Cálculo de la Temperatura

The soft X-ray cameras are situated at two toroidally spaced locations. It applies 208 photodiodes spectrally filtered by a Be-foil [8] in front of the entrance window. It permits the transmittance of photons from about 1 keV on [22]. The detector responsivity is related to the thicknesses of the depletion layer and the Be filter, as well as to the optical transmittance characteristics of Be.

Comparison ofXUV and soft X-ray diagnostics: The discharge under consideration was introduced in Chapter III, where the series of minor disruptions was considered (Figure III.1).

Regard Figure IV.10, on the right: The soft X-ray signal is traced in blue, theXUV C- X views3 at Φ = 0 and Φ =π in red and green. All the three lines of sight cover poloidally the same plasma area. The energy expulsion at each minor disruption is featured by falling soft X-ray signals. An analysis of the expulsion itself is therefore hardly possible. Contrarily, the XUV diodes capture the energy bursts nicely. The amplitude is time- shifted, because the C-X views observe the heat pulse when it arrives in the divertor (this delay was also depicted by Figure III.3). The left hand side: The mode oscillations, which are clearly identified by theXUV channel, are not featured by the soft X-ray diagnostic, which is indeed not surprising due to the cooling. Under predisruptive plasma conditions, the XUV diagnostic is advantageous over the soft X-ray diagnostic for mode analysis.

The combination of the soft X-ray and the XUV diagnostics give a rough hint at the temperature evolution during a disruption. This is of great importance in MGI ex-

3

The C-X channel is an almost vertical line of sight of the top view camera, which crosses the Centre and the X-point.

Eth

Ip

C−X XUV top view Φ=π

C−X XUV top viewΦ=0

Minor disruption

XUV − Soft X−ray comparison, centre channels

soft X−ray mode locking magnetic mode (kHz) t (s)

ZOOM _{Minor disruption}

t =2.8682 s

5 10 15 20

0

Figure IV.10: Soft X-ray -XU V comparison. Minor disruption series in discharge #24413. On the left: Two poloidally overlapping views through the central plasma from the side (side view

cameras). Lower left: Spectrogram of the above-shown XU V signal. Right: The final three

minor disruptions traced by signals from vertically viewing channels (C-X passes through the

centre and the X-point). Symbols: Eth thermal plasma energy, Ip, plasma current.

periments, where temperature measurement by the ECE4 _{is impossible due to the high}

densities.

4

Chapter V

Methods For Plasma Radiation

Analysis

The fundamental methods for radiation analysis are described in this chapter. TheXUV

diagnostic is applicable for 2-D radiation tomography at one toroidal position (Φ = π). The aims of tomography and the problems arising from the restriction in the number of lines of sight are explained in the first section. The subsequent section presents the calculation guideline for evaluating the total radiation loss under consideration of the toroidal asymmetry. The final section considers examples for the application of these methods for analysing disruptive plasmas. On the basis of the introduced methods and the diagnostic information given above, resistive foil andXUV behaviour in a disruption is documented and the chain of predisruption events in an edge cooling disruption is explained.

V.1

Radiation tomography

The aim of radiation tomography during an MGI scenario is to reconstruct the behaviour of radiative structures. In the subsequent chapter, the focus lies on studying the dynamics of a radiation front formed during the injection in an MGI scenario. Capturing the radial penetration velocity of the radiation front towards the core and finding poloidally concentrated impurity amassments are the tasks. Additionally, tomograms are used for converting radiant power flux densities to local emissivity.

V.1.1

Basic accuracy limitations

Tomography in a tokamak is based on stationary cameras. In this point, it basically differs from tomography in medicine, where the camera head rotates around the patient and thus consists of a large number of lines of sight. However, for plasma radiation recon-

struction, two approaches are commonly used for enhancing the image quality: Rotation tomography a) and the principle of including physically reasonable boundary conditions

b).

a) Rotation tomography simulates a quasi-rotation of the cameras around the plasma column [22]. This is possible in cases of the known periodicy of a magnetic mode, which rotates around the torus. Then, the program calculates synthetic signals for virtual cam- eras positioned around the cross-sectional poloidal area at one toroidal location. Those synthetic signals are used in a subsequent step to reproduce the mode radiation again - now, with a higher accuracy. This is finally equivalent to e. g. magnetic resonance to- mography with the difference that in the tokamak application, the lines of sight ensemble is partly virtual.

b) The diffusion on magnetic flux surfaces is used as a boundary: Under normal confine- ment conditions, the ratio between perpendicular diffusion coefficient D⊥ and parallel diffusion Dk is only 10−6. Henceforth, the density along a considered flux surface can be considered as a constant [20].

However, both methods a) as well as b) can not be applied for reconstructing disrup- tion radiation. Rotation tomography is out of question, because in regard of a disruptive plasma, we are interested also in incoherent effects in various spectral regions. The aid of physical boundary conditions also can not be applied because of an enhanced per- pendicular transport by turbulence and island formation often present in disruptions. Consequentially, disruption radiation reconstruction must be carried out without any physical boundary conditions and without virtual lines of sight. As a consequence, arte- facts and inaccuracies will arise, which are explained and discussed with respect to both the intentions pursued by applying tomography as well as the allowed tolerances.

V.1.2

Maximum entropy tomography

The maximum entropy tomography system bases on the code package employed on W7-AS [23] and further developed for soft X-ray tomography on ASDEX Upgrade [22]. Imagine the plasma cross-sectional area, which is covered by the visual ranges of the crossedXUV

lines of sight. In this respective plane, a grid is defined. The grid forXUV reconstructions cover - in contrary to the soft X-ray grid1 _{- the entire vessel cross-sectional area and}

consists of 1073 squared pixels. A detector is illuminated by the pixel k, if the location of the pixel was within the respective visual range. The detector sees a bunch of pixels. Each one contributes to the measurement signal depending on the distance from the pinhole and depending on the distance between pinhole and detector. The contribution is normalised

1

Soft X-rays are radiated in the core - the edge plasma is not considered. The regions relevant for XU V, however, concern the edge as well as the core.