P. Buratti

Global and pedestal confinement in JET with a Be/W metallic wall

Type I ELMy H-mode operation in JET with the ITER-like Be/W wall (JET-ILW) generally occurs at lower pedestal pressures compared to those with the full carbon wall (JET-C). The pedestal density is similar but the pedestal temperature where type I ELMs occur is reduced and below to the so-called critical type I–type III transition temperature reported in JET-C experiments. Furthermore, the confinement factor H98(y,2) in type I ELMy H-mode baseline plasmas is generally lower in JET-ILW compared to JET-C at low power fractions Ploss/Pthr,08 2, the confinement in JET-ILW hybrid plasmas is similar to that in JET-C. A reduction in pedestal pressure is the main reason for the reduced confinement in JET-ILW baseline ELMy H-mode plasmas where typically H98(y,2) = 0.8 is obtained, compared to H98(y,2) = 1.0 in JET-C. In JET-ILW hybrid plasmas a similarly reduced pedestal pressure is compensated by an increased peaking of the core pressure profile resulting in H98(y,2) ≤ 1.25. The pedestal stability has significantly changed in high triangularity baseline plasmas where the confinement loss is also most apparent. Applying the same stability analysis for JET-C and JET-ILW, the measured pedestal in JET-ILW is stable with respect to the calculated peeling–ballooning stability limit and the ELM collapse time has increased to 2 ms from typically 200 µs in JET-C. This indicates that changes in the pedestal stability may have contributed to the reduced pedestal confinement in JET-ILW plasmas. A comparison of EPED1 pedestal pressure prediction with JET-ILW experimental data in over 500 JET-C and JET-ILW baseline and hybrid plasmas shows a good agreement with 0.8 < (measured pped)/(predicted pped,EPED) < 1.2, but that the role of triangularity is generally weaker in the JET-ILW experimental data than in the model predictions.

Improved confinement in JET high β plasmas with an ITER-like wall

The replacement of the JET carbon wall (C-wall) by a Be/W ITER-like wall (ILW) has affected the plasma energy confinement. To investigate this, experiments have been performed with both the C-wall and ILW to vary the heating power over a wide range for plasmas with different shapes. It was found that the power degradation of thermal energy confinement was weak with the ILW; much weaker than the IPB98(y,2) scaling and resulting in an increase in normalized confinement from H98 ~ 0.9 at βN ~ 1.5 to H98 ~ 1.2−1.3 at βN ~ 2.5 − 3.0 as the power was increased (where H98 = τE/τIPB98(y,2) and βN = βTBT/aIP in % T/mMA). This reproduces the general trend in JET of higher normalized confinement in the so-called hybrid domain, where normalized β is typically above 2.5, compared with baseline ELMy H-mode plasmas with βN ~ 1.5 − 2.0. This weak power degradation of confinement, which was also seen with the C-wall experiments at low triangularity, is due to both increased edge pedestal pressure and core pressure peaking at high power. By contrast, the high triangularity C-wall plasmas exhibited elevated H98 over a wide power range with strong, IPB98(y,2)-like, power degradation. This strong power degradation of confinement appears to be linked to an increase in the source of neutral particles from the wall as the power increased, an effect that was not reproduced with the ILW. The reason for the loss of improved confinement domain at low power with the ILW is yet to be clarified, but contributing factors may include changes in the rate of gas injection, wall recycling, plasma composition and radiation. The results presented in this paper show that the choice of wall materials can strongly affect plasma performance, even changing confinement scalings that are relied upon for extrapolation to future devices.

Purely growing precursors and sawtooth trigger mechanisms

Purely growing sawtooth precursors have been identified in high current, high-density discharges of the Frascati Tokamak Upgrade. On the basis of the observed helical structure and growth rate, these precursors are identified as the early non-linear stage of the m = 1 instability in the semicollisional regime. The precursor stage is followed by a fast collapse during which the growth rate of plasma displacement increases by an order of magnitude. During the precursor, a transition from semicollisional to collisionless reconnection regimes occurs, as the non-linear resistive width falls below the collisionless skin depth; such a cross-over is proposed as the trigger mechanism for the fast collapse phase. This simple scheme, in which the crossing of a linear stability boundary triggers the precursor, while a non-linear effect triggers the fast collapse, emerges in plasmas where the diamagnetic rotation frequency is smaller than the precursor growth rate. Diamagnetic effects complicate the picture as they reduce precursor growth rate, but the basic mechanisms are likely to be the same in all cases.

Axisymmetric oscillations at L-H transitions in JET: M-mode

L to H transition studies at JET have revealed an n = 0, m = 1 magnetic oscillation starting immediately at the L to H transition (called M-mode for brevity). While the magnetic oscillation is pres ...

On the measurement of the threshold electric field for runaway electron generation in the Frascati Tokamak Upgrade

Experiments have been carried out to evaluate the threshold electric field for runaway generation during the flat-top phase of ohmic discharges in the Frascati Tokamak Upgrade tokamak. An investigation of the conditions for runaway electron generation and suppression has been performed for a wide range of plasma parameter values. The measured threshold electric field is found to be significantly larger ( ∼2−5 times) than predicted by the relativistic collissional theory of runaway generation, ER=neu2009e3u2009lnΛ/4πu2009e02u2009meu2009c2, and can be explained to a great extent by an increase of the critical electric field due to the effect of the electron synchrotron radiation losses. These findings are consistent with the results of an ITPA joint experiment to study the onset, growth, and decay of relativistic runaway electrons [Granetz et al., Phys. Plasmas 21, 072506 (2014)]. Confirmation of these results for disruptions with high electric field might imply significantly lower requirements on electron densities for suppre...

MHD marking using the MSE polarimeter optics in ILW JET plasmas

In this communication we propose a novel diagnostic technique, which uses the collection optics of the JET Motional Stark Effect (MSE) diagnostic, to perform polarimetry marking of observed MHD in high temperature plasma regimes. To introduce the technique, first we will present measurements of the coherence between MSE polarimeter, electron cyclotron emission, and Mirnov coil signals aiming to show the feasibility of the method. The next step consists of measuring the amplitude fluctuation of the raw MSE polarimeter signals, for each MSE channel, following carefully the MHD frequency on Mirnov coil data spectrograms. A variety of experimental examples in JET ITER-Like Wall (ILW) plasmas are presented, providing an adequate picture and interpretation for the MSE optics polarimeter technique.

EFD-C(13)03/32 ITER Like Wall Impact on MHD Instabilities in JET Discharges

1Consorzio RFX, EURATOM-ENEA Association, Corso Stati Uniti 4, 35127 Padova, Italy 2Euratom/CCFE Fusion Association, Culham Science Centre, Abingdon, OX14 3DB, UK 3Associazione EURATOM-ENEA sulla Fusione, C.R. Frascati, Roma, Italy 4Association EURATOM-CEA, CEA/DSM/IRFM, Cadarache 13108 Saint Paul Lez Durance, France 5Associacao EURATOM/IST, Instituto de Plasmas e Fusao Nuclear, Instituto Superior Tecnico, Av Rovisco Pais, 1049-001 Lisbon, Portugal 6FOM institute DIFFER, EURATOM association, P.O. Box 1207, Nieuwegein, Netherlands 7Max-Planck-Institut fur Plasmaphysik, EURATOM-Assoziation, D-85748 Garching, Germany 8Associazione EURATOM-ENEA sulla Fusione, Universita di Roma, Italy