TJ-II:Observation of suprathermal ions with Neutral Particle Analyzers during electron cyclotron heating in the TJ-II stellarator: Difference between revisions

TJ-II:Observation of suprathermal ions with Neutral Particle Analyzers during electron cyclotron heating in the TJ-II stellarator (view source)

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→‎Description of the activity, including motivation/objectives and experience of the proponent (typically one-two pages)

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 == Description of the activity, including motivation/objectives and experience of the proponent (typically one-two pages)==
-The plasmas in the TJ-II stellarator are created and maintained using two gyrotrons tuned to the second harmonic of the electron gyrofrequency. Additional heating can be applied using neutral beam injection or the plasma can be maintained with microwave power to produce a pure ECRH discharge. In the case of pure ECRH plasmas the majority ions are not directly heated by external sources rather by collisions with the hot electrons, hence their population distribution function is considered Maxwellian. The bulk ion temperature is clearly detached from the electron temperature, the usual value for the ion temperature is around 80 eV whereas the electron temperature is about 1 keV. Under such conditions the count rates in the high energy channels (> 1 keV) of the TJ-II neutral particle analyzers (NPA) are at the background level, indicating the abscence of ions in the high energy tail. However, suprathermal ions have been found in TJ-II ECRH plasmas using spectroscopic methods. Thus it is necessary to confirm their presence with the NPA diagnostics.
+Suprathermal ions have been detected using optical spectroscopy techniques in TJ-II [1] but without conclusive results when using NPA diagnostics [2]. One experiment of the 2016 campaign, designed to investigate this population,  consisted in operating one gyrotron along all the discharge while modulating the second gyrotron at 100% of its power [3] and changing the power deposition position in a shot-to-shot basis. No neutral beam power was injected. When both gyrotrons were on an indication of suprathermal ions was observed by the NPA diagnostic, see figure 1.
+In the shot plotted in figure 1 the NPA was tuned to 200 eV in the lowest energy channel. As can be seen, the count rate in the lower energy channels (NPA 01 – NPA 06, 200 eV to 446 eV) dropped, as did the Halpha signal. In contrast, in the higher energy channels (NPA 07 – NPA 09, 540 eV to 826 eV) the count rate fell when the gyrotron was turned off. This can be a indication of the existence of suprathermal ions. The amount of suprathermal ions depends on the position of the plasma heating, so not all discharges presented the same behaviour.
-In a recent experiment, we modulated the full power of one of the two gyrotrons to produce two clearly separate phases of the ECRH plasmas, one with full power, the other with half power. Also the radial position, where the microwaves heat the plasma, was varied during the experiment. During the experiments the NPA was tuned to scan high energy ions. As a result, in some configurations, signal levels above the normal backgroud levels were detected in the high energy channels when full power was applied. This can be an indication of the presence of suprathermal ions in the plasma.
+[[File:NPA42626.png]]
+A possible explanation for the suprathermal ions is a parametric decay of the injected waves [4].  In order for this to occur, a hollow profile of the density is necessary, as in TJ-II ECR heated plasmas.
-One possible explanation for the generation of suprathermal ions is a parametric decay insta- bility of the heating wave in a local maximum of the density <ref>A.E.Z. Gusakov and A.Yu.Popov, Plasma Physics and Controlled Fusion 60, 025001 (2018)</ref>. We have designed an experi- ment to inject pellets in the ECRH plasma to modify the density profile while scanning the high energy ion tail with the NPA diagnostics in different positions to investigate the influence of the density profile on the suprathermal ion population.
+In this proposal we want to reproduce the results of the 2016 campaign and in addition, change the position of the NPA in order to check the radial extension of the suprathermal ion population. Also it is intended to inject hydrogen pellets at suitable moments along the discharge in order to change the density profile, i.e. to test the validity of the assumption in [4] of the necessity of a local maximum of a non-monotonous density profile to produce the parametric decay.
+The discharges would be similar to those of the 17/11/2016 shots between 42617 and 42647 with one gyrotron heating along all the discharge and the other modulated 100 % with 20 ms period. In one of the phases with both gyrotrons on the pellet will fire so the density profile will change. The difference on the NPA signal will show the presence or not of suprathermal ion population in each phase of the discharge.
+It will be necessary a full day to achieve the objectives which implies changing the heating position and the NPA position between discharges and similar plasma parameters in all discharges.
 == If applicable, International or National funding project or entity ==