Abstract
The understanding of the physics underlying the L-H transition has strong implications for ITER experimental reactor and demonstration power plant (DEMO). In many tokamaks, including JET, it ...has been observed that, at a particular plasma density,
n
e,min
, the power necessary to access H-mode P
L-H
is minimum. In the present work, L-H transitions of JET deuterium plasmas heated by neutral beam injection (NBI) are studied for the first time by means of a power balance analysis to characterize the main contributions in the transition, through integrated transport modelling. In the pulses analysed, we do observe a minimum of the L-H power threshold in density, indicating the presence of density branches and of
n
e,min
. Electron and ion heat fluxes at the transition are estimated separately. The electron/ion equipartition power results in favour of the ions, as shown by QuaLiKiz quasilinear gyrokinetic simulations, which predict a larger ion transport that causes
T
e
>
T
i
. The resulting edge ion heat flux also shows a clear change of slope below
n
e,min
, similarly to ASDEX-Upgrade (AUG) NBI pulses (Ryter
et al
2014
Nucl. Fusion
54
083003). JET NBI data are compared to radio-frequency heated AUG and Alcator C-mod pulses (Schmidtmayr
et al
2018
Nucl. Fusion
58
056003), showing a different trend of the power, coupled to ions at the L-H transition with respect to the linearity observed in the radio-frequency heated plasmas. The presence of
n
e,min
and the role of the ion heat flux is discussed in the paper, although it seems it is not possible to explain the presence of a P
L-H
minimum in density by a critical ion heat flux and by the equipartition power for the JET NBI-heated plasmas analysed.
Abstract
The work describes the pedestal structure, transport and stability in an effective mass (
A
eff
) scan from pure deuterium to pure tritium plasmas using a type I ELMy H-mode dataset in which ...key parameters that affect the pedestal behaviour (normalized pressure, ratio of the separatrix density to the pedestal density, pedestal ion Larmor radius, pedestal collisionality and rotation) are kept as constant as possible. Experimental results show a significant increase of the density at the pedestal top with increasing
A
eff
, a modest reduction in the temperature and an increase in the pressure. The variations in the pedestal heights are mainly due to a change in the pedestal gradients while only small differences are observed in the pedestal width. A clear increase in the pedestal density and pressure gradients are observed from deuterium to tritium. The experimental results suggest a reduction of the pedestal inter-edge localized mode (inter-ELM) transport from deuterium to tritium. The reduction is likely in the pedestal inter-ELM particle transport, as suggested by the clear increase of the pedestal density gradients. The experimental results suggest also a possible reduction of the pedestal inter-ELM heat transport, however, the large experimental uncertainties do not allow conclusive claims on the heat diffusivity. The clear experimental reduction of
η
e
(the ratio between density and temperature gradient lengths) in the middle/top of the pedestal with increasing
A
eff
suggests that there may be a link between increasing
A
eff
and the reduction of electron scale turbulent transport. From the modelling point of view, an initial characterization of the behaviour of pedestal microinstabilities shows that the tritium plasma is characterized by growth rates lower than the deuterium plasmas. The pedestal stability of peeling-ballooning modes is assessed with both ideal and resistive magnetohydrodynamics (MHD). No significant effect of the isotope mass on the pedestal stability is observed using ideal MHD. Instead, resistive MHD shows a clear increase of the stability with increasing isotope mass. The resistive MHD results are in reasonable agreement with the experimental results of the normalized pedestal pressure gradient. The experimental and modelling results suggest that the main candidates to explain the change in the pedestal are a reduction in the inter-ELM transport and an improvement of the pedestal stability from deuterium to tritium.
We report on a detailed study of magnetic fluctuations in the JET pedestal, employing basic theoretical considerations, gyrokinetic simulations, and experimental fluctuation data to establish the ...physical basis for their origin, role, and distinctive characteristics. We demonstrate quantitative agreement between gyrokinetic simulations of microtearing modes (MTMs) and two magnetic frequency bands with corresponding toroidal mode numbers n = 4 and 8. Such disparate fluctuation scales, with substantial gaps between toroidal mode numbers, are commonly observed in pedestal fluctuations. Here we provide a clear explanation, namely the alignment of the relevant rational surfaces (and not others) with the peak in the ω* profile, which is localized in the steep gradient region of the pedestal. We demonstrate that a global treatment is required to capture this effect. Nonlinear simulations suggest that the MTM fluctuations produce experimentally-relevant transport levels and saturate by relaxing the background electron temperature gradient, slightly downshifting the fluctuation frequencies from the linear predictions. Scans in collisionality are compared with a simple MTM dispersion relation. At the experimental points considered, MTM growth rates can either increase or decrease with collision frequency depending on the parameters thus defying any simple characterization of collisionality dependence.
Abstract
The required heating power,
P
L
H
, to access the high confinement regime (H-mode) in tritium containing plasmas is investigated in JET with ITER-like wall at a toroidal magnetic field of
B
...t
=
1.8
T and a plasma current of
I
p
=
1.7
MA.
P
L
H
, also referred to as the L-H power threshold, is determined in plasmas of pure tritium as well as mixtures of hydrogen with tritium (H-T) and mixtures of deuterium with tritium (D-T), and is compared to the L-H power threshold in plasmas of pure hydrogen and pure deuterium. It is found that, for otherwise constant parameters,
P
L
H
is not the same in plasmas with the same effective isotope mass,
A
e
f
f
, when they differ in their isotope composition. Thus,
A
e
f
f
is not sufficient to describe the isotope effect of
P
L
H
in a consistent manner for all considered isotopes and isotope mixtures. The electron temperature profiles measured at the L-H transition in the outer half of the radius are very similar for all isotopes and isotope mixtures, despite the fact that the L-H power threshold varies by a factor of about six. This finding, together with the observation of an offset linear relation between the L-H power threshold,
P
L
H
, and an effective heat diffusivity,
χ
e
f
f
, indicates that the composition-dependent heat transport in the low confinement mode (L-mode) determines, how much power is needed to reach the necessary electron temperatures at the edge, and hence
P
L
H
.
Abstract
The isotope effect on intrinsic rotation was studied at the Joint European Torus (JET) tokamak. With the unique capability of JET to operate with tritium (T), for the first time, experiments ...in hydrogen (H), deuterium (D) and T in Ohmic plasmas were compared. Two rotation reversals per isotope type are observed in plasma density scans spanning the linear and the saturated Ohmic confinement regimes. A clear isotope mass dependence is observed at the higher densities. The magnitude of the core rotation was found to depend on isotope mass, with stronger co-current rotation observed in H. Change on intrinsic rotation characteristics coexist with a stronger thermal energy confinement in T.
The first systematic investigation of core electron thermal transport and the role of local ion temperature gradient/trapped electron mode/electron temperature gradient (ITG/TEM/ETG)-scale core ...turbulence is performed in high temperature, low collisionality H-mode plasmas in the DIII-D tokamak. Wavenumber spectra of L-mode and H-mode density turbulence are measured by Doppler backscattering. H-mode wavenumber spectra are directly contrasted for the first time with nonlinear gyrokinetic simulation results. Core ITG/TEM-scale turbulence is substantially reduced/suppressed by
E
×
B
shear promptly after the L–H transition, resulting in reduced electron thermal transport across the entire minor radius. For small
k
θ
ρ
s
, both experiment and nonlinear gyrokinetic simulations using the GYRO code show density fluctuation levels
increasing
with
k
θ
ρ
s
in H-mode (
r
/
a
= 0.6), in contrast to ITG/TEM-dominated L-mode plasmas. GYRO simulations also indicate that a significant portion of the remaining H-mode electron heat flux results directly from residual intermediate/short-scale TEM/ETG turbulence. Electron transport at substantially increased electron-to-ion temperature ratio (
T
e
/
T
i
⩾ 1,
r
/
a
⩽ 0.35) has been investigated in ECH-assisted, quiescent H-mode plasmas. A synergistic increase in core electron and ion thermal diffusivity (normalized to the gyro-Bohm diffusivity) is found with applied ECH. From linear stability analysis, the TEM mode is expected to become the dominant linear instability with ECH due to increased electron-to-ion temperature ratio and a reduction in the ion temperature gradient. This is consistent with increased electron temperature fluctuations and core electron thermal diffusivity observed experimentally. The reduced ion temperature gradient likely results from a reduction in the ITG critical gradient due to increased
T
e
/
T
i
and reduced
E
×
B
shear. These studies are performed at collisonality (
,
r
/
a
⩽ 0.6) and address transport in electron heat-dominated regimes, thought to be important in ITER due to α-particle heating.
The EUROfusion JET-ILW pedestal database is described, with emphasis on three main issues. First, the technical aspects are introduced, including a description of the data selection, the datasets, ...the diagnostics used, the experimental and theoretical methods implemented and the main definitions. Second, the JET-ILW pedestal structure and stability are described. In particular, the work describes the links between the engineering parameters (power, gas and divertor configuration) and the disagreement with the peeling-ballooning (PB) model implemented with ideal magnetohydrodynamics equations. Specifically, the work clarifies why the JET-ILW pedestal tends to be far from the PB boundary at high gas and high power, showing that a universal threshold in power and gas cannot be found but that the relative shift (the distance between the position of the pedestal density and of the pedestal temperature) plays a key role. These links are then used to achieve an empirical explanation of the behavior of the JET-ILW pedestal pressure with gas, power and divertor configuration. Third, the pedestal database is used to revise the scaling law of the pedestal stored energy. The work shows a reasonable agreement with the earlier Cordey scaling in terms of plasma current and triangularity dependence, but highlights some differences in terms of power and isotope mass dependence.
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