Cross section of C-Mod ion sensitive probe with all parts to scale. The Collector is recessed about an ion Larmor radius behind the Guard. Because the electron Larmor radius is so much smaller than the ion, this prevents any electron from streaming directly along the magnetic field to the Collector.
Cross section of the Alcator C-Mod tokamak with a near double null plasma equilibrium. The red line is the last closed flux surface, the boundary between open and closed magnetic flux surfaces. Inset is a close-up view of the ISP scanning probe which scans through the plasma from behind the limiter shadow to the last closed flux surface.
Exploded view of the ion sensitive probe. The Collector, Guard, and Langmuir probe are located and electrically isolated with precision-machined (±15 μm diameter) alumina ceramic cylinders. Depths are adjusted by filing the length of the cylinders down to ±5 μm. The stainless steel nut holds the entire assembly in the TZM head. Behind the head is the standard Alcator C-Mod scanning probe arm. The ceramic pins and mica insulators insure that the plasma-exposed probe head remains floating from ground.
Rear view of the ion sensitive probe sub-assembly. The cylindrical ceramic sleeves can be seen behind the metal parts. Molybdenum wires with a cross-hatch pattern cut into their tips plug into each of the four holes.
Plasma-facing view of the assembled probe head. Although not part of this assembly, the location of the Blob Langmuir probe is shown.
Results of finite element simulation in COMSOL of the ion sensitive probe plunging the scrape-off layer to a peak heat flux of 0.7 GW/m2. Top panel is surface temperature at its peak (t = 25 ms). Bottom panel is peak temperature of the tungsten Langmuir probe and TZM Guard over the plunge. Although the tungsten Langmuir probe reaches the highest temperature, scan depth will be limited by all of the other TZM parts due to their lower melting temperature.
Diagram of the new Grid Card circuit. A high voltage ground plane is driven by a PA94 op-amp. This voltage sets the voltage on “grid” and “mirror grid.” It also can be used to bias the coaxial shields of the cables. The PA94 responds to the difference between a programmed voltage and a reference voltage (which can be another probe bias, a floating probe, or ground) at its inputs. Variable gain transimpedance amplifiers measure the grid current. A “mirror” grid can be implemented to negate the displacement current due to the probe capacitance. The card outputs measurements of the high voltage (divided by 40) as well as the probe current (at both 1 and 40 times, for a larger dynamic range).
Diagram of the ISP electronic system. An analog output sends waveforms to the Grid card and sweep power supplies. Each Langmuir probe card may be controlled by a different sweep power supply. In this setup the Collector was biased with respect to the Guard. The Collector coaxial cable shield was biased at the same voltage as the center conductor to reduce displacement current. A variable capacitor attached to the mirror circuit was used to offset the remaining displacement currents.
Removal of the residual displacement current from current measurement. Applied voltage and Collector current before the plasma are shown in the top panels. A fast Fourier transform is taken from the signals to get the complex circuit impedance. This impedance is then applied to the during-plasma voltage to get the circuit current. The circuit current is then subtracted from the total current to get that due to the plasma alone. Although this process is not important near the seperatrix, where plasma current dominates, it is important in the far scrape-off layer where the signal is low and the circuit response distorts the measurement signal.
Typical voltage and current measurement during a spatial scan of the ion sensitive probe. Sign convention: positive current is ions collected, negative current is electrons collected. As the probe scans into the plasma, the Langmuir probe and Collector I-V characteristics exhibit broader knees—indicating increasing temperature. With the Collector voltage negative with respect to the Guard voltage (in this case V C = V G − 10 V), only positive current is found on the Collector.
I-V characteristics demonstrating aid of normalizing to density fluctuations. A Langmuir probe very close to the ion sensitive probe is kept in ion saturation to measure the density fluctuations (panel (a)). A large density fluctuation (blob) hits the probe as the ISP is going through the exponential part of its sweep. The minimum error fit to the raw Collector current is dominated by this density fluctuation. Normalizing the Collector current to the density fluctuations improves the fit (panel (c)).
SOL profiles of ion and electron temperatures in a 5.4 T, 0.58 MA, ohmic-heated, deuterium plasma. Two profiles of ion temperature are shown: the dashed line is from fits to the ISP I-V. The solid line is from fits to the ISP I-V normalized to the Blob LP current. The density fluctuation normalization reduces scatter of the data across the SOL. Ion temperature remains higher than electron temperature because of its poorer thermal conductivity.
Comparison of different methods of determining the plasma potential from ISP measurements. Middle and bottom panels are from consecutive voltage sweeps within the same probe scan. Top panel is from the same sweep as the bottom. Top panel is the Guard current. Middle panel is with the Collector biased negative with respect to the Guard. Bottom panel is the Collector biased positive with respect to the Guard, allowing electrons to E × B drift onto the Collector. Neither ISP measurements of plasma potential (start of the exponential decay and the knee where the current decays to zero) are near the plasma potential calculated from the domed Langmuir probe (V p = V f + 2.7T e). Note that the floating potential of the Guard is a good indicator of the knee, especially when the Collector is biased above the Guard and collecting net electrons.
Article metrics loading...
Full text loading...