Equal Electron Temperatures Research Articles

Radial transport in the far scrape-off layer of ASDEX Upgrade during L-mode and ELMy H-mode

The radial turbulent particle flux and the Reynolds stress in the scrape-off layer (SOL) of ASDEX Upgrade were investigated for two limited L-mode (low confinement) and one ELMy H-mode (high confinement) discharge. A fast reciprocating probe was used with a probe head containing five Langmuir probes. One of the probe pins was protruding radially above the other pins. The radial electric field component was derived from the difference of the floating potentials of the protruding pin and another pin nearby. The poloidal electric field component was derived from the difference of the floating potentials of two probes on the same poloidal meridian. In both cases, equal electron temperatures on all probe pins had to be assumed. Of the other pins one was biased to ion saturation (−70 V), whereas one was swept to record the current–voltage characteristic. A detailed statistical analysis of the plasma conditions in L- and H-modes and, more specifically, also during and in between type-I edge-localized mode (ELM) events is presented. We found evidence for similarities in the statistics of transport in L-mode, during ELM events and in between ELMs, suggesting that in all three cases the transport is undertaken by blobs and filamentary structures, respectively. During the intervals in between ELMs the blob structures appear to be relatively smaller as compared with the L-mode.

Capped probe for ion temperature diagnostic in a magnetized plasma

A simple ion temperature measurement method in a magnetic field using an electrostatic capped probe is presented and tested. The probe with the axis oriented parallel with the magnetic field is a small cylinder whose flat tip is insulated with a cap to inhibit electron inflow parallel to the field. The ion collector current, essentially less affected, becomes comparable to or larger than the saturation electron current, and the usual retarding potential technique can be used to measure ion temperature. Measurements were made in a diffused helium plasma with the density 1010 ∼ 1013 cm−3 at the gas pressure ∼mTorr and the field 0.8 ∼ 3.5 kG. Ion temperatures obtained are 0.8 ∼ 2 eV, which nearly equal electron temperatures, and are compared with data from the ion probe and Doppler profile measurements. Availabilities of the method and limiting conditions for the temperature determination are also discussed.

Isotopic effects in the establishment of thermal equilibrium of stationary H2- and D2-glow plasmas

In stationary glow plasmas, thermal equilibrium of the electron gas with the neutral gas is established, if, with increasing gas pressure, the diffusion lifetime of plasma electrons becomes high enough. In D2-plasmas, thermal equilibrium sets in at higher pressures than in H2. Equal electron temperatures are measured in H2- and D2-glows, if the corresponding gas pressures are in the ratiopd2/Ph2=1.22. This isotopic effect of electron temperature and of thermal equilibrium is compared with calculations of the Boltzmann equation, taking into account the influence of ambipolar diffusion on the electron velocity distribution. There is a good agreement between measurement and theory.