Fast electron temperature diagnostic based on Langmuir probe current harmonic detection on DIII-D

A method for the measurement of electron temperature with high spatial and temporal resolution was recently implemented on a fast reciprocating probe on the DIII-D tokamak. The technique, previously used on the Torus Experiment for Technology Oriented Research, is based on detection of harmonics generated in the current spectrum of a single Langmuir probe driven by high-frequency sinusoidal voltage. The method was implemented on DIII-D with a drive frequency of 400 kHz and with improvements such as fully digital processing and active voltage feedback, thus allowing temperature measurements with a bandwidth of up to 200 kHz. Probe voltage and current were recorded at the sampling rate of 5 MHz and the amplitudes of the first (400 kHz) and the second (800 kHz) current harmonics were extracted by digital filtering. Digital processing does not introduce any phase delays (as analog detection does), making the technique suitable for correlation measurements. Electron temperatures obtained from the harmonic diagnostic are in reasonable agreement with swept double probe and Thomson scattering data. First measurements of the conductive turbulent heat flux in DIII-D have been performed. The measured values of the heat flux are consistent with the power balance.