Beurskens, M.

Eich, T.

Frassinetti, L.

Joffrin, E.

Loarte, A.

Maddison, G.

Rimini, F.

Saibene, G.

Solano, E.

Thomsen, H.

Zarzoso, D.

English

MPG.PuRe

ELM size analysis in JET advanced tokamak and hybrid scenarios D. Zarzoso1, M. N. A. Beurskens2, L. Frassinetti3, T. Eich4, E. Joffrin1,8, A. Loarte5, G. Maddison2, F. G. Rimini8, G. Saibene7, E. R. Solano6, H. Thomsen4 and JET EFDA contributors§	  JET-­EFDA,	  Culham	  Science	  Centre,	  OX14	  3DB,	  UK	  1	  CEA,	  IRFM,	  F-­13108	  Saint-­Paul-­lez-­Durance,	  France.	  2	  EURATOM/CCFE	  Fusion	  Association,	  Culham	  Science	  Centre,	  Abingdon,	  OX14	  3DB,	  UK	  3	  Association	  EURATOM-­VR,	  Alfvén	  Laboratory,	  Stockolm,	  Sweden	  4	  Association	  EURATOM-­Max-­Planck-­Institut	  für	  Plasmaphysik,	  D-­85748	  Garching,	  Germany	  5	  ITER	  organization,	  F-­13067	  Saint-­Paul-­lez-­Durance	  Cedex,	  France	  6	  Asociación	  EURATOM-­CIEMAT	  para	  la	  Fusión,	  Madrid,	  Spain	  7	  Fusion	  for	  Energy	  Joint	  Undertaking,	  08019	  Barcelona,	  Spain	  8	  EFDA-­CSU,	  Culham	  Science	  Centre,	  Abingdon,	  OX14	  3DB,	  UK	  ELM size analysis for standard ELMy H-mode plasmas (baseline scenario) has been the subject of multi machine studies [1]. In the projection from current devices towards ITER baseline scenarios, the empirically found negative correlation of ELM losses normalized to the pedestal stored energy ( ) with pedestal neoclassical collisionality ( ) would imply ELM losses of 20% of the pedestal energy at the ITER relevant . In addition to the baseline scenarios, Advanced Tokamak (AT) and hybrid scenarios allowing an improved core confinement are under investigation in existing devices. This paper sets out to characterise the normalised losses of type I ELMs in the range  for a data base of 26 baseline, 13 AT and 56 hybrid plasmas. The pedestal energy has been calculated as [1]  (1) where kB is the Boltzmann constant and the electron density and temperature at the top of the pedestal are calculated with a modified tanh fit performed on the new high resolution Thomson scattering (HRTS) data. The ion density is calculated as under the assumption that carbon is the only impurity present in the tokamak and the ion pedestal temperature is calculated as , with  taken from the core charge                                                  § See the Appendix of F. Romanelli et al., Proceedings of the 22nd IAEA Fusion Energy Conference 2008, Geneva, Switzerland 37th EPS Conference on Plasma Physics P2.171exchange (CX). ELM losses are measured by , which depends only on external magnetic measurements of the poloidal field and the plasma toroidal flux. It reflects only the perpendicular component of the plasma pressure and includes the effect of thermal and fast particles. The energy drop is obtained assuming that the plasma energy increases linearly after the ELM, which means that a linear regression on  can be performed before and after the pedestal collapse and the difference of the values yields the ELM loss. This method removes difficult to measure effects around the ELM crash, possibly due to eddy currents induced in the vacuum vessel and rapid plasma movements on a millisecond timescale which can impact diamagnetic measurements. An alternative method is to calculate the ELM energy loss from the pre- and post-ELM electron kinetic profiles from the HRTS system as described in [2].   All the discharges analysed in this paper exhibit regular type I ELMs of steady frequency and no large compound ELMs. In addition, the time windows of interest have been chosen outside of any detectable core MHD activity (such as neoclassical tearing modes) by diamagnetic measurements. Fig. 1 shows an example of  signal averaged over many ELMs and synchronised to the ELM crash time for the hybrid discharge #77922 - with ELM frequency of 17.3 Hz, Ip = 1.7 MA, Bt = 2.3 T. Fig. 2 illustrates the measured frequency of the ELM energy loss for the same hybrid discharge. Analysis of the dependence of the ELM losses on  (as defined in [1]) for 26 baseline discharges is carried out. The database covers the following range in dimensionless parameters:  ( ),  ( ),  ( ) and triangularity (  and ). In Fig. 3 circles and triangles represent the values when  is used for the ELM losses calculation and black points represent the JET baseline discharges analysed in [1] (these data have been taken Fig. 1: plasma energy evolution for a single JET hybrid discharge. Green lines show the computed average at a given time and the ELM energy drop is inferred from the distance between the horizontal lines. Fig. 2: histogram of the frequency of the ELM energy loss for a single JET hybrid discharge. A sample of 90 ELMs has been used with a standard deviation of 20%. 37th EPS Conference on Plasma Physics P2.171directly from this reference). Each point represents one single discharge and the values are averaged over a time window of typically several seconds during a steady phase in the discharge. The general negative correlation is observed in agreement with [1], corroborating the consistency of our data analysis. The correlation  of the ELM losses with dimensionless parameters  is greater than 0.8 for each. When we analyse the AT and hybrid discharges, with a similar range of variation of , we find a weaker correlation coefficient of < 0.5 between the normalized losses and collisionality. This is illustrated in Fig. 4. In general, for high triangularity, the losses are larger in AT and hybrid discharges than in baseline plasmas. For low triangularity, hybrid and baseline plasmas exhibit similar ELM energy losses. We have isolated nine discharges from gas-scan sessions and observed that a modification of gas-fuelling may result in a strong modification of the ELM energy losses without considerably affecting the collisionality. This seems to be in agreement with [2], where it was shown that the gas-fuelling strongly affect the ELM characteristics. Table 1 gives a summary of some of the characteristics of each discharge. For these nine discharges, the dependence of the normalized ELM losses on collisionality is shown in Table 1: summary of some of the characteristics of the AT and hybrid gas-scan discharges used in this paper Shot Bt (T) Ip (MA) PNBI (MW) Gas-rate (1021 e/s) 75957 2.7 1.8 19.9 - 75960 2.7 1.8 20.0 6.0 75959 2.7 1.8 19.6 7.0 75954 2.0 1.7 18.4 - 75964 2.0 1.7 18.3 7.51 75963 2.0 1.7 18.3 7.62 75962 2.0 1.7 18.3 10.64 76746 2.0 1.7 18.6 10.64 76748 2.0 1.7 14 25 Fig. 3: normalizad ELM energy losses versus pedestal plasma collisionality for new JET baseline discharges (circles). Comparison is made with the discharges analized in [1] (black points). Fig. 4: ELM losses versus collisionality for JET AT and hybrid discharges (closed symbols). Comparison is made with baseline discharges (open symbols). 37th EPS Conference on Plasma Physics P2.171Fig. 5. Further analysis on the ELM penetration has been carried out. We have observed that, for baseline plasmas and , the ELM affected area extends up to 20% of the minor radius, whereas for un-fuelled AT and hybrid plasmas this region is larger (up to 50% of the minor radius, which is consistent with the results published in [3]). However, the ELM penetration seems to be modified by increasing the gas-rate. Indeed, Fig. 6 shows two hybrid discharges (left) and two AT discharges (right) with collisionalities between 0.1 and 0.3. We observe how the ELM affected area can be strongly reduced with a small change in collisionality, resulting in a decrease of the ELM energy losses. In conclusion, it has been found that, although the  ordering is well reproduced for JET baseline scenarios, for AT and hybrid discharges no clear correlation exists. A possible dependence of the ELM affected area on the gas-fuelling rate has been observed, which partially explains the high dispersion observed in Fig. 4. However, additional experimental observations, such as the difference in energy between baseline and un-fuelled AT/hybrid plasmas, need to be explored. Acknowledgements This work was supported by EURATOM and carried out within the framework of the European Fusion Development Agreement. The views and opinions expressed herein do not necessarily reflect those of the European Commission. References [1] A. Loarte et al., Plasma Phys. Control. Fusion 45 (2003) [2] M. N. A. Beurskens et al., Nucl. Fusion 48 (2008) [3] Y. Sarazin et al., Plasma Phys. Control. Fusion 44 (2002) Fig. 6: pre- (blue lines) and post-ELM (red lines) normalized pressure profiles reconstructed from HRTS and CX. Dashed lines represent the standard deviation region. Solid lines represent the averaged profiles. Fig. 5: ELM losses versus collisionality for JET AT and hybrid gas-scan discharges. 37th EPS Conference on Plasma Physics P2.171

ELM size analysis in JET advanced tokamak and hybrid scenarios

http://hdl.handle.net/21.11116/0000-0008-AF74-1

ELM size analysis in JET advanced tokamak and hybrid scenarios

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