Through compact toroid (CT) injection experiments on the TEXT-U tokamak (with BT ≃ 10 kG and IP ≃ 100 kA), it has been shown that theacceleration electrode configuration, particularly in the vicinity of the toroidal field (TF) coils of the tokamak, has a strong effect on penetrationperformance. In initial experiments, premature stopping of CTs within theinjector was seen at anomalously low TF strengths. Two modifications werefound to greatly improve performance: (a) removal of a section of the innerelectrode and (b) increased diameter of the `drift tube' (which guides theCT into the tokamak after acceleration). It is proposed that the primary drag mechanism slowing CTs is toroidal flux trapping, which occurs when a CT displaces transverse TF trapped within the flux conserving walls of the acceleration electrodes (ordrift tube). For a simple two dimensional (2-D) geometry, a magnetostatic analysis produces a CT kinetic energy requirement of 1/2ρv2 ⩾ α(B02/2μ0), with α = 2/(1-a2/R2) adimensionless number that is dependent on the CT radius a normalized by the drift tube radius R. For a typical CT, this can greatly increase the required energies. A numerical analysis in 3-D confirms the analytical result for long CTs (with length L such that L/a ≳ 10). In addition to flux trapping, the CT shape is also shown to affect theenergy criterion. These findings indicate that a realistic assessment of the kinetic energyrequired for a CT to penetrate a particular tokamak TF must take intoaccount the interaction of the magnetic field with the electrode walls of the injector.
Experimental studies of the dynamics of compact toroid injected into the JFT-2M tokamak