Verlag des Forschungszentrums Jülich

JUEL-3727
Nicolai, Albert
Equilibrium properties and main design parameters of a long pulsed spherical torus
58 S., 1999



At tight aspect ratios tokamak plasmas possess features not present at large aspect ratios, such as natural elongation, triangularity, a natural divertor configuration and a considerable bootstrap effect. However, the pulse duration in ST's (Spherical Tori) seems to be considerably smaller than in conventional takamaks due to the much smaller flux swing of the primary solenoid in ST's.

To explore the requirements for a tight aspect ratio device with long pulse durations, a 95% bootstrap driven device (I_p » 2.5 MA) is considered operating at a [beta] = 18%. Here [beta] is defined as the ratio of the volume averaged pressure to the volume averaged energy density of the (total) magnetic field. (The quantity bvac = 45% which is the ratio of the volume averaged pressure to the volume averaged energy density of the toroidal vacuum field, is considerably larger. The seed current is assumed to be driven by neutral injection which partly provides the plasma heating as well.

Ignoring the plasma initialization and the ramp up phase in a first step, the flat - top state is analyzed by means of the TORUS- II code.

In this code a combination of a 'semifree' boundary equilibrium code with a consistent description of the bootstrap current is used.

The calculations are based on the aforementioned plasma current, the aspect ratio A = 1.2, the centerpost current I_cp = 3.0 MA and the mean proton density [n_p] = 5.9 10¹³ cm^-3. Some typical results are:
1. At the mean plasma temperatures [T_e] = [T_i] » 1500 eV the prescribed bootstrap fraction f_boot =95% is reached for the minor half axis a = 1.30 m.

2. Without the action of the shaping coils the natural elongation is only k_nat = 1.6. The triangularity is then [delta] = 0.6, and the Shafranov - shift is d_SH = 0.35 m. To achieve an elongation [epsilon] = 2.0, the current I_div =0.3 MA in the divertor coils is needed.
3. Due to the poloidal [beta]_p » 1 a magnetic well is generated at the outboard side. Here the geodesic curvature of the field lines becomes small and a partly omnigeneous plasma state is reached.

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