Andrew, P.

Basse, N.

Breger, P.

Budny, R.

Core, W.

Ehrenberg, J.

Jarvis, O.

König, R.

Lauro-Taroni, L.

Loghlin, M.

Marcus, F.

Matthews, G.

O'Mullane, M.

Sadler, G.

Strachan, J.

von Hellermann, M.

Watkins, N.

Zastrow, K.

English

MPG.PuRe

PARTICLE TRANSPORT IN STEADY-STATE ELMY H-MODESSTUDIED BY TRACE TRITIUM INJECTION DURING JET DTE-1K.-D. Zastrow, P. Andrew, N.P. Bassea), P. Bregerb), R. Budnyc), W.G.F. Core,J. Ehrenbergd), M. G. von Hellermann, O.N. Jarvis, R.W.T. Könige), L. Lauro-Taronif),M.J. Loughlin, F.B. Marcus, G.F. Matthews, M.G. O’Mullaneg), G.J. Sadler,J.D. Strachanc), and N. Watkinsh)JET Joint Undertaking, Abingdon, United Kingdoma)Ørsted Laboratoriet, Niels Bohr Institutet for AFG, Denmark b) DG-XII/C/5, Brussels, Belgiumc) Princeton Plasma Physics Laboratory, Princeton, USAd) DG III/C/4, Brussels, Belgiume) Max-Planck Institut für Plasmaphysik, Garching, Germanyf) JCMM, Department of Meteorology, University of Reading, Reading, United Kingdomg) Department of Physics and Applied Physics, University of Strathclyde, United Kingdomh) AEA Technology, Harwell, United Kingdom1.  IntroductionTrace amounts of tritium (1-5 % of the deuterium content) have been puffed into steady-stateELMy H-Mode discharges (puff duration 50 msec, τE in the series ranges from 0.2-0.5 sec).The transport properties of the edge and core plasma, and the dynamic response of the tritiumfraction in the wall, determine the evolution of the 14 MeV neutron emission in space andtime following these puffs. These experiments represent the first measurement of fuel iontransport in an ITER relevant regime. In this paper, we outline the method used to analyse thedata, and identify which transport parameters can be measured.2.  Measurements and analysis techniqueThe neutron profile monitor has been absolutely calibrated to provide 2.5 MeV and 14 MeVneutron brightness profiles and neutron yields with associated error bars (counting statisticsplus 10% relative calibration). The rate of forward and back scattering events that give rise toadditional signals are calculated using a Monte-Carlo neutron transport code (MCNP) [1].The integration time of the data points is 50 msec.We derive transport coefficients for tritium by performing a least-squares fit of modelparameters to the chordal data (see Fig. 1) during the transient phase of the profile timeevolution. This approach also enables the error propagation from the neutron data through tothe model parameters, taking into account covariances in model parameters.We use a 11/2-D transport model (SANCO) with diffusive and convective terms. Thetransport coefficients are set radially constant within five zones (see Fig. 3). The innermostzone (0.0<r/a<0.35) represents a sawtooth average, and the outermost zone (0.95<r/a<1.00)represents the transport barrier on one hand, and an ELM average on the other hand. Theintermediate zones have their borders at r/a=0.55 and 0.75. In each zone at least two neutronchords have their minimum r/a along their line of sight.02468 r/a=0.04 χ2( 4)=2.140246r/a=0.17 χ2( 5)=1.270246 r/a=0.22 χ2( 3)=1.10012345r/a=0.35 χ2( 6)=1.0101234519.5 20.0 20.5 21.0t(sec)r/a=0.39 χ2( 2)=1.250.00.51.01.52.02.53.0 r/a=0.52 χ2( 7)=1.120123r/a=0.56 χ2( 1)=0.960.00.51.01.52.0 r/a=0.67 χ2( 8)=0.930.00.20.40.60.81.0r/a=0.81 χ2( 9)=1.490.000.050.100.150.2019.5 20.0 20.5 21.0t(sec)r/a=0.92 χ2( 10)=1.12(1014  n/m2 sec)(1014  n/m2 sec)JET-# 42529K-DZ 98Fig. 1: Achieved fit for each channel, indicating the minimum r/a value along each line-of-sight and theachieved χ2 (Overall χ2=1.37). Note, that for the innermost channel (#4), the fit is worse since the sawtoothoscillations are not included in the model. For the outermost two channels, there is a strong contribution to thesignal from forward and back scattering. For channel #9, the true brightness is indicated by the dashed line.For channel #10 the signal is entirely due to scattering events.3.1.   Fast particle model and neutron reactivitiesThe measured 2.5 MeV brightness profiles and those predicted for the thermal-thermal andbeam-thermal reactions do not agree, see Fig. 2. Specifically, the codes in use at JET(TRANSP and CHEAP) over predict the signal levels in the outer channels (0.5<(r/a)Min<0.8)of the neutron profile, by up to a factor two. For the outermost channel with (r/a)Min=0.92, thecodes predict up to a factor ten more than is measured. We therefore modify the predictionfor beam-thermal reactions, see Fig. 2. The magnitude of the correction is outside the errorsintroduced into the model by the input data (mainly dilution and electron temperature, notingthat to first order beam-thermal emissivity is independent of electron density).Three more observations support the introduction of this correction. 1) The systematicover prediction of the 2.5 MeV yield increases from typically 20% at 50% thermal-thermalfraction to 40% at 90% thermal-thermal fraction. 2) When we use the uncorrected function inthe analysis of the 14 MeV signal, we predict a burst of neutrons in channels #9 and #10,overshooting the measured signal by a factor up to 4 and 10, respectively. This burst can notbe removed by changes in model parameters. 3) When we use the tritium transportcoefficients to predict the deuterium equilibrium profile, with only the edge influx as aparameter to match the central density, we obtain good agreement with the observed profileshape. If, however, we use the uncorrected beam-thermal emissivity, a strong inward pinch isrequired to explain the 14 MeV brightness profile. Its magnitude is inconsistent with thedeuterium equilibrium profile.0.00.51.01.5TRANSPCHEAPFitJET # 42529, t=18.0-19.5 sec(1014  n/(m2  sec))0.0 0.2 0.4 0.6 0.8 1.00.00.20.40.60.81.01.21.4(r/a)MinMeasured/PredictedK-DZ 980.0 0.2 0.4 0.6 0.8 1.00.00.20.40.60.81.01.2 1 2 3 4  5  6  7  8  9  10JET # 42529r/aK-DZ 98Fig. 2: Left: Measured and predicted 2.5 MeV brightness profile with and without correction to beam-thermalemissivity, Right: Radial function used to correct the beam-thermal emissivity. The closest approach of the lineof sight for the channels of the horizontal camera is marked.3.2.  Tritium fraction in neutral influxThe 14 MeV neutron signals several seconds after the puff, before the end of the steady-statephase of the discharge, are higher than just before the puff. Two explanations are possible. 1)The signal may still be in the tail of an exponential decay, which would return to the originalvalue several confinement times later. 2) The fraction of tritium in the neutral influx hasincreased as a result of the puff. The analysis based on the latter assumption gives asignificantly better fit to the data. The transport coefficients are only able to describesimultaneously the rate of rise of the signal, and a higher steady-state level after the puff, ifthe tritium wall flux is increased in the model. The exact shape in time of this increase isunknown, and we use a step function with the final level as a further free parameter of themodel.3.3.  Significance of pinch term and mass dependence of transportThe pinch term is not significant when the analysis is based on the 14 MeV neutron dataalone. Specifically, we obtain an inward pinch for 0.75<r/a<0.95 of v=–0.55±0.50 m/sec,whilst further inside the plasma the value of v is smaller and the error bar encompasses v=0.In this paper we report results obtained based on the assumption v=0 for all radii.Using the derived tritium diffusion coefficient to predict the deuterium equilibriumprofile with beam fuelling (known) and wall influx (free parameter), requires v=-0.5 m/secfor 0.75<r/a<0.95. Therefore, the transport coefficients (D and v) for tritium and deuteriumare the same within the measurement uncertainties.3.4.  Nature of transport barrierWe have explored whether the dynamic behaviour in these transient experiments can be usedto distinguish, if the transport barrier is diffusive or convective. For this series of experimentsthe two cases were indistinguishable. Specifically, both assumptions result in different sets offree parameters (see Fig. 3), but are able to describe the 14 MeV neutron emission equallywell. Any combination of diffusive and convective transport barriers is also consistent withthe neutron data. The exact values of D and v in the barrier region depend on the choice ofbarrier width.Fig. 3: Diffusion coefficientfor the two extreme cases.Open circles: Convectivetransport barrier,v=0 for 0.0<r/a<0.95, v=-31±12 for 0.95<r/a<1.0,(χ2=1.42).Filled circles: Diffusivetransport barrier,v=0 for 0.0<r/a<1.0,(χ2=1.37)0.0 0.2 0.4 0.6 0.8 1.00.11.0r/aD(m2 /sec)JET-# 42529K-DZ 984.  Conclusions1) The beam-thermal fusion reactivity is lower for r/a>0.5 than is calculated by TRANSP andCHEAP.2) The fraction of tritium in the steady-state wall influx increases after the puff.3) Diffusive transport only (r/a<0.95) can describe the 14 MeV neutron data, a pinch term isnot significant. A weak interior pinch is required for the deuterium profile shape.4) There is no evidence from these experiments for a mass dependence on the transport forhydrogen isotopes.5) An edge transport barrier is required to describe the tritium behaviour. The barrier could bediffusive, convective, or any combination thereof.Reference[1] M.J. Loughlin et al.: 12th Topical Conf. on High Temp. Plasma Diagn., Princeton, 1998.

Particle Transport in Steady-State ELMy H-Modes Studied by Trace Tritium Injection during JET DTE-1

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Particle Transport in Steady-State ELMy H-Modes Studied by Trace Tritium Injection during JET DTE-1

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