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10-15 May 2021
Nice, France
Europe/Vienna timezone
The Conference will be held virtually from 10-15 May 2021

Helium doped plasmas on FTU

13 May 2021, 08:30
4h
Nice, France

Nice, France

Regular Poster Magnetic Fusion Experiments P5 Posters 5

Speaker

Dr Cristina Mazzotta (ENEA, Fusion and Nuclear Safety Department, C. R. Frascati, via E. Fermi 45, 00044 Frascati, Roma, Italy)

Description

The possibility to exploit the properties of medium and light impurities in contaminating plasmas is explored in many tokamaks devices [2-5]; since is crucial to fix the conditions that favor a strong increase of particle confinement while minimizing the amount of impurities needed, as well as to favor the so called "plasma detachment". The light doping action represents a good method to increase the core electron density, often without any undesirable central impurity accumulation; meanwhile the amount of impurities, and the relative edge radiation, have to be kept below the threshold where a disruptive MHD activity is generated.
In order to complete experimental observations regarding the electron density peaking in doped plasma referred in FTU [4], a series of experimental sessions have been performed in the last two campaigns by injecting Helium on the L-mode plasma scenarios. As first results reported in this work, it has been revealed that not only the total amount of inflated Helium, but also speed of injection, as well as edge conditioning, can influence the impurity effects. Examples are exposed in the figure 1.
VUV spectroscopy measurements help to evaluate the Helium presence, that triggers the particles inflow; indeed, only a small fraction of electrons could be attributed to the ones stripped by Helium ionization in the total contribution of the electron density rise. In the same phase, the attempt to interpret the radiation losses and effective charge traces, in respect to concurrent different impurities, is treated too. Further observations regard the behavior of the temperature profiles, also in comparison with previous works in which Neon impurity was used [4,6]. Generally, these plasmas, as consequence of the Helium injection, frequently excite some instabilities, such as MARFEs and MHD, that affect measurements and provoke strong perturbations, in some cases they appear simultaneously.
Considerable results have been achieved in the search of the best conditions to obtain a very high electron density peaking, by reaching the value of 5, all are detailed in this document.
A JETTO analysis adds interesting considerations about particle transport and improvement in confinement. Furthermore, a set of different scenarios has been experimentally performed. One of them, consisting in a scan of plasma current (principal parameters: $I_p$=250-500 kA, $B_T$=5.3 T, $n_{e_0}$=0.2-1 $10^{20}$ m$^{-3}$, $T_{e0}$=1-4 keV), has confirmed that the highest peaking is reached at low current. Another one explores the relationship between the increase of the electron density and the plasma position inside of the wall chamber, by varying plasma shape and magnetic configuration.

Times traces of some principal parameters for four FTU plasmas at $B_T$ = 5.3 T and $I_p$ = 250 kA. Figure at left; keeping constant the final released amount of Helium (ΔP = 40 mbar), the variation of valve voltage produces a different speed in the gas inflation. (a) The pulse #42720, (red traces) has the quicker injection than #42721 pulse (pink traces). (b) Line-averaged electron density ($10^{20}$ m$^{-3}$): the fastest and larger increase of ne occurs when the valve is at maximum voltage. (c) Opposite behaviour for the electron temperature (keV) during the injection phase: the plasma is colder when the speed of the gas puff is greater. (d) The signals of radiation losses (MW) follow the electron density. (e) The effective charge is comparable in the pulses and its measurements report that, unfortunately, all these plasmas contain various impurities also before Helium insufflation. (f) The density peaking starts to rise at least 100 msec after the Helium puff (at 0.4 s) and growths with the same trend, until reaching values over 4. Figure at right; keeping the same valve voltage = 145 V, the amount of Helium gas is reduced, in order to identify the minimum ΔP$_{H_e}$ necessary to observe relevant modifications of the plasma. The blue traces indicate the pulse with maximum impurity amount (#43334), the cyan ones refer to the less doped pulse (#43335); for this last one (ΔP =13 mbar) the effects are just observable, identifying, in this way, a lower threshold around ten mbar.

References
[2] A. Messiaen et al 1996 Phys. Rev. Lett. 77 2487
[3] G. Telesca et al 2000 Nucl. Fusion 40 1845
[4] C. Mazzotta et al 2015 Nucl. Fusion 55 073027
[5] K. A. Razumova et al 2017 Plasma Phys. Rep. 43 1043–1051
[6] N. A. Kirneva et al Effect of the impurity injection on plasma confinement in T-10 tokamak 44th European Physical Society Conference on Plasma Physics, June 2017, Belfast, United Kingdom. P4.169

Affiliation ENEA, Fusion and Nuclear Safety Department, C. R. Frascati, via E. Fermi 45, 00044 Frascati, Roma, Italy
Country or International Organization Italy

Primary author

Dr Cristina Mazzotta (ENEA, Fusion and Nuclear Safety Department, C. R. Frascati, via E. Fermi 45, 00044 Frascati, Roma, Italy)

Co-authors

Dr Gianluca Pucella (ENEA, Fusion and Nuclear Safety Department) Dr Gerarda Apruzzese (ENEA, Fusion and Nuclear Safety Department) Luca Boncagni (ENEA, Fusion and Nuclear Safety Department) Dr Lorella Carraro (Consorzio RFX) Carmine Castaldo (ENEA, Fusion and Nuclear Safety Department) Dr Silvio Ceccuzzi (ENEA, Fusion and Nuclear Safety Department) Dr Silvia Cesaroni (Departement of Industrial Engineering, Università di Roma "Tor Vergata", ) Dr Cesidio Cianfarani (ENEA, Fusion and Nuclear Safety Department) Dr Gerardo Claps (ENEA, Fusion and Nuclear Safety Department) Dr Bruno Coppi (Massachusetts Institute of Technology) Dr Ocleto D'Arcangelo (ENEA, Fusion and Nuclear Safety Department) Dr Claudio Di Troia (ENEA, Fusion and Nuclear Safety Department) Dr Basilio Esposito (ENEA, Fusion and Nuclear Safety Department) Dr Lori Gabellieri (ENEA, Fusion and Nuclear Safety Department) Dr Edmondo Giovannozzi (ENEA, Fusion and Nuclear Safety Department) Dr Matteo Iafrati (ENEA, Fusion and Nuclear Safety Department) Dr Giorgio Maddaluno (ENEA, Fusion and Nuclear Safety Department) Dr Massimo Marinucci (ENEA, Fusion and Nuclear Safety Department) Dr Silvia Palomba (Departement of Industrial Engineering, Università di Roma "Tor Vergata") Dr Maria Ester Puiatti (Consorzio RFX) Dr Afra Romano (ENEA, Fusion and Nuclear Safety Department) Dr Luca Senni (ENEA, Fusion and Nuclear Safety Departmentv) Dr Onofrio Tudisco (ENEA, Fusion and Nuclear Safety Department) Dr Vito Konrad Zotta (Department of Astronautical, Electrical, and Energy Engineering, “Sapienza" Università di Roma)

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