7. ICRF Driven Poloidal Asymmetries 87
7.4. Fast ions transport due to sawtooth crashes
however the experimental dependence (dashed line in Fig. 7.10b) indicates a steeper growth for low density and two times weaker asymmetry for higher density. A similar discrepancy was observed with the TORIC-SSFPQL model as well. Finally, a change of the asymmetry with the minority fraction and Zeff is indicated in Fig. 7.10c-d, and further investigation will be focused on the experimental verification of this dependence.
a) b)
c) d)
TORIC-FFPMOD Experimental fit
Figure 7.10.: Poloidal asymmetry of the tungsten SXR radiation modeled by TORIC-FFPMOD and Dendy’s model as a function of a) ICRF power, b) electron density, c) minority fraction and d) effective charge. The other plasma parameters was set according to the discharge #30812 at 4.65 s (see Fig. 7.2).
7.4. Fast ions transport due to sawtooth crashes
One of pioneering applications of the measured poloidal asymmetries is the investigation of the fast particle redistribution by sawtooth crashes. The fast particle transport in AUG has been analyzed in [85], including the fast NBI deuterium ions further accelerated by ICRF. However, the fast hydrogen ions have a too low charge exchange cross-section to be detected by the fast ion D-alpha (FIDA) diagnostic, and the perturbation of the electrostatic potential is one of the few direct manifestations of these energetic ions
7. ICRF Driven Poloidal Asymmetries
in the plasma. Additionally, from the measured poloidal variation of the SXR radiation the poloidal variation of the minority fraction can be inferred from Eq. (5.41), which provide information about the pitch angle distribution [129].
The detailed examination of the asymmetry variation due to sawtooth crashes was performed on a subset of the database examined in the previous section. The subset consists of four discharges with large sawtooth crashes. In the database discharges
#30811 and #30812, heated by 2.5 MW of NBI and 4.3 MW of ICRF and discharges
#32469 and #32470 heated by a 2.5 MW of ICRF were included. All discharges had the same value of the plasma currentIp =1 MA, a moderate value of the edge safety factor q95 = 4.5, and low electron densities between 2–6·10−19m−3. The sawtooth crashes had a similar size with their inversion radius estimated fromTe measurements to be aboutρinvtor =0.4. The largest difference between the discharges was observed in the sawtooth period duration. The fast rotation and high pressure of the passing fast NBI ions in discharges #30811 and #30812 increased the sawtooth period to 110 ms.
The sawtooth period in the discharges #32469 and #32470 was only 30 ms with a 5 ms long precursor and post-cursor phase.
The change in the poloidal asymmetry caused by the sawtooth crash, corrected for the centrifugal asymmetry using Eq. (5.41) is depicted in the upper row of Fig. 7.11 for the first large sawtooth crash after switching on the ICRF. A significant reduction of the asymmetry after the crash can be observed in the discharges #30811 and #30812, while the asymmetry in the discharges #32469 and #32470 is almost unaffected. In the lower row of Fig. 7.11, the FIDA signal normalized to the signal from the beam emission spectroscopy (BES) for the same crash is shown. The FIDA data are available only for discharges #30811 and #30812, where beam Q3 was used and the measurements demonstrate a flattening of the peaked fast NBI ions profile.
The same conclusion can be drawn from Fig. 7.12, where the fast particle asymmetry was evaluated from the SXR asymmetry for each sawtooth crash. While in the discharges
#30811 and #30812 a 20–30 % reduction of the poloidal asymmetry was observed, the change in the discharges #32469 and #32470 was below 10 %, despite the large magnitude of the observed asymmetry. The cause of this difference is in the sawtooth activity and lower ICRF power. The long sawtooth period of 110 ms and 4.3 MW of the ICRF power in discharges #30811 and #30812 caused a peaked temperature anisotropy profile close to the IC resonance. The particle distribution was then strongly affected by the enhanced transport during each sawtooth crash. For the discharges
#32469 and #32470, the 30 ms long sawtooths period was significantly shorter than the anisotropy relaxation time τaniso ≈ 150 ms estimated using the FFPMOD code.
Consequently, the peaked temperature anisotropy profile cannot built- up before the next sawtooth crash and the asymmetry stays almost unaffected.
7.4. Fast ions transport due to sawtooth crashes
ICRH
ICRH ICRH ICRH
Mixing radius Mixing radius
Mixing radius Mixing radius
SXR asym. w/o CF asym.
pol
FIDA / BES [a.u.]
0.00 0.05 0.10 0.15
0.0 0.2 0.4 0.6 0.8 1.0 0.00
0.04 0.08 0.12
4.532s 4.535s 4.573s
4.585s
0.0 0.2 0.4 0.6 0.8 1.0 E = 25-60keV ξ = 0.5-1
pol
SXR asym. w/o CF asym. a) b)
d) c)
e) f)
Figure 7.11.: a–d) The change in the asymmetry profile evaluated for the first sawtooth crash after switching of ICRF. The centrifugal asymmetry before and after the crash has been removed according to the Eq. (5.41). e–f) The signal from the fast ion D-alpha diagnostic (FIDA) for the energy range 25–60 keV and pitch range 0.5–1, normalized to the beam emission
spectroscopy (BES) signal for each LOS of discharges #30811 and #30812.
fm fm * -
*-Figure 7.12.: Reduction in the poloidal asymmetry of the minority ions fm−fm∗ during a sawtooth crash evaluated for all major crashes within ICRF phase of the each discharge.
Each crash is depicted by a separate color.
7. ICRF Driven Poloidal Asymmetries
7.5. Conclusions
In this chapter, the first evidences of ICRF driven poloidal asymmetries in the tungsten density on AUG were presented. High-resolution SXR emissivity profiles were employed for detailed validation of the parallel force balance of heavy impurities and the fast minority ions accelerated by ICRF. The response of the accelerated ions was modeled using the the TORIC code coupled with the FFPMOD and the SSFPQL Fokker-Planck solvers, and interpreted using Dendy’s model (Eq. (5.45)).
A very good match between the experimentally measured and the modeled asymmetry was found in the detailed investigation of a low-density discharge with ICRF heating on the low-field side. On the contrary, a discrepancy has been observed when a high-field side heated discharge was examined. It was shown that the temperature anisotropy from TORIC-SSFPQL was slightly overestimated, and consequently the modeled poloidal asymmetry exceeded the experimentally measured value. Such a discrepancy could indicates limitation of the particle trapping model in SSFPQL. In the contrary, TORIC-FFPMOD provided results consistent with the measurement.
Further investigations focused on studying the fast particles content in ICRF heated plasmas. A database was analyzed to identify the most relevant parameters deter-mining the poloidal asymmetries. Inverse squared electron density and weak electron temperature scaling was found, consistently with the physical expectation based in the Fokker-Planck equation. The influence of the electron density and temperature was found consistent with the physical expectation. However, the comparison with TORIC-FFPMOD a significant discrepancy for the moderate density cases (ne >4·1019m−3), where the modeled asymmetry exceeded the experiment by about a factor of two.
The cause of this discrepancy is unknown and modeling using alternative ICRF codes will be necessary.
Finally, the poloidal asymmetries were used to examine the fast particle redistribution during sawtooth crashes. Two similar groups of discharges were investigated with a significantly different sawtooth period. When the sawtooth period was much shorter than the typical fast particles relaxation time, the fast particle distribution was kept flat by frequent crashes, and the asymmetry was almost unaffected. On the other hand, slow sawtooth crashes resulted in a periodical peaking of the trapped fast particles close to the ICRF resonance and the crash reduced the asymmetry by 20–30 %, confirming the significant effect of the sawtooths on the fast particles transport.
In conclusion, the measurement of the heavy ions poloidal asymmetries provides a stringent test beneficial for validation of the ICRF heating codes. The discrepancies observed in the high density, and HFS heated case motivates for a further development of these codes. Moreover, the understanding of the poloidal asymmetries is critical for the prediction of neoclassical transport, as will be demonstrated in the next chapter.