Description

Temporal evolution of a typical discharge of the CASTOR tokamak is shown in figure [main-param]. Before discharge, the vacuum vessel is evacuated down to the pressure of \(10^{-4}\) Pa and filled by working gas (hydrogen). After that, the power supplies are connected to the toroidal magnetic field coils. Since now, the toroidal magnetic field \(B_{tor}\) starts increasing (detailed behaviour is described in appendix [ch_appendix_Btor]). When \(B_{tor}\) reaches the range of 0.8 – 1 T (which is usually 10 – 25 ms after switching it on), the primary transformer winding is automatically connected to its power supplies (capacitor banks) and the toroidal electric field \(E_{tor}\) is induced within the vacuum vessel. The \(B_{tor}\) is measured by an open loop fixed at the top of the vessel. The measured voltage is termed a loop voltage \(U_{loop}\), its temporal evolution is shown in the second panel of figure [main-param]. The \(E_{tor}=U_{loop}/2\pi R\) starts to accelerate free electrons, which are produced by an electron gun placed in the limiter shadow. Some free electrons are always present due to the cosmic radiation,

but their amount is not sufficient for a fast and reproducible breakdown (the moment of ignition of the discharge). After breakdown, the electron density \(n_e\) increases exponentially, as shown in figure [main-param], right bottom panel. After \(\sim1\) ms from breakdown, the working gas is completely ionized. Simultaneously, the plasma current \(I_{pl}\) increases to the rate of \(2~\rm{MA}/\rm{s}\), which is determined by the primary circuit parameters. The slope \(dI_{pl}/d{\rm t}\) has to be kept relatively low to negate the skin effect, which could drive the current only on the surface of the plasma column. In this case, the radial profile of plasma current and plasma current density gets a hollow shape, plasma gets unstable and consequently disrupts.

After the plasma current reaches values of \(\sim10\) kA, it tends to remain constant for the next \(20-30\) ms. During this quasistationary phase of discharge, the loop voltage is \(2-3\) V, as shown in figure [main-param]. It is interesting to realize that the plasma current in the range of 10 kA is driven by toroidal electric field \(1~\rm{V}/\rm{m}\) only. The quasistationary phase is exploited for physical measurements. After the \(20-30\) ms, the primary winding of the transformer is set to be short circuited. The plasma current exponentially decays. This is called a soft termination of discharge or a “soft landing".