2. The ASDEX Upgrade tokamak 9
2.3. Basic Diagnostic
AUG is equipped with two pairs of ICRH antennas, where each antenna of a pair is situated on the opposite sides of the torus (see Fig. 2.2a). During the AUG tungsten program, these antennas were coated by tungsten as well. However, a significant increase in the impurity influx was observed when ICRH heating was used. The strong electric fields in the antenna have accelerated light impurities and increased sputtering of tungsten at the antenna limiters which enhanced the tungsten source. Therefore, limiters of two of the four antennas were coated with boron in 2012. The second antenna pair was redesigned to reduce the RF image currents in the antenna frame and refurbished in 2015. The 3 strap antenna concept was employed [12], effectively reducing the W source by a factor of 2, on the level equal to that of the boron coated antennas. Moreover, by an intentional dephasing of the central and side straps of this antenna, it is possible to modulate the tungsten influx while the heating is kept constant. Therefore, this new antenna also provides a unique tool for W transport studies and modulation transport experiments similar to the one of A. Janzer [13] can be performed.
2.3. Basic Diagnostic
The ASDEX Upgrade tokamak is equipped with an extensive set of diagnostics to monitor the plasma behavior. This can be illustrated by the fact, that about 150 shotfiles and 30 GB of data are recorded for a typical discharge. In the next sections, the most relevant AUG diagnostics for this thesis will be described.
2.3.1. Electron Density
The line integrated electron density is commonly measured by interferometers at tokamaks. Light passing the plasma experiences a small time delay, proportional to the line integrated electron density as the refractive index is changed by the electron density. The phase shift of the laser beam is determined from the interference with a reference signal, and the density is obtained by unwrapping the phase of the measured signal. AUG is equipped with five lines-of-sights (LOSs) of the deuterium cyanide (DCN) laser interferometer system, operating at 195µm. Additionally, there is a two color system using three CO2lasers (at 10.6µm) and HeNe lasers to compensate for mechanical vibrations of the tokamak vessel [14]. The unfolding is facilitated by the Integrated Data Analysis (IDA) code developed by R. Fisher [15]. Well determined edge profiles are necessary for a reliable unfolding of the line integrated density measurements and therefore the steep pedestal density profile is measured by a dedicated lithium beam diagnostics. In addition to that, ne is measured independently by the 16 channel core
2. The ASDEX Upgrade tokamak
Thompson scattering (TS) system [16]. The density is proportional to the intensity of the scattered light and in contrast to the interferometers, these measurements are local.
2.3.2. Electron Temperature
The electron temperature is measured by electron cyclotron emission (ECE). The gyrating electrons on their orbits emit photons due to the cyclotron radiation on their gyro-frequency and higher harmonics. If the plasma density is high enough, the plasma can be considered as optically thick at these frequencies, and radiate as the black-body.
The radiation intensity is then approximated by the Rayleigh-Jeans formula I = ω
2kTe 8π3c3.
In such a case, the radiation intensity is proportional to the electron temperature Te, independent of the electron density. The frequency of the radiation is given by the Larmor frequency and hence depends on the local value of the magnetic field.
Therefore, the radial coordinate of the radiation origin can be directly deduced. The toroidal positions of the ECE measurements in a typical AUG discharges are depicted in Fig. 2.2.
The electron cyclotron emission is measured on AUG by a 1D ECE Heterodyne radiometer operating in the X-mode at the second harmonic [17]. This radiometer has 60 channels which are sampled at 1 MHz rate on different consecutive frequency bands.
Therefore, this diagnostics provides measurements of the electron temperature with rather high temporal and spatial resolution. The level of accuracy is expected to be about 7% in the absolute value of the temperature due to the calibration uncertainty, limited amplifiers stability, non-linearity and other issues. The observed uncertainty in the position of these measurements is about 1 cm, but it will be reduced when a new model for a warm resonance position and ray tracing will be applied [18].
The electron temperature profiles are also routinely monitored by the vertical Thomp-son scattering diagnostics (VTA) [19]. This diagnostic measures the Doppler broad-ening of the light from the very intensive laser pulses scattered by the free electrons in the plasma. However, due to a larger uncertainty and low laser repetition rate, the profiles from the ECE diagnostics are usually preferred in the present work.
2.3.3. Ion Temperature and Rotation
The temperature, rotation, and density of the light impurities are measured by the active charge exchange spectroscopy (CXRS) system. This experimental technique relies on the charge exchange process between a donor neutral D0 provided by the NBI and an
2.3. Basic Diagnostic
impurity IZ+ in the plasma:
IZ++D0 −→I(Z−1)+∗ +D1+.
The electron captured by the impurity stays for a short time in the excited state and then experiences a radiative decay leading to a cascade of transitions to the ground level. The light emitted during specific transitions is analyzed spectroscopically, and the impurity velocity and temperature with the corresponding statistical uncertainty can be evaluated from the Doppler shift and broadening of the measured spectral line.
The impurity temperature is assumed to be equal to the bulk ions, due to significant collisional energy exchange. As predicted by neoclassical theory, the toroidal velocity of the light impurities is slightly lower than of the bulk ions, but the difference is typically less than 5% in the core. The neoclassical correction can be found using the TRANSP (NCLASS) code or an analytical formula [20].
In 2010, an improved core CXRS system (CER) was installed at ASDEX Upgrade [21]. The profiles are now measured along 25 lines of sight (LOS) which cross the NBI path from Q3 beam slightly above the mid-plane between the magnetic axis and the pedestal top (see Fig. 2.2). The CXRS spectra are routinely measured with 10 ms integration time on impurities present in the plasma, typically either B, C or N. After a boronization for wall conditioning the intensity of the boron line dramatically increases and, therefore, the visible boron CX line B V 7-6 (494.467 nm) is preferred. When the boron concentration decreases, the carbon line C VI 8-7 (529.059 nm) can be utilized.
In the nitrogen seeding experiments, the line NVII 9-8 (566.937 nm) must be measured because the carbon and boron spectra are partially disturbed by the nitrogen lines.