Adamek, J.

Bilkova, P.

Bohm, P.

Cavalier, J.

Devitre, A.

Dimitrova, M.

Elmore, S.

Faitsch, M.

Hacek, P.

Havlicek, J.

Imrisek, M.

Komm, M.

Krbec, J.

Panek, R.

Peterka, M.

Popov, T.

Seidl, J.

Tomes, M.

Weinzettl, V.

MPG.PuRe

First detachment studies on COMPASS tokamak using nitrogen seedingM. Komm1, J. Cavalier1,J. Seidl1, J. Adamek1, P. Bilkova1, P. Bohm1, A. Devitre2,M. Dimitrova1, S. Elmore3, M. Faitsch4, P. Hacek1,5, J. Havlicek1, M. Imrisek1,5, J. Krbec1,6,M. Peterka1,5, R. Panek1, T. Popov7, M. Tomes1,5, V. Weinzettl1 and the EUROfusion MST1 team81 Institute of Plasma Physics of the CAS, Za Slovankou 3, 182 00 Prague 8, Czech Republic2 Universidad de Costa Rica, San Jose, Costa Rica3 Culham Sci Ctr, CCFE, Abingdon OX14 3DB, Oxon, England4 Max-Planck-Institut für Plasmaphysik, 85748 Garching b. München, Germany5 MFF Charles University, V Holešovičkách 2, 180 00 Prague 8, Czech Republic6 FNSPE, Czech Technical University in Prague, Břehová 7, Czech Republic7 Sofia Univ St Kliment Ohridski, Fac Phys, 5 J Bourchier Blvd, Sofia 1164, Bulgaria8 See the author list of "Meyer et al, Overview of progress in European Medium Sized Tokamakstowards an integrated plasma-edge/wall solution", accepted for publication in Nuclear FusionIntroductionPartial detachment is considered to be a mandatory regime for ITER and next-step devices,as it allows to dissipate a substantial part of the heat flux carried by the plasma particles andsignificantly reduce the heat fluxes deposited on the divertor target tiles. Despite that, moststudies of power deposition in contemporary tokamaks were performed in attached plasmas[1].Detached regimes tend to be more difficult to access and diagnose - for example the infraredthermography (the principal diagnostics for such studies) suffers from interference caused byincreased bremsstrahlung in the infrared spectrum in detached plasmas.COMPASS has a combination of a relatively short connection length to the target and anopen divertor geometry, which was considered unfavourable for detachment access. In orderto facilitate access to partially detached regime of operation, impurity seeding in the divertorregion can be employed. In this paper, we report on first results from the impurity seedingexperiments, which were performed in Ohmic plasmas.Nitrogen injectionNitrogen was introduced into the vessel through a pre-programmed piezzo valve in a sin-gle toroidal location at the inner divertor leg (R = 469 mm). The valve was connected to a gasreservoir with gas pressure of 2.1 bar. In the first test, the valve was opened on 100% (which cor-responds to flow 1.21×1021 particles per second) during 10 ms in a low-power Ohmic discharge#13700 (Ip = 200 kA, ne = 4×1019 m−3, BT = 1.38 T). The injection was monitored by fast vis-ible camera RIS1, which showed a clear formation of an emissive ring in the divertor (see Fig.44th EPS Conference on Plasma Physics P1.118b)Figure 1: Response to the 10 ms nitrogen injection as seen on divertor probes (A) with the location ofthe outer strike point indicated by yellow line and valve waveform by white dashed line. Formation ofthe radiative ring is seen on fast visible camera (B).1B). An array of Langmuir and Ball-pen divertor probes[3] (dubbed new divertor probes) wasused to evaluate the effect of nitrogen injection on the target plasma parameters. The response isclearly seen in Fig. 1A, where the radial profiles of electron temperature Te = (VBPP−VLP/1.4)are plotted. At t=1115 ms (which is approximately 15 ms after start of injection), the targettemperature sharply drops from 80 eV to less than 5 eV across the whole outer target and itgradually recovers 40 ms after the valve was closed (tstop =1110 ms).Following the initial test of injection, nitrogen was injected at varying amount in a series ofotherwise identical discharges (with main parameters same as in #13700). The waveform of thegas had two parts: (i) a short 10 ms full opening of the valve, which ensured that the valve wasnot stuck, followed by (ii) 100 ms injection at varying level of flow at 2.0 (#13729), 3.7 (#13730)and 4.5 (#13731) ×1020 molecules per second (for comparison the deuterium gas puff in thedischarge was on average 1.5 ×1019 molecules per second). Fig. 2 shows the evolution of Te,ion saturation current density jsat and the electron density ne measured by a set of new divertorprobes near the outer strike point (Rprobe = 484 mm). For comparison, also measurements at thesame radial position obtained by an array of 39 Langmuir swept probes (dubbed old divertorprobes ), which were analyzed using the first derivative probe technique[4], is over-plotted.It can be seen that introduction of nitrogen causes a significant drop of Te and jsat , where themagnitude of the drop increases with increasing amount of injected gas. For the lowest amountof injected nitrogen, the Te measured by the new probes exhibits slow quasi-regular oscillations44th EPS Conference on Plasma Physics P1.1181100 1120 1140 1160 1180 120001020304050607080Time [ms]Te [eV]1100 1150 120005101520253035404550Time [ms]j sat [kA/m2]1100 1150 120000.10.20.30.40.50.60.70.80.91Time [ms]ne [x1019 m−3]  ΓN2 = 2.0x1020 s−1ΓN2 = 3.7x1020 s−1ΓN2 = 4.5x1020 s−1Valve [a.u.]c)b)a)Dα (HFS)Figure 2: Response to N2 injection observed by probes at outer strike point - Electron temperature (A),ion saturation current density (B) and electron density (C). Measurements by new probe array plotted aslines, measurements by old probe array by circles.(also seen on Dα signal, as shown in Fig. 2A), which resemble a behavior seen in the JETdivertor [5]. The profiles of jsat measured by the two diagnostics are in good agreement, withdifferences < 50%. However, the measured electron temperatures are distinctively different.The temperature prior to nitrogen injection differs by a factor of 4x - the new probes are likelyto overestimate the Te as it is higher than the separatrix Te (Te,sep ∼ 40 eV) measured by HighResolution Thomson Scattering (HRTS) (see Figure 3A). On the other hand the old probes areunable to capture the slow temperature oscillations at low levels of nitrogen injection (whichcorrelate with HFS Dα signal).Detachment characterizationDecrease of divertor temperature below 5 eV is one of the indications of detached plasmaoperation, as well as reduced flow of charged particles to the divertor targets [2]. Low tempera-ture plasma permits formation of considerable neutral pressure in the divertor, which allows theincoming plasma particles to recombine before hitting the divertor tiles. Indeed, the jsat showsa distinct reduction (see Fig. 2B), when nitrogen is injected.However, the decrease of jsat is only by a factor of three in case of largest injection, whichsuggests that it is driven mainly by the reduction of divertor temperature, rather than by reduc-tion of the plasma density. Indeed, the density during the second stage of injection measured bythe new divertor probes near the outer strike point is practically identical to its value prior toinjection (see Fig. 2C). Closer inspection of temperature profiles obtained by HRTS system (seedetailed view in Fig. 3) confirms that the plasma is being significantly cooled already inside the44th EPS Conference on Plasma Physics P1.1180 0.2 0.4 0.6 0.8 1 1.2020040060080010001200ψTe [eV]a)  ΓN2 = 2x1020 m−3ΓN2 = 3.7x1020 m−3ΓN2 = 4.5x1020 m−30 0.2 0.4 0.6 0.8 1 1.20123456ψne [x1019 m−3]b)0.9 1 1.1020406080  Figure 3: Profiles of electron temperature (A) and electron density (B) observed by Thomson scattering.Dashed line show profiles prior to injection, solid lines during injection (t=1140 ms).LCFS (ψ < 1). In Fig. 3, the dashed lines correspond to profiles prior to injection, while solidlines are during the injection at t=1140 ms.ConclusionsNitrogen is usually used to dissipate power in the divertor region on larger tokamaks, whichis due to its favourable radiative properties. On COMPASS, however, it can propagate into theconfined plasma causing significant cooling in the edge. As a consequence, the electron tem-perature and ion saturation current density at the outer divertor target diminish, however thepresence of significant amount of recombination processes in the divertor was not confirmed.Substantial differences between different divertor probe diagnostics were observed and is cur-rently under investigation.AcknowledgementsThis work was supported by the Czech Science Foundation projects GA16-14228S and GA15-10723Sand co-funded by MEYS project number 8D15001. This work has also been carried out within the frame-work of the EUROfusion Consortium (MST1) and has received funding from the Euratom research andtraining programme 2014-2018 under grant agreement No 633053. The views and opinions expressedherein do not necessarily reflect those of the European Commission.References[1] T. Eich et al., Nucl. Fusion 53 (2013) 093031[2] P.C. Stangeby, IoP Publishing Ltd, Bristol (2000)[3] J. Adamek et al., submitted to Nucl. Fusion, (2017)[4] Tsv. K. Popov et al., Plasma Phys. Control. Fusion 51 (2009) 065014[5] A. Loarte et al., Phys. Rev. Let. 83 (1999) 3657-366044th EPS Conference on Plasma Physics P1.118

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First detachment studies on COMPASS tokamak using nitrogen seeding

First detachment studies on COMPASS tokamak using nitrogen seeding

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MPG.PuRe

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