Advances on the High Field Ultralow Aspect Ratio Tokamak

Full text: Spherical tokamaks (STs) offer a potential more economical approach for attain fusion energy and has been also proposed as a neutron source for fusion materials studies. Therefore, it is important to explore the limit of ST's compactness in order to address the potential benefits for both tasks. This is the objective of the medium-size high-field ultralow aspect ratio tokamak (HF-ULART) proposal, which aims to explore very high β under the minimum toroidal field as a target plasma, and then explore higher pressures values using the combined minor and major radius adiabatic compression (AC) technique. This might be one of potential economical pathways scenario for at least an ultracompact pulsed neutron source based on the ST concept. The major characteristics of the target plasma are: R_o = 0.51 m, a = 0.47 m, aspect ratio A = 1.1, k = 2, δ ∼ 0.8, B(R_o) = 0.1 T (0.4 T max), I_p = 0.5 MA (2 MA, max), n_e(0) ∼ 1 x 10²⁰/m³ , T_e(0) ∼ 1 keV, and discharge duration of ∼ 100 ms. The vessel is spherical, made of SS, and insulated from the expected natural diverted (ND) plasma by thin (few cm) tungsten (W) semispherical limiters. The central stack is made of cooper cover by a thin (∼ 2 mm) W sleeve. No internal PF coils or solenoid is envisaged. This helps the compactness due to the close plasma-vessel fitting, and possibly provides wall stabilization as previously envisaged in the RULART proposal, while also potentializes easier H-mode (small edge neutral source volume), which has already been observed in the ULART Pegasus device operating in ohmic ND plasmas, using inboard gas fuelling. The initial source for heating is provided by I_p generated from RF in combination with transient coaxial/local helicity injection techniques, as both have been successfully demonstrated in STs. By applying the AC technique over a high-β plasma, i.e., I_p = 0.5 MA, B(R_o) = 0.1 T, R_o = 0.51 m, a = 0.47 m, A = 1.1, k = 2, δ ∼ 0.8, qψ(Peng) = 22, T_e/T_i = 263/486 eV, n_e(0) ∼ 0.15 x 10²⁰/m³ , lead to final following values: I_p = 1.0 MA, B(R_o) = 0.61 T, R_o = 0.33 m, a = 0.28 m, A = 1.2, k = 1.6, δ ∼ 0.1, qψ(Peng) = 12, T_e/T_i = 1.9/3.4 keV, n_e(0) ∼ 2.8 x 10²⁰/m³ . Other preliminary analysis will be also presented. They include the time scales of the AC to respect the expected energy confinement time, the neutron yield and standard MHD stability calculations, and some fixed and free-boundary equilibrium simulations by VMEC and FIESTA codes, respectively. (author)