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 m-cll as to reduce both the injected particle-wall surface interaction and the impurity content in the plasma. SMBI can bt considered a significant improvement over conventional gas puffing. With a penetration depth of hydrogen particles greater than 15 cm, thy rate of increase of electron density, d (n) over bar (e)/dt, was up to 7.2 x 10(20) m(-3) s(-1) without disruption, and the highest plasma density was (n) over bar (e) = 8.2 x 10(19) m(-3). The density profile peaking factor Q(n) reached a maximum value of more than 1.67 after SMBI. The energy confinement time tau (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 tu the Hagena empirical scaling law; of clustering onset, Gamma* = kd(0.85)P(o)/T-o(2.29). If Gamma* > 100, clusters will form. In the present experiment Gamma* is about 127.