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#HXR measurement on GOLEM tokamak – dependence on pressure II
Date: 11.4.2013
Shots: 11799-11822
Goal: The goal of this measurement was to further investigate how HXR count depends on pressure.
##Description
Similar measurement was already conducted, however its results were quite puzzling, because the HXR count was highly influenced by strange tokamak behaviour (see report for details). Our goal now was to try to avoid the strange phenomenon observed at the time, so that the results are more reliable. There was a glow discharge before shot 11802 (i.e. before the actual measurement commenced). A proposal was made that the discharge could prevent the issue from occuring, and it was also needed to check whether anything else changes. The settings of the tokamak parameters were not changed during the session (apart from pressure) and are noted below.
Preionisation ON
C~BT~=1000 V triggered at 5.0 ms
C~CD~=500 V triggered at 8.0 ms
C~BD~ and C~ST~ were not charged.
The experimental setup was similar as it was it the previous measurement. An HXR scintillator detector was placed in proximity of the tokamak. The measured data were processed the same way as before, the same MATLAB script was used to count number of peaks in the signal.
##Measured data
![](tab1.png)
Tab. 1 lists all relevant shots performed during the session. First shot (11801) was made just before the glow discharge. The HXR count dependence on pressure is plotted in Fig. 1.
![
Fig. 1: HXR dependence on pressure](/TrainingCourses/FTTF/2012-2013/HXR/hxr2.png)
There are several things that should be noted. Firstly, the sudden drop in loop voltage that was troubling during the previous measurement was not observed. It is possible that the glow discharge performed before the measurement contributed to this. Secondly, plasma was achieved for much higher pressures than usually. The plasma was observed even for pressure above 40 mPa, which is something that does not happen very often. The glow discharge can again be the reason.
The data now clearly show the trend, and there is no need to do any alternations to see it. Except for very low and very high pressures, the HXR count shows approximately linear decrease with pressure, which is exactly what was expected.
##Conclusion
Due to the fact that the phenomenon observed during previous measurement was avoided, this measurement was more successful and the results more conclusive. The HXR count decreases linearly with pressure, as predicted by theory. The rate at which the runaway electrons are slowed is proportional to density, which is proportional to pressure. The linear behaviour apparently fades for high pressures, where the decrease is slower. However, it should be noted that pressures as high as observed in this session are not seen very often, usually plasma does not appear at all. The fact that it appeared this time may be attributed to the fact that there was a glow discharge prior to the measurement. Even at these pressures, some runaways are produced.
The presence of the glow discharge prior to the session clearly puts the tokamak in some sort of unusual state, although it is unclear what exactly happens (there is likely less impurities on the walls, but other effects may arise). This state appears to prevent the sudden drop in U~loop~ that was observed previously. It may also enable the plasma to exist for higher pressures than without the discharge. These statements however may not be true – more research would have to be done in order to test whether the glow discharge really has the above mentioned effects.
#HXR measurement on the GOLEM tokamak – Spatial distribution of HXR radiation.
Date: 20.6.2013
Shots: 12611-12629
Goal: The mail goal of the measurement was to examine the spatial distribution of HXR emission, namely to scan the dependence of HXR emission on the vertical position of the detector.
##Description
The measurement required that significant amount of HXR photons is present. With respect to this requirement, the machine parameters were set as follows (see shot
12611 to see the settings):
Preionisation ON
C~BT~=600 V triggered at 5.0 ms
C~CD~=500 V triggered at 5.5 ms
C~BD~ and C~ST~ were not charged.
Requested pressure p=12 mPa
These parameters were kept constant during the whole session, and the vertical position of the detector was changed. Glow discharge was performed in the tokamak after shot 12622.
##Experimental setup
HXR radiation was directly measured by a NaI crystal scintillator detector mounted on the tokamak in a way that allowed shifting in the vertical direction. The analog signal was collected and digitized by the NIturbo data acquisition system. By plotting the raw signal in time, Fig. 1, stand-alone peaks can be observed, each corresponding to an incident photon. A simple MATLAB script was written to identify and therefore count the number of peaks in each shot. This script was used to post-process all shots within the session.
![
Fig. 1: An example of raw HXR signal from the detector plotted against time.](/Diagnostics/Radiation/HXR/sessions/1212FTTFpraktika/signal.png)