Hydrogen cluster-like behaviour during supersonic molecular beam injection on the HL-1M tokamak

A new method of gas fuelling, pulsed supersonic molecular beam injection (SMBI), has been successfully developed and used in the HL-1M tokamak. SMBI is an attempt to enhance the penetration depth and the fuelling efficiency, as well as to reduce both the injected particle-wall surface interaction and the impurity content in the plasma. SMBI can be considered a significant improvement over conventional gas puffing. With a penetration depth of hydrogen particles greater than 15?cm, the rate of increase of electron density, de/dt, was up to 7.2 ? 1020m-3?s-1 without disruption, and the highest plasma density was e = 8.2 ? 1019?m-3. The density profile peaking factor Qn reached a maximum value of more than 1.67 after SMBI. The energy confinement time ?E measured by diamagnetism is 10-30% longer than that with gas puffing with the other discharge conditions kept the same. SMBI has recently been improved to enhance the flux of the beam and to allow a survey of the cluster effect within the beam. A series of new phenomena show the interaction of the beam (including clusters) with the toroidal plasma, which indicates that hydrogen clusters may be produced in the beam according to the Hagena empirical scaling law of clustering onset, ?* = kd0.85P0/T02.29. If ?* > 100, clusters will form. In the present experiment ?* is about 127.