Spontaneous Self-Organization in a Helicon Plasma Device: Instabilities, Bifurcation, Hysteresis and Plasma Detachment

We observe a spontaneous, self-organized global transition in the plasma dynamics via a transport bifurcation
¹
in the linear helicon plasma device, Controlled Shear Decorrelation eXperiment, with increasing magnetic field (B). For B < B
crit
, the plasma is dominated by density gradient driven resistive drift waves rotating in the electron diamagnetic drift direction. For B > B
crit
the plasma exhibits steepened density and ion temperature gradients and both azimuthal and parallel velocity shear layers, along with multiple, radially separated plasma instabilities: high azimuthal mode number fluctuations rotating in the ion diamagnetic drift direction at the core (signatures of ITG modes), resistive drift waves at the density gradient and turbulence driven sheared flows near the edge. After the transition, we observe very rich plasma dynamics such as formation of a radial internal transport barrier, inward particle transport, formation and propagation of blobs, intermittency etc. Simultaneously, we also observe axial plasma detachment from the end of the device along with formation of the helicon core. Using spectroscopy, we find that the detachment also follows the hysteresis curves associated with the transport bifurcation, which led to steepening of the profiles. We show that this spontaneous self-organization is universal, but the value of B
crit
can depend on the source parameters (pressure, gas flow rate, rf power). This study allows us to access new regimes to study plasma turbulence and transport, helicon core formation as well as plasma detachment for divertor relevant physics.