The ion temperatureTi, the toroidal plasma rotation in the core and at the edge, vtor and
vtor,edge, and the poloidal rotation at the edge, vpol,edge, is measured using the charge
exchange recombination spectroscopy system (CXRS) at ASDEX Upgrade [97]. Addi- tionally the impurity density can be inferred from the intensity of the observed radiation. This method uses charge exchange collisions of fast neutrals of the NBI beam with fully ionised impurity ions in the plasma, which are in thermal equilibrium with the main ion species (typically deuterium). During the charge exchange collision the electron is trans- ferred from the neutral atom to the impurity ion. The impurity ion remains in an excited state, falls into the ground state and thereby emits a photon with a specific frequency. Electrons in different excited states form a particular spectrum for each element. The emission process is so fast, that the measured light holds the information of the fully ionised atom, such as the Doppler-shift due to the velocity of the ion. The temperature of the observed impurity ion can be deduced from the Doppler width of the spectral lines. The velocity along a line of sight can be calculated from the Doppler shift of the observed peak to their natural emission line frequency. Depending on the impurity content of the plasma, typically boron B5+ or carbon C6+ are observed.
The CXRS diagnostic at ASDEX Upgrade consists of four different systems: One toroidal core system looking at beam number 8 has a rather poor resolution and low sensitivity and is only used in this work if the other systems are not available (CHZ). The other three systems are looking at beam 3: all systems deliver the ion temperature and impurity concentrations in the region covered by the optics. Additionally the toroidal core system (CEZ) delivers the toroidal rotation of the core plasma, the toroidal edge system (CMZ) measures the toroidal rotation at the edge and the poloidal edge system (CPZ) provides the poloidal rotation at the edge. In this work the ion temperature data of the edge systems are combined with the data of the core system to one single profile if both measurements are available.
The core systems CEZ and the toroidal edge system CMZ have been upgraded recently to increase the spatial resolution and the signal level, which allows a higher temporal resolution [97]. The core system has a radial resolution of 2 to 2.5 cm. It covers the plasma cross section at the midplane on the low field side from the plasma edge up to the core with 30 lines of sight. The minimum exposure time can be as low as 4 ms
4.3. CHARGE EXCHANGE RECOMBINATION SPECTROSCOPY
if the concentration of the observed impurity species is high enough to obtain a decent signal to noise ratio. The edge system has 8 LOS covering the outermost 9 cm of the plasma up to the separatrix which leads to a radial separation of the channels of 1.1 cm. The radial extension of the measurement region is only 3 mm. For that reason a radial sweep of the separatrix position on the low field sideRout by about 1.5 cm is performed
regularly to utilise the full capacity of the diagnostic. To this end the ion temperature and rotation profiles are swept past the measurement volumes to gain a better radial resolution and an overlap of the measurements of the channels. This allows a cross calibration between channels and a precise evaluation of the gradients of ion temperature, density and rotations. The new poloidal edge system covers the outermost 10 cm of the plasma with 8 LOS, which leads to a radial separation of the channels of 1.3 cm and a radial extension of the measurement of only 5 mm. For the poloidal system the radial resolution is also increased by performing radial sweeps of the low field side plasma position. The exposure time is 2.2 ms for both edge diagnostics.
As mentioned in chapter3.1the beam voltage and thus the beam power of the NW NBI injector was reduced in order to vary the NBI heating power in smaller steps. The volt- age of the SW NBI was kept at its maximum value of 60 keV to increase the signal of the CXRS measurements. A lower acceleration voltage would lead to a stronger attenu- ation of the beam and it would not reach the core of the plasma. This would hamper ion temperature and plasma rotation measurements in the central part of the plasma, which would mean the loss of one of the major diagnostics of this work. However, the ion tem- perature and rotation should also be measured in periods of the discharges with low NBI powers, which did not allow the use of the full 2.5 MW heating power from one source with full acceleration voltage. For that reason 16 ms long beam blips with full voltage from beam 3 of the SE NBI injector were introduced allowing CXRS measurements up to the core [98]. Owing to the short duration of the beam blips and a duty cycle of 15 %, their energy deposition could be kept small enough to leave the plasma temperature and rotation as unperturbed as possible while allowing reasonableTi and vrot measurements.
For each analysed phase with beam blips the time averaged power injected by the beam blips<PNBI,blips>is less than 10 % ofPaux,total and the on-time of the beamτblipis less
than 15 % ofτE. The timing of the 16 ms beam blips was done in a way that the first half
of the first 4 ms frame of the CXRS core system lies in the rising edge of the beam power. The second half is in the full power phase in which the beam is still stabilising. Due to the accompanying fluctuations this frame cannot be used for the analysis. The next three frames provide Ti, vtor, vtor,edge and vpol,edge measurements and are used for the analy-
sis. The last frame is covering the switch-off of the beam and is discarded. However, the introduction of beam blips prevents the analysis of the radial electric fieldEr (see chapter
5.4). Despite the timing of the beam blips with the radial sweeps of the plasma edge the data points are radially not dense enough to allow a sufficient precise evaluation of the edge impurity density gradients. Additionally the signal to noise ratio drops considerably when using beam blips. For that reason the analysis of the electric field was only possible in discharge phases with a constant NBI power of 2.5 MW from beam 3.
dition e.g. the density and beam voltage (NBI beam attenuation), the boron concentration (signal strength) or stability of the NBI beam (beam blips). For the analysed discharges typically the errors range from 3 % around mid radius to 10 % in the center and at the edge for good measurement conditions. For beam blip analysis these errors can increase to 20 to 30 %. The systematic error of the rotation measurement is slightly higher by 1 to 2 % arising from an additional source of error from the wavelength calibration.
The toroidal and poloidal measuring locations of all four systems are shown in figure3.1
and figure3.2as orange (CHZ), green (CEZ), red (CMZ) and blue (CPZ) crosses.