Devynck, P.

Giroud, C.

Jacquet, P.

Lehnen, M.

Lerche, E.

Maddison, G.

Matthews, G.

Neu, R.

Stamp, M.

Van Eester, D.

English

MPG.PuRe

*See the Appendix of  F. Romanelli et al., Proceedings of the 24th IAEA Fusion Energy Conference 2012, San Diego, US  Comparing the bulk radiated power efficiency in carbon and ITER-like-wall environments in JET.  P. Devynck1,  G. Maddison2,  C. Giroud2, P. Jacquet2, M. Lehnen3, E. Lerche4, G.F. Matthews2, R. Neu5, M.F. Stamp2, D. Van Eester4 and JET-EFDA Contributors* JET-EFDA, Culham Science Centre, Abingdon, OX14 3DB, UK 1CEA-IRFM F-13108 Saint-Paul-lez-Durance, France,  2CCFE  Culham Science Centre, Abingdon, OX14 3DB, UK, 3 ITER Organization, 13115 Saint Paul-lez-Durance, France,4 ERM-KRS 1000, Brussels, Belgium,5 Max-Planck-Institut für Plasmaphysik D-85748 Garching, Germany 1) Abstract We use a parameter βr for all plasmas that allows detecting the pollution of the plasma bulk by highly radiative impurities. This parameter is defined as the radiative loss of the mixture of impurities relative to their mean Z2 and was used in the past to characterize the efficiency of radiative mantles in Neon seeded discharges [1,2]. We show that this parameter, though global, is very sensitive to the presence of highly radiative impurities in the bulk of the discharge. In the carbon environment of JET, the value of  βr  is around  10-40 MW.m6, indicating the absence of highly radiative impurities in the plasma. In the ILW machine, the value of βr is found to depend on the type of additional heating and confinement state of the plasma. We observe that neutral-beam injection (NBI) introduces little W into the plasma, with a βr between 2 and 3 10-40 MW.m6. Ion-cyclotron radio-frequency (ICRF) waves yield a βr of order 5 in L-mode and  10-39 MW.m6 in H-mode when no edge-localized modes (ELMs) are present.  2) The definition of βr. Following  references [1,2]  a parameter characterizing the quality of cooling of the impurities  in the bulk of the plasma can be written as:  βr=Pradbulk/(Zeff-1) ne2          (1)           where ne is the line-averaged density provided by high-resolution Thomson scattering and Zeff is calculated from bremsstrahlung emission measured along a horizontal line-of-sight crossing the plasma centre  (i.e. not passing through the div rtor region). Prad in the bulk is evaluated by bolometry. In JET, we neglect the bremsstrahlung a d cyclotron radiation; however, in a machine such as ITER, they will have to be calculated or measured and removed from the total radiated power before evaluating βr. We give the expression for βr in a general case with different types of impurities in a deuterium plasma. First the bulk radiated power can be written as:  kitkiikikikke LcbanVnP ∑∑= impbulkrad , In this expression nimp is the total impurity density, ne the electron density, ai=ni/nimp the fraction of impurity ions with charge Zi, bi=nei/ne, the fraction of the density in the volume 41st EPS Conference on Plasma Physics P1.009where the ion with charge Zi radiates and ci=Vi/V the fraction of the volume in which the same ion radiates. Lti is the radiative cooling function for the same ionwith charge Zi. We suppose in this expression that the electron temperature Te is homogenous in the volume where the ion of charge Zi is radiating, as well as the density of impurity ions and electrons. Here k denotes different types of impurities.  We can write the expression for Zeff as: )1(11 impeff −+= ∑ ∑kikik ikikeZZannZ  If we combine these expressions in relation (1), we find ∑ ∑∑ ∑−εε=βk ikikikikk ikitkikikikZZaLcbaV )1(impimpr    (2)     where ∑=εkkkknnimpimpimp .  We notice that  βr/V has the dimension of Lt and can be expressed in W.m3  3) The physical meaning of βr The radiative loss parameter of an impurity k is defined in reference [3]   as Sk= )/(r̂ad kek nnP , where radP̂  is radiated power density (W/m3). It can be calculated using the same notations used in section 2. We find in this case that for an impurity k,  Sk ≈ itkiikiki Lcba∑  where i takes account of the different ionization states. Hence βr can be expressed as a function of the radiative lossparameter  Sk of the impurities as: ∑ ∑∑−εε≈k ikikikikkkkZZaSV )1(βimpimpr .   (3) For W, the radiative loss parameter increases moderately for Te above 100 eV (about a factor of 2 between 100 eV and 3 keV), while it decreases by a factor of 10 for carbon in the same range. In the case of W pollution, the rather weak dependence of SW with Te leads to the conclusion that an increase of βr must be associated with an increase of the relativ concentration of W in the bulk Wimpε  (even if this increase is not sufficient to have a measurable impact on Zeff). The second point is that the value of βr is liable to be very resilient to Te changes with tungsten pollution. Finally, in the ILW changes in the values of βr will always indicate a change in the bulk impurity mixture. 4)  Carbon environment  in  H-mode. In Figure 1, βr is plotted for an H-mode shot  with ELMs.  Two heating phases are present, the first with 9 MW of NBI power, the second with 19 MW of NBI power. Figure 1 shows that the 9 MW phase has a value of βr close to  10-40 MW.m6 ,value usually obtained in L-mode. At 19 MW, βr decreases to 0.5 10-40 MW.m6. This decrease may be attributed to the behaviour of the radiative loss parameter of the light impurities and carbon (SC) in particular. As the 41st EPS Conference on Plasma Physics P1.009additional heating power is increased, region. As a consequence, the radiative loss parameter of carbon decreases (numerator of relation (2)), thus yielding lower even less efficient at radiating in the bulk when heating power below 10 MW, the 5) ILW transition from LFigure 2notice first that the value of MW.m6NBI heating, close to those measured in carbon pulses. During the H(10s<t<14s), additional heating powerthat in this scenario, NBI heating increases moderately the amount of highL to H-impurities does not increase either. 6  ILW transition from L to H illustrates the behavior of  . We have measured βr increases from 2.2 to an average 3.7 10mode. The fact that Right) LRight) value of βr values. This si- to Hvalues as low as 1.3 10 is increased from 1.5 MW to 10 MW. This result illustrates the fact ZeffLeft) scenario of shot 79521, βr calculated during the NBI additional heating phase.eft) scenario of shot 82835Teβr value is the same in L or H-mode with NBI only.βr when there is an Lβr during the L remains unchanged also indicates that the pollution by low -mode with ICRF onlyFigure 1βr calculated during the additional heating phases. increases in the whole bulk including the pedestal mply indicates that the lowT-mode phase (8s< carbon Figure 2 in ILW, Le is increased. We  -H transition with NBI heating. We -40 MW.m6 40 MW.mIn Figure 3environment-H transition-Z observe-mode.  t 10s) is for some ILW plasmas during 6, a 68% increase, though the , a plasma where only ICRF is .   , NBI heating.impurities become  that at NBI around 2.2 10-mode phase -Z impurities from    -40 -Z 41st EPS Conference on Plasma Physics P1.009used triggers an Ldrops at the transition, commonly observed in the JET database when ICRF trigge s Hone of the highest values obtained so far for immediately after the ICRF power has reached the thr shold, it can be speculated that this is partly the effect of the transport change. As transport decreases in the bulk after the H transition, the amount of impurities there increases. 7) ConclusionWe have shown that a global timethe presence of highly radiative impurities in tokamak discharges. It can be calculated for all plasmas regardless of the scenario and of the confinement state. We havin the carbon environment with ones in the ILW. The very low values obtained in the carbon environment clearly indicate the absence of significant radiation from highly radiative impurities in the bulk plasma, the ILW environment, we observe that plasmas heated with NBI have relatively low of order 2 to 3 times those measured in the carbon environment. During the Lβr parameter is observed to increase power. In the case of ICRF heating, the plasmas systematically yield  5 10-40 in LThis work was supported by EURATOM Development Agreement. The views and opinions expressed herein do not necessarily reflect those of the European Commission. References:[1] G. Telesca et al, Nucl. Fusion, [2] L. Carraro et al., Nucl. [13] R. Neu et al, 2010 proc.Korea),vol.IAEA-t=17.27s. The energy stored in the discharge is not sufficient to trigger ELMs.  After βr increases from 5s  -mode and 10 -CNLeft) scenario of shot 81240Right) value of H transition. This is visible in the plasma traces where the D_alpha signal -39 MW.m Fusion,  24rd IAEA Fusion Energy Conf. (Daejon,Rep. of -180, EX/Dto 10, a 100% increase. -d pendent parameter can give reliable information about with an average value of moderately even with a sevenfold increase of the NBI6  in H-and carried out within the framework of the European Fusion 40, No. 11,184540 (2000), No. 12, 1983-P3-24.  βr calculated during the additional heating phases.βr. As the level of  mode if no ELMs are present. Figure 3 in ILW, L- H βr around  10βr≈10-40 mode transition -39 -modes without ELMs. It is βr jumps to 10ee compared JET shots  MW.m6. When switching to  , ICRF heating.MW.m6 is the level -39 β-H transition the βr values of order  MW.m6 r values          41st EPS Conference on Plasma Physics P1.009

Comparing the bulk radiated power efficiency in carbon and ITER-like-wall environments in JET

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Comparing the bulk radiated power efficiency in carbon and ITER-like-wall environments in JET

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