Transient impurity events in JET with the new ITER-like wall
Summary (2 min read)
1. Introduction
- Such events are visible as sharp increases in the total radiated power similar to what caused by laser ablation experiments.
- The impurities responsible have been studied using vacuum-ultra-violet (VUV) spectroscopy.
- Correlations with auxiliary heating power, plasma geometry (limiter or diverted magnetic configurations, as well as positions of the strike-points) external causes such as arcs from the lower-hybrid antennas or reciprocating probe plunges and disruptions have been investigated.
- Moreover, studies of the dust mobilized by disruptions examined using the high resolution Thomson scattering (HRTS) diagnostic and their correlations with TIEs are reported (section 3).
2. Transient impurity event analysis
- The occurrence of transient impurity events has been analysed in all phases of plasma discharges with current Ip > 1 MA.
- None of these events have been seen to lead to permanent changes in plasma conditions (e.g. permanent loss in confinement).
- For each event, the values and variations of the ohmic and auxiliary heating power, plasma current, plasma geometry (high- or low-field-side limiters, strike point positions for diverted plasmas), excursion in radiated power and total injected deuterium rate have been tabulated.
- In order to correctly normalize the event distributions, a database containing the total plasma operation time (in steps of 10 ms) for each geometrical configuration and each correlation quantity listed above has been created.
- Its error bars are evaluated through propagation of the statistical error of the number of events, equal to its square root.
2.1. Evolution during the campaign
- The discharges have been binned according to pulse number (bin size = 200), but each bin is independently normalized to the total plasma time in that bin.
- In the phases before, almost pure ohmic or ion cyclotron resonance frequency (ICRF) heated discharges had been performed.
- This suggests a correlation with input power with threshold at >.
- Since most TIEs occur in diverted configuration (see section 2.2), this is most probably a correlation with power delivered to the divertor.
- Since the normalization of the number of events has been performed over total plasma-time, this increase is probably caused by simple cumulative effects due to higher fraction of total time operated at high input power.
2.2. Geometrical dependencies and impurity identification
- Similarly, configurations with outer strike point on tile 7 account instead for only 6 events, while the outer strike point has never been positioned on tile 8, so no statistics is available.
- Since most high power scenarios were performed in configuration [3, 5], its ∼ 30% higher probability with respect to [4, 6] is most probably due the power dependence of TIEs.
- The presence of this element has been further cross-checked with the second spectrometer which measures the quasi-continuum spectral features of tungsten ions W 27+ − W 35+ [6].
- Since both stainless steel (material of the support structures) and Inconel (support structure of the divertor W mono-block tiles) are made up of Ni, Fe and Cr (although in different percentages), dust particles from these parent structures could lead to events of either of these elements.
- These events could be due to light impurities not distinguishable through VUV or visible spectroscopy (e.g. Be or C) or to the elements listed above but in conditions unfavorable for their detection by spectroscopic means (e.g. too low photon flux for detection through the VUV spectrometers).
3. Disruptions and HRTS dust analysis
- Since dust could be one of the possible causes of TIEs, it is important to try and correlate the results obtained in the present analysis with studies performed with other complementary diagnostics.
- This has been done by analysing the effects of disruptions on the redistribution of dust particles using the High Resolution Thomson Scattering (HRTS).
- This clean-up phase has also been observed in-situ by optical spectroscopy in the visible range with reduction of carbon, oxygen and other impurities in the first phase of operation [11].
- This suggest that the dust initially present in the machine before the first plasma (or generated before JPN = 81000 for the TIE database) has been slowly eroded, or ablated when interacting with the hot plasma [12], or transported to remote, inaccessible areas of the machine.
- This dilution of dust would then lead to a lower probability of detectable TIEs as well as lower number of detectable dust particles from the HRTS diagnostic).
4. Conclusions and Outlook
- The transient impurity events observed in JET from the installation of the ILW occur mainly in the divertor configuration, independently of strike-point position.
- None of these events has lead to permanent changes in plasma conditions such as long lasting confinement degradation.
- Their occurrence shows a input power threshold >.
- Further analysis will be necessary to understand what the divertor conditions are and what role they play in TIE generation.
- Analysis with the HRTS diagnostic on dust particles mobilized by disruptions has revealed that the quantity of dust in the JET-ILW is lower by an order of magnitude than in JET-C.
Did you find this useful? Give us your feedback
Citations
83 citations
67 citations
52 citations
50 citations
28 citations
References
186 citations
177 citations
173 citations
165 citations
146 citations
Related Papers (5)
Frequently Asked Questions (15)
Q2. What is the origin of W in limited plasmas?
In limited plasmas, W could originate from W-coated neutral beam shine-through protection tiles as well as from few outer poloidal limiters, recessed inner limiter tiles and restraint ring protection tiles.
Q3. How many events occur in limited plasmas?
The probability of TIEs occurring in limited plasmas is very low (≤ 10 mHz) and only ∼ 7% of all events occur in this configuration.
Q4. What is the effect of disruptions on the redistribution of dust particles?
Following a disruption, light scattered from dust particles (through Mie scattering) that have been heated or ablated by the laser beam is also seen [9].
Q5. What is the likely cause of the events of the divertor?
Since both stainless steel (material of the support structures) and Inconel (support structure of the divertor W mono-block tiles) are made up of Ni, Fe and Cr (althoughin different percentages), dust particles from these parent structures could lead to events of either of these elements.
Q6. What is the reason for the increase in the probability of TIEs occurring in limited plasmas?
Since the normalization of the number of events has been performed over total plasma-time, this increase is probably caused by simple cumulative effects due to higher fraction of total time operated at high input power.
Q7. How many events have been detected by the divertor spectrometer?
Reciprocating probe (RCP) plunges have shown to be the cause of 15 events (< 1%), while arcs at the lower hybrid (LH) antennas are responsible for 7 events (< 0.5%).
Q8. What is the probability of TIEs occurring in a divertor?
Since the horizontal part of the divertor is a deposition zone for material coming from the main chamber, dust which accumulates in this region could partly be the cause of the occurrence of TIEs.
Q9. What is the effect of a MARFE on plasma discharges?
Depending on their composition and size, if such particles reach the plasma core they can perturb it with intense radiation spikes which have been seen to lead to MARFE (stranding for MultifacetedAsymmetric Radiation From the Edge) instabilities or disruptions, especially in long pulse operation [4].
Q10. Why is the probability of TIEs increasing with the fraction of total time spent at higher heating?
Due to the fact that the normalization has been performed over total plasma-time in each specific configuration, the TIE-probability shown in these plots will increase with the fraction of total time spent at higher heating powers.
Q11. How many events have been detected by the HRTS diagnostic?
The number of events detected by the HRTS diagnostic has been seen to decrease constantly during the experimental campaigns with the ILW, dropping of a factor ∼ 4 from the start of the first plasmas to the end of the campaign.
Q12. How is the probability of a TIE normalized?
In order to correctly normalize the event distributions, a database containing the total plasma operation time (in steps of 10 ms) for each geometrical configuration and each correlation quantity listed above has been created.
Q13. What is the effect of disruptions on the dust distribution in the ILW?
Correlation with disruption force reveals that the number of detected dust particles increases strongly with disruption force (figure 3) and confirms the results shown in [10].
Q14. Why is dust production and control a major concern in future tokamaks?
The understanding of dust production and control in future tokamaks such as ITER is a major concern not only for safety reasons and for diagnostic interpretation (see e.g. [1, 2, 3]), but also for reliable plasma operation.
Q15. What is the probability of TIEs occurring in a limited plasma?
Configurations with the outer strike point on vertical tile 7 seem to be less prone to TIE-occurrence than those with the outer strike point on the horizontal tiles 5 or 6 (1.9 mHz vs. 40 − 50 mHz).