Dual Langmuir-probe array for 3D plasma studies in TORPEX

Abstract

We have designed and installed a new Langmuir-probe (LP) array diagnostic to determine basic three-dimensional (3D) features of plasmas in TORPEX. The diagnostic consists of two identical LP arrays, placed on opposite sides of the apparatus, which provide comprehensive coverage of the poloidal cross section at the two different toroidal locations. Cross correlation studies of signals from the arrays provide a basic way to extract 3D information from the plasmas, as experiments show. Moreover, the remarkable signal-to-noise performance of the front-end electronics allows us to follow a different approach in which we combine information from all probes in both arrays to reconstruct elementary 3D plasma structures at each acquisition time step. Then, through data analysis, we track the structures as they evolve in time. The LP arrays include a linear-motion mechanism that can displace radially the probes located on the low field side for experiments that require fine-tuning of the probe locations, and for operational compatibility with the recently installed in-vessel toroidal conductor.

Meandering particle bunches and a link between averages of time series of particle counts and higher-order moments

“…This E-field, in fact, is just the gradient of the plasma potential V p . We use, as a proxy for V p , 2D-resolved floating potential measurements 39,40 rescaled such that the magnitude of the gradients agrees with E-field fluctuation profiles obtained with a triple-probe 41 . The fast ions act as tracers and do not affect the fields.…”

Millimeter-Wave Beam Scattering by Field-Aligned Blobs in Simple Magnetized Toroidal Plasmas

“…A blob example is shown in Figs. 1(b)-1(d), where 2D measurements of the electron density n e are computed from ion-saturation current data obtained using the hexagonal turbulence imaging probe (HEXTIP) diagnostic [33] in combination with triple probe electron temperature measurements [34]. The evaluated density is cross-checked with the experimentally determined location of the upperhybrid resonance [34].…”

“…Figure 2(a) shows that the time-averaged detected profile is not strongly affected by the presence of the plasma. Figure 2(d) shows the 2D distribution of the coefficient of the cross-correlation at zero time lag between δPðx; tÞ measured with the detecting antenna at x ¼ 5 cm and δn e ðx; tÞ obtained with HEXTIP [33] :δn e ðx; tÞ and δPðx ¼ 5 cm; tÞ are positively correlated in some regions and negatively correlated in others. When the cross-correlation is computed with a nonzero time lag between δPðx; tÞ measured at x ¼ 5 cm and δn e ðx; tÞ, the same structures are obtained, but they are shifted vertically.…”

Optically isolated millimeter-wave detector for the Toroidal Plasma Experiment

“…In a typical experiment, we activate the mmw source, then the HWs, wait 0.1 s for the magnetron to stabilize, and record 0.2 s of the output of the OIMD, or of the bare SDD, using a 16-bit D-tAcq ACQ196CPCI data acquisition card 22 at 250 k samples/s (the card includes filtering for proper anti-aliasing) 16 . Since the SDD operates in square-law regime, the recorded voltage signal is proportional to the detected EM power which, in the absence of noise, is just the mmw power reaching the detector.…”

“…In the typical Simple Magnetized Torus (SMT) configuration, the magnetic field has a small vertical component B z ≤ 5 mT and a dominant toroidal component B φ ≈ 70 mT. SMT plasmas exhibit turbulent features that are well suited for studies of turbulent plasma structures 11,12,16 .…”