Diagnostics/Radiation/HXR/sessions/1212FTTFpraktika/in
dex
HXR measurement on GOLEM tokamak – dependence on
pressure
Date: 9.1.2013Shots: 10527-10564
Goal: The mail goal of the measurement was to find the dependence of HXR emission
on pressure of the working gas and compare the results to similar measurement
conducted earlier.
Description
The parameters were set as follows (see shot 10531 to see the
settings):Preionisation ONCBT=800 V triggered at 5.0 msCCD=400 V triggered at 7.0
msCBD and CST were not charged.These parameters were kept constant throughout
the session, only parameter that was changing was the pressure (6-25 mPa).
Some shots exhibit strange behaviour, which will be referred to as the „anomaly“.
Typically 7-8 ms after plasma breakdown, the loop voltage suddenly drops and plasma
current rapidly increases. After typically 1-2 ms these parameters return to normal
and the plasma ceases to exist (see shot 10537). This behaviour however occures
quite often. See shot 10458 to see FastCamera data, which imply that plasma
violently changes position during this final stage of the shot.The anomaly is usually
accompanied by significant HXR emission, and it was therefore suspected that it
could affect this measurement. All the shots were therefore analysed and those
showing this phenomenon will be treated separately.
Experimental setup
HXR radiation was directly measured by a NaI crystal scintillator detector placed near
the tokamak vacuum vessel (Fig. 1). The analog signal was collected and digitized by
the NIturbo data acquisition system. By plotting the raw signal in time, Fig. 2, standalone 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.
Additionally, a routine determining the energy of incident photons based on their
height was also implemented but still needs some refinement. For calibration,
Caesium-137 was used. In this way, absolutely calibrated HXR radiation spectra can
be obtained.
Fig. 1: Photograph of the scintillator detector placed near the tokamak.
Fig. 2: An example of raw HXR signal from the detector plotted against time.
Measured data
Tab. 1 lists all shots performed during the session with available HXR data. HXR data
from shots 10543 to 10556 are unavailable due to technical issues. The table also
contains information on whether plasma was achieved and whether the anomaly
described above took place during the particular shot.
Tab. 1: All shots with available HXR data
As can be seen, the pressure was not changed continuously from the beginning of
the examined interval to its end in one go. Furthermore, due to the aforementioned
issues, first column was measured some time before the second one, and 15 shots in
total were performed in between. What is also worth mentioning is that the anomaly
described above only occurs in the first batch of shots, there is not a single occurence
in the second one. Let us now have a look at a chart of HXR(p) dependence (Fig. 3).
Fig. 3: Dependence of HXR count on the pressure
The data from the second batch show approximately linear decrease (as shown by
the linear fit). This behaviour was assumed and corresponds with the measurements
conducted earlier.
As for the data from the first batch, it is easy to recognize shots with the anomaly and
those without, as the two groups show different HXR behaviour. Those with the
anomaly generally show higher HXR – as noted earlier, the anomaly itself appears to
be a source of runaway electrons, hence the higher HXR. The anomalous shots too
show approximately linear decrease for pressures higher than about 10 mPa. The
HXR count for lower pressures is lower.
What is especially troubling is the fact, that three out of the four non-anomalous
shots from the first batch show different HXR count than shots from the second
batch, although the pressure and all the other parameters are the same. A hint to
explain this behaviour can be found when looking at the order in which the shots
were performed. The shots that differ from the second batch are shots 10531, 10533
and 10534, one of the first shots in the session. These were followed by 5 anomalous
shots, after which non-anomalous shot 10541 was performed – this shot appears to
already lie on the same line as shots from the second batch. Before the measurement
started (shot 10531) there were only 4 plasma shots in the session. Previous
tokamak operation was 6 days earlier. This would suggest that the behaviour of the
tokamak at the beginning of the session is different than after several shots are
performed.
In order to get more conclusive data a way to get rid of the anomalous HXR was found.
Because the anomaly always appears after quite well defined time elapses, the HXR
count from all the shots was calculated again, but this time the final stage of the shot
was not taken into account. Any HXR that result from the anomaly are therefore not
counted (but nor are any other HXR resulting from another process). The result can
be seen in Fig. 4. The linear fit again belongs to second batch data only.
Fig. 4: Dependence of HXR count on the pressure – calculated before the onset of the
anomaly
Nearly all points now lie approximately on the fit line from the second batch. Only
exceptions are shots 10531, 10533, 10534 and 10538. The first three are the shots
from the beginning of the session, as noted above, the last one is a shot with the
lowest pressure during which plasma was still achieved. The dependence is most
likely not linear in this low-pressure area.
Conclusion
The dependence of HXR emission on the working gas pressure appears to be linear
and decreasing with pressure. This behaviour was assumed, corresponds with
measurements conducted on the tokamak before, and is not surprising in any way.
Two more interesting things can however be mentioned. Firstly, some shots exhibit
strange phenomenon (described above) which significantly affects HXR emission.
During this particular session it appeared only in the first half of the measurement,
then it completely disappeared. Its nature is unknown, it may be some instability-
driven rapid process. Understanding of the exact reason and mechanism of this effect
may be important to understand the tokamak behaviour better.
Secondly, shots from the very beginning of the session appear to behave differently
than those from the end. Given that the tokamak was not operated for almost a week,
the likely explanation is that several shots are needed to clean the vessel, which would
mean there is cleaner plasma with less impurities in the later shots. It might therefore
be wise to conduct several (at least 10) shots at the beginning of each session to test
diagnostics etc. before actual physical research begins.
What would be very helpful in order to investigate reasons behind these things is
density measurement on the tokamak. Had these data been available for this session,
more clues to explain the issues could be found. Density is however not directly
measured as of today, and the data therefore could not help.