Rate of production of water vapour in low-power, high-repetition-rate discharge cleaning

The effects of low-power, high-repetition-rate discharge cleaning in a tokamak have been investigated by measuring the water vapour pressure at different values of the filling pressure, plasma current and repetition rate. As the filling pressure and repetition rate increase, the water vapour pressure also increases, while the optimum value of the plasma current for the production of H2O in the JFT-2 tokamak is 5 ∼ 10 kA. The time variation of H2O was measured and it was found to decrease as the t-0.9 law. These results are explained by a water particle balance equation derived from a simplified model and by the plasma density and temperature measured by a double probe in the scrape-off layer. (author)