SYSTEMATICS OF DENSITY INCREASE AND RECYCLING BEHAVIOR IN JET H-MODES

The behaviour of plasma density has been studied in JET beam heated H-modes in the X-point configuration. The X-point tiles have been conditioned (in addition to the normal conditioning involving initial baking to 500-degrees-C; constant 300-degrees-C temperature between pulses and overnight helium glow cleaning) by the running of helium tokamak discharges (He conditioned) or by beryllium gettering. The particle removal rate in the Ohmic X-point phase is compared for He-conditioned and Be-gettered tiles, indicating that Be-gettering offers substantial initial advantages. The density rise in the H-mode is analysed for JET discharges to show three phases; an initial transient rise with density increase greatly exceeding the beam fuelling; a second phase with more gradual density rise and third phase (occurring in higher power longer pulses) where strong ingress of carbon fuels the plasma. The systematics of the first two phases are discussed using plasma edge measurements. The recovery from the "carbon bloom" phase of fuelling by the use of strong gas puffing is described. In the post recovery phase, which now lasts for approximately 3 s or more in JET H-modes, it is shown that the fuelling characteristics can be modelled using a two particle-reservoir model with reservoirs in the tiles and the plasma, yielding similar parameters to those obtained in JET limiter discharges with beam heating.