# Pedestal instabilities during the L-H transition in ASDEX-Upgrade plasmas

F Clairet, Anna Medvedeva, G D Conway, C Bottereau

► To cite this version:

F Clairet, Anna Medvedeva, G D Conway, C Bottereau. Pedestal instabilities during the L-H transition in ASDEX-Upgrade plasmas. Plasma Physics and Controlled Fusion, 2025, 67 (3), pp.035030. 10.1088/1361-6587/adb887. hal-05404569

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# Pedestal instabilities during the L-H transition in ASDEX-Upgrade plasmas

To cite this article: F Clairet et al 2025 *Plasma Phys. Control. Fusion* **67** 035030

View the [article online](#) for updates and enhancements.

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# Pedestal instabilities during the L-H transition in ASDEX-Upgrade plasmas

F Clairet<fnref n="1" />,*, A Glasser<fnref n="2" />, G D Conway<fnref n="3" />, C Bottereau<fnref n="1" />, the ASDEX Upgrade team<fnref n="4" /> and EUROFusion Exploitation Team<fnref n="5" />

E-mail: frederic.clairet@cea.fr

Received 21 November 2024, revised 13 February 2025

Accepted for publication 20 February 2025

Published 3 March 2025

<figure><img src="image_2.png" /></figure>

CrossMark

## Abstract

The high confinement regime, or H-mode, is the result of a transport barrier at the plasma edge, where the pressure rises sharply to form a pedestal. This paper reports on a collection of experimental observations of instabilities and turbulence occurring in this decisive and narrow edge region. Using the high sampling rate enabled by an ultra-fast frequency sweeping reflectometer, a high spatial and temporal resolution analysis of density, gradients, and turbulence allows new and thorough insights on plasma instabilities arising from the microturbulence and flow competing system during the I-phase to the edge localized modes and blob-filaments during the established H-mode. The measurements provide the observation on how different instabilities can arise in the pedestal during the L-H transition and address their specific turbulent natures.

Keywords: L-H transition, pedestal, turbulence, reflectometry, tokamak

## 1. Introduction

The H-mode [1] is an improved high confinement regime in magnetic confinement devices foreseen for plasma fusion. The improved core plasma performance is strongly dependent on the behavior of the edge plasma parameters owing to the formation of an edge transport barrier for particles and heat, which is characterized by a strong pressure gradient. However, improved regimes present several inconveniences, such as plasma impurity accumulation and the deleterious effects of burst plasma events, such edge localized modes (ELMs), on material erosion of the internal structures, which questions the confinement stability and performance over long duration. Explanations for the low-to-high confinement (L-H) transition process and the establishment of the edge transport barrier

are still required and the sophisticated synergies between turbulence drive and regulation effects more clearly identified. In fact, as the gradients of the kinetic profiles (i.e., density and temperature) steepen, complex physical processes occur during the formation of the transport barrier. The steepened gradients act as a free energy source to drive turbulence, which generates zonal flows that help saturate the turbulence. Additionally, the zonal mean field $E \times B$ flow, determined by momentum transport, plays a stabilizing role in suppressing the turbulence.

Among the instabilities and competing processes developing in the pedestal, we find edge coherent modes, whose origin is not fully established [2], the I-phase (a stationary improved L-mode regime) oscillations [3–5], also called M-modes on JET [6, 7]—seen as a competition and feedback loop process between turbulence and zonal/GAM flows [8, 9]—which may also be linked to type-III ELMs [10]. Finally, in high-performance H-modes, type-I ELMs [11] are induced by transgressions across the peeling-ballooning limit [12, 13] driven by an interplay between the edge pressure gradient and the current density, as well as driving density filaments. The

<footnote n="1">¹CEA, IRFM, F-13108 St. Paul-lez-Durance cedex, France</footnote>

<footnote n="2">²Aix Marseille University, CNRS, Centrale Marseille, M2P2 Marseille, France</footnote>

<footnote n="3">³Max-Planck-Institut für Plasmaphysik, 85748 Garching, Germany</footnote>

<footnote n="4">⁴ See Zohm et al 2024 (https://doi.org/10.1088/1741-4326/ad249d) for the ASDEX Upgrade team.</footnote>

<footnote n="5">⁵ See Joffrin et al 2024 (https://doi.org/10.1088/1741-4326/ad2be4) for the EUROFusion Exploitation Team.</footnote>

<footnote>* Author to whom any correspondence should be addressed.</footnote>
ballooning nature of these ELM-like instabilities is known to occur at a high-pressure gradient [14]. The H-mode regime in particular is thus characterized by the presence of these periodic instabilities producing a sudden loss of particles and energy transported through the separatrix, which are a concern for the integrity of the plasma-facing components [15]. Nevertheless, the link between the ELMs onset and the filaments remains unclear [16].

While a previous study on JET H-mode plasmas [17] using a fast frequency sweeping reflectometry was undertaken, we have subsequently used an improved diagnostic on AUG with a faster sweeping rate, providing the opportunity to collect high-quality data during 3 years of campaign between 2014 and 2016. This work reports an attempt to track and resolve the temporal and radial evolution of this key pedestal region during the L-I-H transition with the resolution capabilities provided by an ultra-fast frequency swept reflectometer.

After a brief recall of the diagnostic performance and a description of the experiments in section 2, we address in section 3 the I-phase and the limit cycle oscillation (LCO) behavior of the plasma turbulence, then section 4 presents the behavior of the pedestal during ELMs and their associated blob-filaments in section 5. Finally, to conclude and summarize the results in section 6.

## 2. Experimental conditions

### 2.1. Diagnostic setup and plasma discharges

The measurements were performed on ASDEX-Upgrade (AUG) discharges in favorable lower single null configurations (when the ion $B \times \nabla B$ drift is directed towards the active X-point) using an ultra-fast sweeping reflectometer (UFSR) [18] located in the low field side equatorial plane. This system consists of two FM-CW heterodyne reflectometers operating in V and W frequency bands (i.e. 50–75 and 75–110 GHz respectively) and X-mode probing, with a sweeping time of 1 µs and a dead time between sweeps that can be as short as 0.25 µs. The data acquisition system acquires at a rate of 1 Gsample/second, which generates 1000 points per signal (i.e. per sweep), with In-phase and quadrature phase detection, which provides complex signals. On board, 2 Gb of memory can store up to 200 000 signals, thus limiting the acquisition duration to a maximum of 250 ms.

The signals can be analyzed in two ways, either for each frequency sweep to provide radial density profiles or at fixed frequencies from sweep to sweep to provide the frequency spectra of the radial density fluctuations at a sampling rate of 800 kHz. A high radial resolution can be achieved as the swept signals provide approximately the equivalent of 2000 fixed probing frequency steps. These high temporal and subcentimeter radial resolutions make it possible to record the plasma instabilities and their dynamics in the relatively narrow edge pedestal region. In addition to the density profile data, the electron temperature is obtained by either electron cyclotron emission (ECE) [19] or Thomson scattering diagnostics [20].

Due to the limited acquisition time available, we select three discharges, with the time window of the recorded

UFSR data shown in figure 1, where the transitions from the ohmic, L-mode, I-phase, and ELMy H-mode were captured (figures 1(a) and (c)). The I-phase is conveniently identified from the oscillations of the divertor shunt current signal ranging typically between 0 and 3 kHz. On machines like JET [7] or DIII-D [21] the Dα radiation signal is used. These signals are also used for the ELM identification. During AUG discharge #32389 the reflectometry measurements cover the ohmic phase up to the H-mode through a short I-phase between 1.21 s and 1.23 s, but without reaching the ELMing phase. The magnetic field $B_T$ is equal to 2.4 T, the mean density varies from $<n_e \ge 3-5 \times 10^{19} m^{-3}$, the plasma current first rises from $I_p = 0.95-1.05$ MA between $t = 1.05$ and 1.2 s, then is kept constant until $t = 1.3$ s, an additional ECRH power of 0.8 MW starting at 1.1 s, then is increased to 1.45 MW at 1.2 s with an additional neutral power of 2.4 MW. The AUG discharge #33282 measurements miss the ohmic and L-mode phases but capture a rather long I-phase between 0.75 and 0.92 s and the H-mode phase with most probably type-I ELMs as frequency increases [22] with the addition of NBI power observed later on. The magnetic field $B_T$ is also equal to 2.4 T, the mean density varies from $<n_e \ge 2.5-5 \times 10^{19} m^{-3}$, the plasma current rises continuously from $I_p = 0.7-0.95$ MA, an additional ECRH power equal to 1.5 MW is followed by a neutral beam power of 2.4 MW at 0.89 s. Both I-phases exhibit pulsation frequencies in the kHz range identified from the spectral analysis of divertor shunt current signals (figures 1(b) and (d)). Finally, an H-mode discharge #32342 (figure 1(e)) has been specifically studied for its type-III ELM instability, identified as their frequency decreases as the additional power increases. The magnetic field $B_T$ is also equal to 2.4 T, the mean density varies from $<n_e \ge 4-5 \times 10^{19} m^{-3}$, a plasma current of $I_p = 0.82$ MA, an additional ECRH power equal to 2.8 MW with the neutral beam power first equal to 7.5 MW and increased to 10 MW at 1.2 s.

### 2.2. Density fluctuations calculation

The probing electromagnetic wave is reflected from the plasma at the cutoff layer, where the plasma density fluctuations are inferred from the reflected wave amplitude and phase in the heterodyne receiver. In order to estimate the perturbed density behavior, we consider the case of the 1D model. The reflected signal is of the form

$$S = A(t) \exp(i\phi(t)) \quad (1)$$

where A corresponds to the amplitude of the reflected signal and $\phi$ is the phase shift the probing electromagnetic wave experiences when traveling through the plasma. The probing wave records then the plasma density fluctuations during its travel such that amplitude and phase can be written as:

$$A = \langle A \rangle + \delta A \quad \text{and} \quad \phi = \langle \phi \rangle + \delta \phi \quad (2)$$

in the case of the 1D model, the density fluctuations are calculated from the signal phase alone. Taking the series of frequency sweeps at a fixed frequency (F) from sweep to sweep,
<figure><img src="image_3.png" /><figcaption>Figure 1. Plasma experimental conditions during the L-H transition discharges #32389 (a) and #33282 (c). Shaded gray areas represent the 250 ms acquisition time windows of the UFSR. The spectrograms (b) and (d) represent the spectral analysis of the divertor shunt current signals, which exhibit the I-phase frequency oscillation in the kHz range and where the H-mode phase can be considered to occur at the end of the oscillations. H-mode discharge #32342 (e) offers a series of type III ELMs identified on the divertor shunt current signal.</figcaption></figure>

we are then able to calculate the frequency (f) power spectrum of the phase fluctuations at a given cut-off radial position :

$$P_{\delta\Phi}(f)_F = |\text{FFT}(\exp(i\delta\phi(t)))|^2. \quad (3)$$

From Parceval's theorem, one can calculate the root mean square of the phase fluctuations:

$$\delta\phi_{\text{rms}} = \sqrt{\int P_{\delta\phi}(f) df}. \quad (4)$$

It should be noted that we can select the fluctuations according to their frequency domain and that the square of these density fluctuations (a square summable function) represents their energy content. Finally, the density fluctuations are determined according to the linear relationship [23]:

$$\frac{\delta n}{n} = \frac{\sqrt{\tilde{k} \cdot \nabla N_X^2}}{2\sqrt{\pi} k_0} \delta\phi_{\text{rms}} \quad (5)$$

where $N_X$ is the X-mode refractive index, $k_0$ is the vacuum wavenumber of the probing wave, and $\tilde{k}$ is the fluctuation wavenumber seen by the probing wave. Actually, as the wave number of the probing wave ($k_{\text{wave}}$) quickly drops to zero at the cut-off layer, the Bragg backscattering ($\tilde{k} = 2k_{\text{wave}}$) sensitivity to the fluctuations is strongly enhanced [24] so its value is taken, a bit arbitrarily, as a mean value over the thickness of the reflection layer, as approximately equal to $1 \text{ cm}^{-1}$. Such an analytical approach represented by equation (5) provides rather simply a quick but fair estimation of the density fluctuation level, suitable for processing a large amount of data. It also mainly provides a measure of the evolution of the turbulence.

Figure 2(a) shows the radial and temporal evolution of the normalized density fluctuation level ($\delta n/n$) for discharge #32389 during the L-H transition, relative to the fluctuation level calculated during the ohmic phase between 1.08 and 1.1 s. The calculation is performed with an integrated time of 5 ms for density profiles and fluctuations. The radial coordinate is the normalized poloidal flux $\rho_{\text{pol}}$. The main features observed are an increase of the fluctuations by about 20% around mid-radius when the additional ECHR power is turned on at $t = 1.1$ s and a substantial decrease of more than 40% when the plasma enters the H-mode phase around $t = 1.23$ s. It is worth noticing that this decrease first arises at the edge and propagates towards the core (the three small localized increases observed at the edge around $\rho_{\text{pol}} = 0.95$ are due to temporary imperfections in the edge density profile reconstruction crucial for the fluctuation calculation). Figures 2(b) and (c) show how the absolute density fluctuations drop and the density pedestal builds up during the transition. It is worth noting that the fluctuation level stays below a few percent, which lets the linear relationship of equation (5) be valid [25].

The increase of turbulence towards $\rho_{\text{pol}} = 1$ during the H-mode phase will be examined in more detail later in section 4 on the ELMs.
<figure><img src="image_4.png" /><figcaption>**Figure 2.** (a) Evolution of the density fluctuations during an L-H transition of discharge #32389 (calculated with an integration time of 5 ms) relative to the ohmic phase averaged between 1.08 and 1.1 s, i.e. $\frac{\delta n}{n} / \langle \frac{\delta n}{n} \rangle_{1.08-1.1s}$. Thus, the color scale at 0.6 and 1.2 means, respectively, a decrease of 40% and an increase of 20% of the fluctuations compared to the ohmic phase. (b) Density fluctuation radial profile and (c) mean density radial profile evolution from ohmic to H-mode at 4 different times.</figcaption></figure>

### 3. I-phase

The density profiles measured by the UFSR during the density ramp-up phase of discharge #33282, where the I-phase was present between the L and H modes, shown in figure 3, present density oscillations localized around the pedestal. These oscillations exhibit a sawtooth-like temporal character, figure 4(a), with a slow rise followed by a rapid decrease, or small crash, with relatively small density variations of a few $10^{17} \text{ m}^{-3}$ which represents a fluctuation level change of about 1% of the local density and with a radial displacement of about 5 mm. During the I-phase, the amplitude of the density variation increases as the oscillation frequency decreases. The density profiles oscillate around a pivot point located in the edge gradient region, at about 1 cm from the separatrix within the confinement region. It results an alternation of a slow steepening followed by a rapid release of the gradient. We notice that the pivot point of these density oscillations at $R = 2.11 \text{ m}$ ($\rho_{\text{pol}} \sim 0.98$) do not coincide precisely with the maximum of the pressure gradient, which is at 2.10 m.

The total density fluctuation level $(\delta n/n)^2$ shown in figure 5 is calculated throughout the acquisition sequence as a function of time for two frequency bands: high frequencies $f > 20 \text{ kHz}$ (red), low frequencies $f < 20 \text{ kHz}$ (blue), and the whole band (black). At first, there is a slight reduction during the early I-phase, which then stabilizes. Then later, there is a more dramatic drop as the plasma enters H-mode. The intermittent bursts are due to the ELMs, but the fluctuation level always returns to the low H-mode level in between. The most striking result are the large oscillations when discriminating the fluctuations based on their frequencies (see equation (4), i.e. when comparing between the low ($|f|l < 20 \text{ kHz}$) and the high ($|f|l > 20 \text{ kHz}$) spectral components (the 20 kHz limit was

<figure><img src="image_5.png" /><figcaption>**Figure 3.** (a) Time traces of iso-densities near the separatrix (dashed line) during the I-phase with ramp-and-crash like oscillations around a pivot point at $R = 2.11 \text{ m}$ ($\rho_{\text{pol}} \sim 0.98$). (b) and (c) Corresponding averaged profiles of density, temperature and pressure just before a crash between 0.884 and 0.8845 s.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>**Figure 4.** (a) Sawtooth character of a density oscillation at $R = 2.09 \text{ m}$ ($\rho_{\text{pol}} \sim 0.9$) during the I-phase. (b) Corresponding (blue and red) averaged density profiles that oscillate around a pivot point located main toward the pedestal top.</figcaption></figure>

taken a bit arbitrarily). These oscillations disappear during the H-mode phase.

In order to examine in detail this oscillation process during the I-phase, figure 6 shows a closer view of the time sequence of the density and fluctuations around the pedestal top at $R = 2.09 \text{ m}$. Alongside the sawtooth trend in the mean density at the top pedestal (figure 6(a)), are the temporal evolution of the fluctuations. A sliding Fourier analysis (windowed SFFT) of the reflected raw signal, using an integration time
<figure><img src="image_7.png" /><figcaption>**Figure 5.** Density fluctuations at $R = 2.1$ m ($\rho_{pol} \sim 0.97$) for frequencies $|f| > 20$ kHz (in red), $|f| < 20$ kHz (in blue), and for all frequencies $-400 < f < 400$ kHz (in black). The fluctuations are calculated with an integration time window of 100 µs.</figcaption></figure>

of 100 µs, provides the temporal evolution of the turbulence frequency spectrum in the pedestal (figure 6(b)).

Figure 6(c) provides a more detailed view of the modulation of the low and broadband frequency density fluctuations of figure 5. While taking into account the 100 µs time resolution, this modulation exhibits an alternate in opposite phase between the low and high frequency components varying in amplitude by an equal amount, which suggests a reciprocal and almost complete energy transfer between low and high frequency turbulences. Figure 7 illustrates the alternating spectra of figure 6(b). The low frequency spectrum exhibits a peak near zero, or zero-mean frequency, due to the 100 µs time window analysis inducing a frequency resolution of 10 kHz.

This modulation is also observed on the reflected signal amplitude (figures 6(d) and (e)) and extends within 2–3 cm around mainly the pedestal top, between $R \sim 2.09$ and 2.11 m, and varies by about 15 dB. This behavior, having such a notable effect on the reflection efficiency of the probing microwaves, provides some information on the plasma turbulence structure. When the broadband spectrum dominates, the amplitude of the reflected signal is significantly reduced, suggesting increased ‘roughness’ of the plasma cutoff layer that would produce enhanced scattering of the reflected wave in many directions. In contrast, during the narrow-band zero-mean frequency turbulence, the reflection amplitude substantially increases as if the probing wave is reflecting from a smooth or flat surface.

During this sequence, as the amplitude of the zero-mean frequency signal increases, the density rises as the confinement improves, and when the pedestal becomes steeper, the higher frequencies take over, leading to a density crash. The zero-mean frequency then reappears almost immediately, and the I-phase cycling process (LCO) repeats.

In this context, the origin of the low-frequency component in our data is of particular interest. While MHD instabilities such as microtearing modes (MTMs) and kinetic ballooning modes (KBMs) are possible candidates, these typically exhibit frequencies on the order of tens of kHz in the pedestal region. The nearly zero frequency of the observed component, coupled with its dynamic interplay with broadband turbulence, makes it unlikely to be caused by such modes. Furthermore, no evidence of tearing or ballooning modes was observed in the magnetic diagnostic signals during these phases of the

<figure><img src="image_8.png" /><figcaption>**Figure 6.** (a) Density time evolution recorded at $R = 2.09$ m ($\rho_{pol} \sim 0.95$). (b) Time intermittency of the low frequency (zero-mean) part of the spectrogram at $R = 2.10$ m ($\rho_{pol} \sim 0.97$) (horizontal white arrow bar indicates the integration time of 100 µs used for the calculation of the spectra). (c) Density fluctuation amplitudes for high frequencies $|f| > 20$ kHz (red), low frequencies $|f| < 20$ kHz (blue) and the sum of both low and high frequencies (black). (d) Radial and temporal contour plot showing evolution of the reflected signal amplitude. The white dashed line corresponds to the separatrix. (e) Time evolution of the reflected signal amplitude integrated between 2.09 and 2.11 m. (f) Averaged density profile between 0.873 and 0.876 s. The vertical dashed lines are used to guide the eye. $B_T = -2.5$ T, $I_p = 0.82$ MA, $<n_e> \sim 3.7 \times 10^{19} \text{ m}^{-3}$, $T_{e0} \sim 3$ keV, $P_{ECRH} = 1.4$ MW.</figcaption></figure>

discharges. Instead, the observed modulation could be indicative of a zonal flow oscillation (ZFO), driven by turbulent stresses and interacting with broadband turbulence. This interpretation is consistent with the alternating modulation of high- and low-frequency components and the observed changes in the reflection signal amplitude. The low-frequency component may also be influenced by mean-field $E \times B$ velocity oscillations or dithering transitions during the I-phase. However, the energy exchange observed between high-frequency broadband turbulence and near-zero-mean frequency fluctuations strongly supports the role of zonal flows in modulating edge
<figure><img src="image_9.png" /><figcaption>**Figure 7.** Turbulence frequency spectra of figure 5(b) recorded at 0.875 s (in red) and at 0.8755 s (in black) at $\rho_{pol} \sim 0.97$.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>**Figure 8.** (a) Time evolution of the low frequency fluctuation at $R = 2.12 \text{ m} (\rho_{pol} \sim 0.95)$ (the spectra are calculated with $100 \ \mu\text{s}$ of integrated time). (b) Amplitude of the reflected signal. (c) Averaged density profile between 1.21 and 1.22 s. $B_T = -2.5 \text{ T}$, $I_p = 1 \text{ MA}$, $\langle n_e \rangle \sim 3.5 \times 10^{19} \text{ m}^{-3}$, $T_{e0} \sim 4.5 \text{ keV}$, $P_{ECRH} = 1.5 \text{ MW}$, $P_{NBI} = 2.5 \text{ MW}$.</figcaption></figure>

turbulence and transport. Further investigation with a better diagnostics coverage is needed to fully understand the non-linear dynamics of these interactions and to clarify the precise role of zonal flows in the transition to H-mode.

Discharge #32389 allows us to observe a complete L-mode to H-mode transition thanks to a relatively short I-phase. Figures 8(a) and (b) show the time evolution of the oscillation of the zero-mean frequency fluctuation with the reflected signal amplitude. Strong reflected signal amplitude variations of about 20 dB occur during the I-phase and increase each time the zero-mean frequency appears as previously observed. When entering the H-mode, as the I-phase oscillation ends, this zero-mean frequency intermittency stops and remains steady for a while. In figures 9(a) and (b) we observe that around $t = 1.21$ s, during the I-phase, the mean density increases while the density fluctuations decrease before the

<figure><img src="image_11.png" /><figcaption>**Figure 9.** (a) Measured averaged density and (b) density fluctuations amplitude for different integrated frequency domains at $R = 2.12 \text{ m} (\rho_{pol} \sim 0.95)$ during the L-H transition (with an integrated calculation performed over $100 \ \mu\text{s}$).</figcaption></figure>

H-mode settles around $t = 1.23$ s. In addition, during the transition to the H-mode, the mean density rises from the beginning of the I-phase, and the density fluctuations at both low and high frequencies decrease.

Figure 10 shows, for discharge AUG#33282, the transition from the I-phase to the H-mode up to the first ELM. The density fluctuation level decreases by half until a significant increase (blip) due to the ELM at $t \sim 0.96$ s and then returns to the H-mode level. It is noted that while low and broadband frequency fluctuations evolve in opposite phases during the I-phase, they both increase during an ELM, suggesting that the two events might result from different physical processes. At the beginning of the time sequence shown, the amplitude of zero-mean frequency turbulence oscillates (together with the reflected amplitude) then stabilizes at $t = 0.92$ s for a short period (in comparison with AUG#32389 discharge) and is becoming tenuous from $t = 0.93$ s. Moreover, when the ELM event approaches, the reflected amplitude decreases substantially. This is probably due to the appearance of a new instability, such as filamentary activity, that will be discussed in the next two sections.

## 4. ELMs

For AUG discharge #32342 (figure 1(c)), as the H-mode is established, a series of type-III ELMs, previously identified as their frequency decreases as the additional power increases, are observed in the divertor shunt current signal. Figure 11(a) shows the time evolution of the iso-densities during 4 successive type-III ELMs that oscillate around a pivot location. The position of the maximum pressure gradient (the pressure gradient is primarily given by the density pedestal and marginally by the temperature) coincides precisely with the ELM pivot point position (already mentioned in [26]) at about $R = 2.105 \text{ m}$, which can be considered as the radial origin of the crashes in accordance with the theory [14].
<figure><img src="image_12.png" /><figcaption>**Figure 10.** Time sequence of transition from I-phase to H-mode for the AUG#33282 discharge at $R = 2.10$ m ($\rho_{pol} \sim 0.97$). (a) Density fluctuation levels calculated with an integrated time of 100 µs (blue and red lines represent integrated low and broadband frequencies below and above 20 kHz, respectively, while the black line represents the total fluctuation level). (b) Turbulent time-frequency spectrogram also calculated with an integrated time of 100 µs. (c) Amplitude of the reflected signal.</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>**Figure 12.** (a) Average density profile between 1.23 and 1.25 s. (b) Time evolution of iso-densities during a series of type-III ELMs. (c) Divertor shunt current signal showing the ELM timing events. (d) Close view of the localized increase of the density oscillations on top and bottom of the pedestal before the ELM. $B_T = -2.5$ T, $I_p = 0.83$ MA, $\langle n_e \rangle \sim 4.5 \times 10^{19}$ m$^{-3}$, $T_{e0} \sim 5$ keV, $P_{ECRH} = 2.8$ MW, $P_{NBI} = 10$ MW.</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>**Figure 11.** Left: (a) Temporal evolution of the radial positions of iso-densities (sliding average over 40 profiles) for a series of 4 consecutive ELMs. (b) and (c) Averaged density, temperature, and pressure gradient profiles measured by reflectometry and ECE just before an ELM between 1.266 and 1.27 s.</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>**Figure 13.** (a) Average density profile between 0.93 and 0.97 s. (b) Time evolution of iso-densities with a type-I ELM occurring at $t = 0.96$ s. (c) Divertor shunt current signal showing the ELM timing event. (d) Close view of the density oscillations before the ELM occured at the top pedestal. (e) Isolated burst events occurring simultaneously along the whole pedestal. $B_T = -2.5$ T, $I_p = 1$ MA, $\langle n_e \rangle \sim 6.5 \times 10^{19}$ m$^{-3}$, $T_{e0} \sim 4$ keV, $P_{ECRH} = 1.5$ MW, $P_{NBI} = 2.5$ MW.</figcaption></figure>

Figures 12 and 13 highlight an increasing amplitude of the density oscillations ending, respectively, by a type-III and type-I ELM crashes. Nevertheless, from figures 12(b) and 13(b), we observe that these density oscillations either occur across the whole pedestal, or are rather localized at the top pedestal. We also note that, despite these perturbations, the plasma confinement further improves as the pedestal continues to grow with the core density increasing and the edge decreasing. A closer look at figures 12(d) and 13(d) shows that these oscillations do not exhibit coherent mode frequencies and induce either a steepening or a flattening of the density profiles. In figure 13(e) we also notice additional individual

fast burst events occurring all across the pedestal. It is known that ELMs are created from MHD instability and pressure gradients, thus, these filamentary structures are probably at the origin of these steadily growing density oscillations leading to the ELM crash [27] and are studied in the next section.

## 5. Blob-filaments

Fast and intermittent events observed in H-mode plasmas during inter ELM periods have been described as blob-filaments
<figure><img src="image_16.png" /><figcaption>**Figure 14.** (a) Iso-density lines exhibit inward and outward particle transport (dotted points at the edge help denote the 1.25 µs temporal resolution). (b) Corresponding density profiles, where the thick black line is the averaged profile. $B_T = -2.5$ T, $I_p = 0.83$ MA, $\langle n_e \rangle \sim 4.5 \times 10^{19}$ m$^{-3}$, $T_{e0} \sim 5$ keV, $P_{ECRH} = 2.8$ MW, $P_{NBI} = 10$ MW.</figcaption></figure>

<figure><img src="image_17.png" /><figcaption>**Figure 15.** (a) Time traces of iso-densities at two radial positions during a single inter ELM period with density bursts oriented inward (negative skewness) and outward (positive skewness). The red dashed line indicates the position of the separatrix. (b) Radial profile of the skewness with a zero crossing (black circle) indicating the position of the origin of the filament burst (max. and min. skewness are identified by a blue and red circle, respectively). (c) Absolute value of the average pressure gradient profile during this time interval.</figcaption></figure>

[28]. These radially propagating structures can make a substantial contribution to the transport of particles and heat in the radial direction, which may also have damaging effects on the plasma facing components. They have been widely observed using Langmuir probes [29] and fast visible cameras [30] and with reflectometry [31, 32], but often with limited radial extent.

The iso-density lines, recovered from UFSR density profiles, illustrate the fast dynamic in the pedestal. During an inter ELM period (figure 14), we were able to observe rapid density events lasting around 10–20 µs in the gradient region close to the separatrix, with particle transport propagating both towards the plasma core and the SOL. Although each density profile is determined independently, the density bursts are visible during multiple profile measurements (the black dots on the outermost contour show the temporal resolution), so we can assess that these are not profile artifacts.

In order to determine where these blob-filaments are created, we used a statistical method by looking at the radial evolution of the skewness defined as the third-order moments of a probability distribution function, which gives a measure of the degree of asymmetry of this distribution with respect to its mean value. The skewness is calculated from the temporal evolution of the radial position of an iso-density, providing information on the blob-filament density radial asymmetry direction. When the density radial variations are oriented towards the plasma core, the skewness is negative and positive otherwise, as shown in figure 15(a). By applying this analysis to several iso-densities, we obtain a radial profile of the skewness from the SOL to the plasma core, as shown in figure 15(b). A skewness equal to zero corresponds to the origin of the

growth of the filaments, we note that this radial position coincides precisely with the maximum of the pressure gradient at $R = 2.10$ m.

The minimum and maximum around the skewness zero contain information about both the radial excursion and the number of blob-filaments, thus, these quantities give an indication of the intensity of these instabilities. We used a series of ELMs to examine the relationship between the appearance of the blob-filaments and the pressure gradient. Figures 16(a) and (b) show the density, temperature, and pressure profiles before and during an ELM with a significant change in the pressure gradient. The skewness and the pressure gradient were calculated during this series (figures 16(c)–(e)) with an integrated time of 0.5 ms (which is a compromise between the requirement for statistics and the ELM’s duration of approximately 4 ms). Figure 16(f) shows the close relationship between the pressure gradient and filament activity. Note the asymmetry between the minimum and maximum skewness comes from a gradient effect, and its variation gives rise to a larger radial density displacement towards the plasma core at the top pedestal than towards the edge.

The increase in the pressure gradient in the pedestal region during the inter-ELM period is indicative of an accumulation of particles and energy. This growing pressure gradient enhances the drive for instabilities, such as drift-Alfvén instabilities and resistive ballooning modes, which are known to trigger blob-filament formation and transport. The increase in the maximum skewness during the inter-ELM period is
<figure><img src="image_18.png" /><figcaption>**Figure 16.** (a), (b) Density, temperature and absolute pressure gradient profiles before (solid line) and during (dashed line) an ELM. (c) Series of three ELMs recorded in the divertor shunt current, with (d) the maximum in the pedestal pressure gradient and (e) the maximum positive (blue) and negative (red) values of the skewness, accounting respectively for inward and outward density radial displacement. (f) Skewness as a function of the pressure gradient determined between 1.2 and 1.3 s (Skewness and the pressure gradient are calculated with an integrated time of 0.5 ms).

$B_T = -2.5$ T, $I_p = 0.83$ MA, $\langle n_e \rangle \sim 4.5 \times 10^{19}$ m$^{-3}$, $T_{e0} \sim 5$ keV, $P_{ECRH} = 2.8$ MW, $P_{NBI} = 10$ MW.</figcaption></figure>

consistent with the expectation that an increasing pressure gradient strengthens the drive for turbulent transport and enhances the formation of coherent filamentary structures that propagate radially outward. However, the asymmetry in the skewness (with a larger radial displacement towards the plasma core at the top of the pedestal) suggests that the growth and propagation of filaments are not uniform across the pedestal. The stronger inward transport near the top of the pedestal may be related to the stabilizing effect of magnetic shear or the presence of zonal flows that regulate turbulence. The observed correlation between the pressure gradient and filament activity supports the idea that pedestal steepening leads to increased blob-filament generation, which in turn contributes to enhanced radial transport. This process plays a key role in regulating pedestal confinement and ultimately influences the onset of the next ELM.

## 6. Summary and discussion

UFSR provides valuable and exceptional information by comparing density fluctuations and density profiles continuously from the SOL to the plasma core during the L-H transition. With its high spatial and temporal resolution, UFSR is particularly well suited to perform measurements in the narrow and crucial pedestal region where instabilities occur, which are critical for the plasma performance. The instabilities investigated in this work are the I-phase LCO, the ELMs during the established H-mode, and the blob-filaments during

the inter-ELM phase. In particular, the I-phase sequence has been described simultaneously in both the radial and temporal domains.

During this cyclical process, we have identified a transfer of energy, in the pedestal edge, between broadband frequency fluctuations and low or zero-mean frequency fluctuations. Assessing a direct transfer without any time lag must be considered according the time resolution of 100 µs. However, these different turbulence frequency bands clearly influence the particle transport, since the broadband frequency fluctuations level increases, the pedestal gradient relaxes rapidly, while the low frequency fluctuation band, participates in a particle transport barrier that gradually steepens the pedestal.

A limit cycle oscillation model was first proposed to explain a 1-2 kHz periodic L-H-L switching (called a dithering H-mode) observed in AUG [33] between two confinement states. A similar LCO (predator-prey) model was later proposed to describe the cyclic behavior of the I-phase. Here, the alternation of the characteristics of the edge plasma turbulence and kinetic profiles looks like as an interactive and competitive process between a low frequency zonal flow/GAM, and the background broadband frequency micro-turbulence [34–39]. One may wonder whether this low frequency component, localized in the pedestal, where the $E \times B$ flow is usually recorded with Doppler reflectometry (but unfortunately not measured during these experiments), could be attributed to the radial oscillation of the zonal flow and its importance in suppressing turbulent particle flux. In both discharges with I-phase, the core and edge densities are higher than for the AUG conditions previously studied in [4] and thus the GAM is generally more collisionally damped, leaving the turbulent energy to couple into a low-frequency (few kHz) ZFO [40]. Nevertheless, the role of some kind of MHD instability (KBM, MTM...) responsible in a stabilization of the broadband frequency turbulence may remain debatable.

The I-phase ends as its oscillation slows down, the low frequency component spectra stabilizes for a while, finally rests, and disappears from our reflectometry signal. Then, the plasma evolves towards an H-mode, which can be defined as a significantly reduced density fluctuation at the edge with a ‘front’ originating from the pedestal and propagating towards the core of the plasma.

Concerning the ELMs, the density behavior might offer similarities with the I-phase having as well a ramp and crash density evolution around a pivot point. However, the ELM density variations have a much larger amplitude, and the cycle repeats on a non-regular basis with a lower frequency ranging from a few Hz to a few hundred Hz. Moreover, we highlighted direct observation of small and rapidly growing density oscillations before the ELM and localized in the pedestal, which are associated with blob-filaments. The behavior of the density variations appears differently for the present studied discharges. The blob-filaments appear as precursor instabilities closely associated with the final ELM crash. There is a radial correspondence between the origin of these precursor filament oscillations and the position of the maximum pressure gradient, suggesting a ballooning character for this instability. These density oscillations evolve temporally in different ways
before the ELM and can be either strictly localized at the top
pedestal in the case of type-I or throughout the pedestal for the
case of type III. However, we do not intend to generalize to all
types of ELM until a thorough analysis with various plasma
parameters and systematic investigation has been carried out.

Data availability statement

The data that support the findings of this study are available
upon reasonable request from the authors.

Acknowledgment

This work has been carried out within the framework of
the EUROfusion Consortium, funded by the European Union
via the Euratom Research and Training Program (Grant
Agreement No 101052200—EUROfusion). Views and opin-
ions expressed are however those of the author(s) only and
do not necessarily reflect those of the European Union or the
European Commission. Neither the European Union nor the
European Commission can be held responsible for them.

ORCID iDs

F Clairet https://orcid.org/0009-0003-1634-7631

A Glasser https://orcid.org/0000-0003-4323-3528

G D Conway https://orcid.org/0000-0002-3947-4268

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