work presents the results of the energy confinement study carried out on the compact spherical tokamak (ST) Globus-M2 with toroidal magnetic field (BT) as high as 0.8 T. A reproducible and stable discharge was obtained with the average plasma density (5-10) 1019 m-3. Despite the increase in the magnetic field, the neutral beam injection (NBI) led to clear and reproducible transition to the H-mode accompanied by a decrease in the turbulence level at the plasma edge. NBI allowed effectively heat the plasma: electron and ion temperatures in the plasma core exceeded 1 keV. In comparison with the previous experiments carried out with BT=0.4 T plasma total stored energy was increased by a factor of 4. The main reason of this phenomenon is a strong dependence of the energy confinement time (τE) on the toroidal magnetic field in the spherical tokamak. It was experimentally confirmed that such kind of dependence is valid for ST with magnetic field up to 0.8 T. It also has been shown that the enhancement of the energy confinement in the Globus-M2 with collisionality decrease is associated with an improvement of both electron and ion heat transport.

Overview of Globus-M2 spherical tokamak results at the enhanced values of magnetic field and plasma current

“…The absolute value of Z eff was in the range 2-2.5 according to the measured bremsstrahlung radiation intensity [16] and verified by ASTAR simulations comparing the calculated and measured loop voltage. Estimated effective ion heat diffusivity χ i values were as high as 1.5-3 m 2 s −1 that is similar to the electron heat diffusivity (χ i = χ e ) and it is 2-3 times higher than the neoclassical ion heat diffusivity values [12].…”

“…From the figure, one can see that observed τ E rise cannot be explained using strong τ E dependence on plasma current only. Accurate analysis of the τ E dependence on I p and B T was recently reported [5,12]. It has been shown that both I p and B T increase, enhancing energy confinement, while the role of B T is more important.…”

First Results of the Implementation of the Doppler Backscattering Diagnostic for the Investigation of the Transition to H-Mode in the Spherical Tokamak Globus-M2

“…After the transition to H-mode, an increase in the plasma density was followed by the rise of the total stored energy W DIA , measured by the diamagnetic loop. Compared to the experiments on Globus-M, carried out with a magnetic field of 0.4 T, the total stored energy increased by a factor of four [ 10 ]. The h-factor also changes after the LH transition and increases from 0.5 to 1.2–1.4, which is the result of the energy confinement time τ E in H-mode doubling and reaching 7 ms.…”

“…It is worthy of note that all of the aforementioned tokamaks differ in parameters and yet still show a similar dependence on the magnetic field, which contradicts the conventional τ E,98y,2 scaling [ 8 ]. It was also determined that there seems to be a strong correlation between the increase in energy confinement time and the decrease in collisionality, which is seen as an indication of favorable plasma parameters for the future [ 10 ]. All this leads to the question of what other properties of the H-mode or the LH transition deviate in spherical tokamaks in comparison to more conventional devices.…”

Fusion pilot plant performance and the role of a sustained high power density tokamak

“…This NSTX-Petty08 scaling formula is a continuous function of ε but is not unique and has considerable physics uncertainty. Obtaining a more definitive low-A confinement scaling [13,14] at reduced collisionality and higher magnetic field and current is a major research goal for NSTX-U [15][16][17][18][19][20], MAST-U [21][22][23], ST-40 [24], and Globus-M2 [25].…”