# Mohamed Ezzat

###### PhD Student

Mohamed Ezzat
Geothermal Energy & Geofluids
Institute of Geophysics

Contact
 Phone +41 44 632 2558 Email mostamoh@ethz.ch

 Dominique Ballarin Dolfin Phone +41 44 632 3465 Email ballarin@ethz.ch

## Publications

Underlined names are links to current or past GEG members

### REFEREED PUBLICATIONS IN JOURNALS

4.
Ezzat, M., B. M. Adams, M.O. Saar, and D. Vogler, Numerical Modeling of the Effects of Pore Characteristics on the Electric Breakdown of Rock for Plasma Pulse Geo Drilling, Energies, 15/1, 2022. [View Abstract]Drilling costs can be 80% of geothermal project investment, so decreasing these deep drilling costs substantially reduces overall project costs, contributing to less expensive geothermal electricity or heat generation. Plasma Pulse Geo Drilling (PPGD) is a contactless drilling technique that uses high-voltage pulses to fracture the rock without mechanical abrasion, which may reduce drilling costs by up to 90% of conventional mechanical rotary drilling costs. However, further development of PPGD requires a better understanding of the underlying fundamental physics, specifically the dielectric breakdown of rocks with pore fluids subjected to high-voltage pulses. This paper presents a numerical model to investigate the effects of the pore characteristics (i.e., pore fluid, shape, size, and pressure) on the occurrence of the local electric breakdown (i.e., plasma formation in the pore fluid) inside the granite pores and thus on PPGD efficiency. Investigated are: (i) two pore fluids, consisting of air (gas) or liquid water; (ii) three pore shapes, i.e., ellipses, circles, and squares; (iii) pore sizes ranging from 10 to 150 μm; (iv) pore pressures ranging from 0.1 to 2.5 MPa. The study shows how the investigated pore characteristics affect the local electric breakdown and, consequently, the PPGD process.

3.
Ezzat, M., D. Vogler, M. O. Saar, and B. M. Adams, Simulating Plasma Formation in Pores under Short Electric Pulses for Plasma Pulse Geo Drilling (PPGD), Energies, 14/16, 2021. [View Abstract]

Plasma Pulse Geo Drilling (PPGD) is a contact-less drilling technique, where an electric discharge across a rock sample causes the rock to fracture. Experimental results have shown PPGD drilling operations are successful if certain electrode spacings, pulse voltages, and pulse rise times are given. However, the underlying physics of the electric breakdown within the rock, which cause damage in the process, are still poorly understood.

This study presents a novel methodology to numerically study plasma generation for electric pulses between 200 to 500 kV in rock pores with a width between 10 and 100 $\mu$m. We further investigate whether the pressure increase, induced by the plasma generation, is sufficient to cause rock fracturing, which is indicative of the onset of drilling success.

We find that rock fracturing occurs in simulations with a 100 $\mu$m. pore size and an imposed pulse voltage of approximately 400 kV. Furthermore, pulses with voltages lower than 400 kV induce damage near the electrodes, which expands from pulse to pulse, and eventually, rock fracturing occurs. Additionally, we find that the likelihood for fracturing increases with increasing pore voltage drop, which increases with pore size, electric pulse voltage, and rock effective relative permittivity while being inversely proportional to the rock porosity and pulse rise time.

2.
Horacek, J., et al., M. Ezzat, et al., EUROfusion MST1 Team, JET Contributors, and MAST-U Team, Scaling of L-mode heat flux for ITER and COMPASS-U divertors, based on five tokamaks, Nuclear Fusion, 60/6, 2020. [View Abstract]This contribution aims to improve existing scalings of the L-mode power decay length, especially for plasma configurations with strike points at the ITER-relevant location - closed vertical divertor targets. We propose 13 new scalings based on data from the tokamaks JET, EAST, MAST, Alcator C-mod and COMPASS, and validate them against the output of the 2D turbulence code HESEL. The analysis covers 500 divertor heat flux profiles (obtained by probes or IR cameras), measured in L-mode discharges with varying 12 global plasma parameters (all well predictable). We find that two previously published scalings [Eich, J.Nucl.Mat. 438 (2013) S72; Scarabosio, J.Nucl.Mat. 438 (2013) S426] (based on outer targets of AUG and JET) describe well all the JET, C-mod and COMPASS profiles, not only at outer horizontal and vertical targets, but surprisingly also at the inner vertical targets. In contrast, EAST, HESEL and especially MAST data are poorly described by these scalings. We therefore derive 13 new scalings describing 85-92 % of the measured decay lengths variability. The reader is suggested to use as many as possible scalings from here, depending on which parameters have available. Despite the fact that the scaling candidates are based on different parameters, predictions for the highest current L-modes in ITER are all very similar. Just prior to the L-H transition, in ITER baseline scenario, all the scalings predict values 2.5-3.5 mm (mapped to outer midplane), shorter for a single scaling based on predicted stored plasma energy. 1.6-2.6 mm is predicted for the COMPASS-Upgrade tokamak. In attached L-mode plasma, our results imply (using significant assumptions) steady-state surface-perpendicular heat flux around 10 MW/m2 for ITER, and 20 MW/m2 for COMPASS-Upgrade.

1.
Ascasíbar, E., et al., M. Ezzat, et al., J. M. García-Regaña, et al., and the TJ-II team, Overview of recent TJ-II stellarator results , Nuclear Fusion, 59/11, pp. 1-13, 2019. [View Abstract]The main results obtained in the TJ-II stellarator in the last two years are reported. The most important topics investigated have been modelling and validation of impurity transport, validation of gyrokinetic simulations, turbulence characterisation, effect of magnetic configuration on transport, fuelling with pellet injection, fast particles and liquid metal plasma facing components. As regards impurity transport research, a number of working lines exploring several recently discovered effects have been developed: the effect of tangential drifts on stellarator neoclassical transport, the impurity flux driven by electric fields tangent to magnetic surfaces and attempts of experimental validation with Doppler reflectometry of the variation of the radial electric field on the flux surface. Concerning gyrokinetic simulations, two validation activities have been performed, the comparison with measurements of zonal flow relaxation in pellet-induced fast transients and the comparison with experimental poloidal variation of fluctuations amplitude. The impact of radial electric fields on turbulence spreading in the edge and scrape-off layer has been also experimentally characterized using a 2D Langmuir probe array. Another remarkable piece of work has been the investigation of the radial propagation of small temperature perturbations using transfer entropy. Research on the physics and modelling of plasma core fuelling with pellet and tracer-encapsulated solid-pellet injection has produced also relevant results. Neutral beam injection driven Alfvénic activity and its possible control by electron cyclotron current drive has been examined as well in TJ-II. Finally, recent results on alternative plasma facing components based on liquid metals are also presented.

Ezzat, M., Advanced neoclassical impurity transport modelling with its experimental comparison for TJ-II , MSc Thesis, Ghent University, 51 pp., 2018. [View Abstract]The absence of the disruptive instabilities, increasing of the confinement time with the ECRH heating and the steady-state operation make stellarator concept as a competitive candidate for future fusion reactors as the tokamak. Impurity accumulation in the core though is one of the stellarator drawbacks because it dilutes the plasma and increases the radiation losses contributing to the plasma collapse. Neoclassical theory predicts a non-ambipolar transport of different species electrons and ions in stellarators due to magnetic field ripples that are produced by the three-dimensional coils structure. Non-ambipolar transport creates, depending on the collisionality of each species, inward (outward) an ambipolar radial electric field for ion (electron) root regimes. Ion root regime has been predicted for the future stellarator reactor scenarios, which imply very likely impurity accumulation. However, outward transport has been observed during an improved confinement regime so-called \textit{HDH mode} at W7-AS (\textit{K. McCormick 2002}) and the \textit{impurity hole} at LHD (\textit{K. Ida 2009}) but without without satisfactory theoritical explanation. Historically, neoclassical treatment considers only the radial component of the electric field, which is a good approximation for the bulk species, but not for the higher charge species like impurity. Recent approaches have considered the tangential component due to the variation of the electrostatic potential within a flux surface which is more important for high charge impurity (see $\textit{J. M. {Garc\'ia-Rega\~na} 2013}$ and reference therein). Advanced modelling of this variational part has been done here for TJ-II plasma introducing a parameter which can be measured indirectly. Direct measurement of the variational part is non-trivial and had been carried only for plasma edge (\textit{M. A. Pedrosa 2015}). Here, the indirect measurements cover the whole cross section by constructing the radiation map at two toroidal planes in TJ-II that carried impurity density distribution and in sequence the variational electrostatic part. Linearized impurity-ion collision operator (in \textit{I. Calvo 2018-Arxiv}) had been employed for impurity simulation because it is higher collisional instead of the usual pitch angel scattering operator (in \textit{C. D. Beidler 2011}) for bulk species with low collisions.