_Applied_  
_sciences_  
_Article_  
An Unsupervised Spectrogram Cross-Correlation Method to  
Assess ELM Triggering Efficiency by Pellets  
[PERSON] 1\({}^{\copyright}\), [PERSON] 1, [PERSON] 2, [PERSON] 1\({}^{\copyright}\), [PERSON] 1\({}^{\copyright}\), [PERSON] 1, [PERSON] 1, [PERSON] 1\({}^{\copyright}\) and JET Contributors \({}^{\dagger}\)  
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Footnote  
a few hundred microseconds before recovering, inducing the expulsion of a considerable amount of particles and energy [4; 5; 6]. The energy and the particle flux released during an ELM involves high loads on the PFC. Some ELM extrapolations for ITER suggest that the energy loss during large ELMs (so-called type-I ELMs) ranges from 5 to 22 MJ and that about half of this energy will be deposited in a region of about one square meter, involving a surface energy density that ranges from 2.5 to 11 MJ m\({}^{-2}\). Such loads are 5-20 times higher than any acceptable values for the PFC [3]. Since future tokamaks will not be able to resist natural ELM loads, strategies to suppress or mitigate their occurrence are being investigated. In general, the types of countermeasures are divided into three approaches: radiating dispersion, ELM suppression, and ELM mitigation and control. The first approach, the radiating dispersion, aims at decreasing the inter-ELM heat loads onto the divertor by using gas seeding. The requirements for the radiated fraction (radiated power over the total output power) are quite stringent, 50% for ITER [7] and 90% for DEMO [8] and at ITER such approach is still under study. ELM suppression is instead based on the stabilisation of the ELM instability by avoiding reaching the boundaries of the peeling-ballooning ELM stability limit, and at ITER, it has been planned to perform it using resonant magnetic perturbation [9]. Finally, the mitigation and control of ELMs intends to trigger ELMs to drive their frequency in order to decrease the peak loads. In fact, large energy ELMs, the so-called type-I ELMs, are characterised by an inverse proportionality between loads and frequency: the higher the ELM frequency, the lower the energy released by a single ELM. Therefore, the main idea of ELM control is to trigger ELMs with a high frequency in order to reduce the peak loads on the PFC. Different approaches have been tested: vertical kicks [10; 11; 12], resonant magnetic perturbations [13; 14; 15], and pellet pace-making [15; 16; 17; 18; 19]. The pellet pacing technique is based on injecting a frozen pellet, usually made up of Deuterium, into the plasma. Such an approach has been investigated in ASDEX-Upgrade, JET, DIII-D, and EAST, and it is considered one of the most relevant tools for ITER ELM control [15; 16; 17; 18; 19]; at the same time, the physics of pellet pacing has been mostly analysed through the non-linear MHD code called JOREK [20; 21; 22; 23]. The pellet injection induces a significant variation in the pressure profile at the edge of the plasma and alters the magnetic topology by causing a strong growth of a chain of modes which facilitate the occurrence of ELMs [24]. Therefore, the inter-ELM elapsing time interval decreases, allowing the control of their frequency.