1 OV/4-3

OVERVIEW OF TJ-II STELLARATOR RESULTS

E. Ascasıbar1, D. Alba1, D. Alegre1, A. Alonso1, J. Alonso1, F. de Aragon1, A. Baciero1, J.M.Barcala2, E. Blanco1, J. Botija1, L. Bueno1, S. Cabrera1, E. de la Cal1, I. Calvo1, A. Cappa1,

D. Carralero1, R. Carrasco1, B. Carreras3, F. Castejon1, R. Castro1, A. de Castro1, G.Catalan1, A.A. Chmyga4, M. Chamorro1, A. Cooper5, A. Dinklage6, L. Eliseev7, T. Estrada1,

M. Ezzat1, F. Fernandez-Marina1, J.M. Fontdecaba1, L. Garcıa3, I. Garcıa-Cortes1, R.Garcıa-Gomez1, J. M. Garcıa-Regana1, A. Gonzalez-Jerez1, G. Grenfell8, J. Guasp1, J.Hernandez-Sanchez1, J. Hernanz1, C. Hidalgo1, E. Hollmann9, A. Jimenez-Denche1, P.

Khabanov7, N. Kharchev10, I. Kirpitchev1, R. Kleiber6, A.S. Kozachek4, L. Krupnik4, F.Lapayese1, M. Liniers1, B. Liu11, D. Lopez-Bruna1, A. Lopez-Fraguas1, B. Lopez-Miranda1, J.

Lopez-Razola1, U. Losada1, E. de la Luna1, A. Martın de Aguilera1, F. Martın-Dıaz1, M.Martınez-Fuentes1, G. Martın-Gomez1, A.B. Martın-Rojo1, J. Martınez-Fernandez1, K.J.

McCarthy1, F. Medina1, M. Medrano1, L. Melon1, A.V. Melnikov7, P. Mendez1, R. Merino1,F. J. Miguel1, B. van Milligen1, A. Molinero2, B. Momo8, P. Monreal1, S. Mulas1, Y.

Narushima12, M. Navarro1, M. Ochando1, S. Ohshima13, J. Olivares1, E. Oyarzabal1, J.L. dePablos1, L. Pacios1, N. Panadero1, F. Parra14, I. Pastor1, A. de la Pena1, A. Pereira1, A.B.

Portas1, E. Poveda1, J. A. Quintana1, F. J. Ramos1, G.A. Ratta1, M. Redondo1, E. Rincon1,L. Rıos1, C. Rodrıguez-Fernandez1, L. Rodrıguez-Rodrigo1, B. Rojo1, A. Ros1, E. Rosa1, E.Sanchez1, J. Sanchez1, M. Sanchez1, E. Sanchez-Sarabia1, S. Satake12, J.A. Sebastian1, R.Sharma15, C. Silva15, E.R. Solano1, A. Soleto1, B.J. Sun1, F.L. Tabares1, D. Tafalla1, H.

Takahashi12, N. Tamura12, A. Tolkachev1, J. Vega1, G. Velasco1, J.L. Velasco1, S. Yamamoto13

and B. Zurro and the TJ-Team1

1National Fusion Laboratory, CIEMAT, Madrid, Spain2Department of Technology, CIEMAT, Madrid, Spain

3Universidad Carlos III, Madrid, Spain4Institute of Plasma Physics, NSC KIPT Kharkov, Ukraine

5Swiss Alps Fusion Energy (SAFE), Vers l’Eglise, Switzerland6Max-Planck-Institut fur Plasmaphysik, Greifswald, Germany

7National Research Centre ‘Kurchatov Institute’, Moscow, Russian Federation8Consorzio RFX (CNR, ENEA, INFN, Universita di Padova, Acciaierie Venete SpA), Padova, Italyy

9University of California-San Diego, San Diego, CA, United States10General Physics Institute, Russian Academy of Sciences, Moscow, Russian Federation

11ENN Energy Research Institute, Langfang, Hebei, China12National Institute for Fusion Science, Toki, Japan

13Institute of Advanced Energy, Kyoto University, Uji, Japan14Rudolf Peierls Centre for Theoretical Physics, University of Oxford, United Kingdom

15IPFN, Instituto Superior Tecnico, Universidade de Lisboa, Lisboa, Portugal

Email contact main author: enrique.ascasibar@ciemat.es

Abstract: The main results obtained in the TJ-II stellarator in the last two years are reported. Themost important topics investigated have been: modelling and validation of impurity transport, validationof gyrokinetic simulations, turbulence characterisation, effect of magnetic configuration on transport,fuelling with pellet injection, fast particles and liquid metal PFCs. It must be noted that work done onTJ-II is relevant for W7-X.

1. INTRODUCTION

The TJ-II stellarator (heliac type, major radius 1.5 m, minor radius ≤ 0.22 m, fourperiods, average magnetic field on axis 0.95 T, plasma volume ≤ 1 m3), is entering itsthird decade of operation. Both ECRH (two gyrotrons, 53.2 GHz, P ≤ 300 kW each,suitable for X2 heating) and NBI (two H0 injectors, E ≤ 30 kV, P ≤ 600 kW each)

2 OV/4-3

heating methods can be used to produce and sustain the discharge. Plasma physicsresearch in TJ-II is focused on some selected areas posing fundamental problems thatmust be solved in the development of reactor-grade magnetic confinement devices, namely:impurity accumulation in stellarators, turbulence, effects of the magnetic configurationon transport in stellarators, plasma core fuelling with pellet injection, fast particle drivenmodes, power exhaust physics and technology, among others. The very wide range ofconfiguration flexibility of the device (edge rotational can be varied from 1.0 to 2.2) andits rather complete set of advanced diagnostics have been instrumental for the plasmaresearch done so far by the TJ-II team. In recent years, a substantial effort has been doneby the team in strengthening its modelling capability in order to tackle the validationof theory in the areas of transport, turbulence, magnetic topology, electric fields, fastparticles and the relations among them. Whenever feasible the modelling results havebeen extended to the largest world class stellarator/heliotrons in operation, namely LHDand W7-X. It must be noted that work done on TJ-II is relevant for W7-X. This paperreports the main results obtained in the last two years and is organised along the list oftopics mentioned in the abstract, each section covering one of them.

2. NEOCLASSICAL IMPURITY TRANSPORT

Impurity accumulation is one of the main problems that limit the plasma performance viaincreased radiative losses and plasma dilution. For stellarator reactor-relevant conditions,accumulation is due to the negative (inwards pointing) radial electric field required tosatisfy the ambipolarity constraint on the neoclassical particle fluxes. But there are someexperimental observations of outward impurity flux not well understood in stellarators[1, 2] that call for improvements of the neoclassical theory. In this context a number ofworking lines exploring several recently discovered effects are being developed.

• Effect of tangential drifts and the non-constant component of the electrostatic po-tential on stellarator neoclassical transport.

Two important ingredients have often been neglected in standard neoclassical theoryand simulations of stellarator plasmas. First, the component of the electrostatic potentialthat is non-constant along the flux surface, that we denote by ϕ1. Only recently hasit been acknowledged that ϕ1 can affect radial impurity transport via the radial E × Badvection of the variations of impurity density along the flux surfaces, which has triggeredthe interest in its calculation. The computation of ϕ1 has been included in some modernneoclassical codes, but the number of simulations in which ϕ1 has been calculated is stillscarce [3]. The systematic analytical description of ϕ1 has received little considerationas well. The second ingredient that has often been missing in stellarator neoclassicalcalculations is the component of the magnetic drift that is tangent to the flux surface,whose effect is essential to correctly determine the radial neoclassical fluxes of the bulkions at low collisionality and small radial electric field values.

In reference [4], low collisionality neoclassical equations that include ϕ1 and the tan-gential magnetic drift have been rigorously derived for sufficiently optimized stellarators,and the radial neoclassical fluxes for the main ions have been computed in the 1/ν,

√ν and

superbanana-plateau regimes. Besides, it has been proven that ϕ1 can become especiallylarge in plasma regimes in which the tangential magnetic drift counts. Therefore, the twoingredients mentioned above must be simultaneously and consistently treated, in general.The orbit averaged code KNOSOS (KiNetic Orbit-averaging-SOlver for Stellarators) has

3 OV/4-3

been developed, that solves the equations derived in [4]. In reference [5], a thoroughanalysis of the solutions for ϕ1 in the 1/ν,

√ν and superbanana-plateau regimes has been

presented. The different asymptotic regimes for ϕ1 have been identified analytically andverified with KNOSOS. Reference [5] used academic configurations and plasma profilesin order to illustrate the different neoclassical regimes. But it has been proven, usingLHD data, that the tangential magnetic drift is also relevant for the calculation of ϕ1 inrealistic stellarator plasmas [6].

• Impurity flux driven by electric fields tangent to magnetic surfaces in stellarators.

The standard argument to explain the accumulation of impurities with Z 1 relieson the, in principle, large inward pinch in the neoclassical radial impurity flux, Γz, causedby the typically negative stellarator radial electric field Er. This argument was proven tobe flawed, at least in some important cases, in [7], where it was shown that if the mainions have low collisionality and the impurities are collisional, then Γz does not depend onEr. This result was interpreted in a positive way because it diminished the prevalence ofEr in Γz and made ”temperature screening” (that is, the reduction or absence of impurityaccumulation thanks to the contribution of the temperature gradient to Γz) more likelythan previously thought. In [7], the effect of ϕ1 on Γz was not included. In [8], we givean analytical calculation of Γz for low collisionality main ions and collisional impurities,incorporating the effect of ϕ1. We show that this effect can be very strong for Z 1 andthat, once it is taken into account, the dependence of Γz on Er reappears. In addition,we prove that such an effect is stellarator specific (i.e., Γz in a tokamak does not dependon Er, whatever ϕ1). A careful analysis of the final expression for Γz in stellarators (thatreduces to the expression derived in [7] when ϕ1 vanishes) reveals that the impact of ϕ1

starts to be important for ϕ1 surprisingly small, e|ϕ1|)/T=O(ε2), where ε is the inverseaspect ratio. Our analytical results have been applied to the calculation of the neoclassicalparticle flux Γz versus Er, for three charge states of tungsten in an LHD plasma (see [8]).The computation of ϕ1 has been carried out with the code EUTERPE. The results showa large difference in the neoclassical impurity flux obtained with and without ϕ1, and astrong dependence of the flux on Er when ϕ1 is included. Although the effect of ϕ1 istypically to increase impurity accumulation, there is a small region in parameter spacewhere its inclusion leads to impurity expulsion.

• Experimental validation with Doppler reflectometry of the variation of the radialelectric field on the flux surface in electron-root stellarator plasmas.

The interest in the role of the electric field tangent to the surface on the transportof impurities, commented in the first bullet above, has dealt in most cases with ion-rootplasmas, whereas electron root conditions have been barely investigated. On the otherhand, since ϕ1, the part of the electrostatic potential that leads to the electric field tangentto the surface, depends parametrically on the flux surface label as well, it can make thetotal radial electric field to vary over the flux surface. These two elements of the problemhave been recently addressed [9] motivated by the observation of strong differences in thetotal radial electric field measured at different points over the same flux surface with DRin TJ-II electron-root plasmas, see Fig. 1, left. In Fig. 1, right, we show the calculatedcontribution to the radial electric field component that arises from the radial dependencyof ϕ1 for a TJ-II electron-root plasma (shots 43391 and 43392). Although the agreementat the specific DR measurements positions has not been quantitatively captured by the

4 OV/4-3

numerical simulations performed with the code EUTERPE (see ref. [9] for details), thefact that those differences occur in electron-root conditions and are practically absentunder ion-root has been well reproduced by the simulations.

-2

0

2

4

0.3 0.4 0.5 0.6 0.7 0.8 0.9

#43391: elestron root, region_1#43392: electron root, region_2#43387: ion root, region_1#43388: ion root, region_2

E r (kV/

m)

ρ

FIG. 1: Left, Er profiles measured by the DR at turbulence scales k⊥ ∼ 6− 8 cm−1, in the twoplasma regions (1 in red and 2 in blue) in both electron and ion root regimes. Right, calculatedcontribution to the radial electric field component that arises from the radial dependency of ϕ1.Plasma regions 1 and 2 are indicated by consistently coloured thick lines in the upper (external)part of the plasma cross section.

3. EXPERIMENTAL VALIDATION OF GLOBAL GYROKINETIC SIMULA-

TIONS

ZFs are recognized as an important mechanism for the self regulation of plasma turbulence.

FIG. 2: Damping rate (γ) vs oscillation fre-quency (Ω) for the experimental oscillations de-tected in [17]. Data are shown for the specifiedTJ-II discharges and for numerical simulations,both with a single (gray squares) and with mul-tiple kinetic species (blue circles), namely, ions+ electrons + C4+ impurity ions [18].

Besides, the appropriate framework tostudy plasma turbulence is the gyrokineticformalism, in which kinetic simulationsare affordable using appropriate codes andsupercomputers. However, these codesshould be carefully validated against exper-iment in order to be reliable. Scale resolvedturbulence measurements are important tovalidate the simulations and to identify thetype of turbulence at work. Some topics inboth areas have been simulated and exper-imentally validated in TJ-II.

• Comparison between measurementsof zonal flow relaxation in pellet-induced fast transients and gyroki-netic simulations.

Linear ZF relaxation in stellarator geome-tries has been studied from different per-spectives. From the analytical point of

5 OV/4-3

view, the long term relaxation of zonal potential perturbations has been studied as aninitial value problem in [11] and expressions for the frequency of the Low Frequency Oscil-lation (LFO), characteristic of stellarators [12], were derived. Semianalytical predictionswere compared to results of numerical simulations carried out with the gyrokinetic codesGENE [13] and EUTERPE [14] in different stellarators (TJ-II, W7-X and LHD). Goodagreement has been found between analytical estimations of the ZF oscillation frequencyand the results from numerical simulations. This cross-benchmark has allowed not onlytesting the analytical calculations but also verifying the codes in this setting.

During pellet injection experiments in TJ-II [15, 16], a sudden global perturbation tothe plasma potential was detected at the radial location of pellet ablation which undergoesa fast oscillatory relaxation. This phenomenon was studied numerically in global lineargyrokinetic simulations with the code EUTERPE, using the experimental plasma profilesas input for the simulations [17]. This constituted the first experimental observation ofthe LFO predicted analytically in [12]. The numerical simulations of oscillation frequencyand damping rate of the potential oscillations reproduced qualitatively the experimentaloscillations, although a quantitative agreement was not found in the first single-speciessimulations. In a second step, the ZF relaxation has been studied numerically in a multi-species plasma with TJ-II configurations [18]. Multi-species collisional simulations carriedout with the experimental plasma profiles from the pellet injection experiments previouslyanalyzed in [17] showed an improved quantitative agreement in frequency and dampingrate between experiment and simulations [18], as shown in Fig. 2.

• Validation of gyrokinetic simulations with density fluctuations spectra measured byDoppler reflectometry.

Global gyrokinetic simulations of electrostatic microinstabilities have been carried outwith the EUTERPE code [19] and validated with dedicated experiments carried outduring the 2017 experimental campaign in TJ-II [20]. The Doppler reflectometry (DR)system provides information on the density fluctuation wave-number spectra as well as theradial electric field and is used as the basic diagnostic for the characterisation of the plasmaedge turbulence. Several ECRH scenarios have been studied in which plasma profiles aremodified by changing the heating power and deposition location. Measurements havebeen compared to numerical simulations in which the experimental plasma profiles areused as input for the simulations.

Electron-driven modes, which are compatible with Trapped Electron Modes (TEM),were found unstable in the plasma edge region in linear simulations. The range of unstablewave-numbers and the radial location of maximum instability found in simulations areconsistent with the relevant wave-numbers and the radial variation of experimental densityfluctuations spectra measured by the DR system. Experiments were carried out bothwith hydrogen and deuterium as base gas, finding no dependency of the power spectrawith the bulk ion mass. In agreement with the experiment, simulations with hydrogenand deuterium show very similar results in respect to the radial location of instabilitiesand unstable wave-numbers, which is consistent with the electron-driven character ofthe unstable modes found in the simulations. The instabilities are localised toroidaland poloidally in simulations, the location of maximum instability being affected by therotational transform. A systematic difference is found between the density fluctuationspectra measured by the DR system at poloidally separated positions on the same flux-surface. As in the case of simulations, the poloidal asymmetry of the fluctuation spectra

6 OV/4-3

-35

-30

-25

-20

-15

-10

-5

0

2 3 4 5 6 7 8 910

region_1region_2

S (d

B)

kperp (cm-1)

a) Standard mag. configuration

ECH 500 kW on-axisne = 0.5 1019 m-3

ρ = 0.74-0.82-35

-30

-25

-20

-15

-10

-5

0

2 3 4 5 6 7 8 910

ECH 500 kW on-axis ne = 0.5 1019 m-3

ρ = 0.76-0.86

region_1region_2

S (d

B)

kperp (cm-1)

b) High iota

FIG. 3: k⊥ spectra measured in the two plasma regions poloidally separated (see FIG. 1 captionfor the explanation of regions) in the standard (left) and high iota (right) magnetic configurations.

measured by the DR system is also largely affected by the rotational transform. Asan example, Fig. 3 shows the perpendicular wavenumber spectra measured at the twopoloidally separated plasma regions in the standard and high iota magnetic configurations.An asymmetry is clearly observed in the intensity of the density fluctuations in the wholek⊥ range. In the standard magnetic configuration, higher turbulence level is measured inregion 1 as compared with that measured in region 2, see Fig. 3, left. This asymmetryreverses in the high iota configuration, see Fig. 3, right. The different intensity in thedensity fluctuation spectra can be related to the poloidal localization of instabilities foundin the gyrokinetic simulations that depend on the rotational transform.

• First measurements of density fluctuations in both positive and negative densitygradient regions of the plasma.

A crucial open issue for next step devices and the fusion reactor is how to achieveparticle fuelling in the core plasma in order to replenish the fused nuclei. Pellet fuellingsystems designed to inject particles at high speed deep into the plasma are one of the maintools to achieve that goal. However, at reactor relevant plasma densities and tempera-tures, pellets will hardly be able to reach the core plasma region because pellet ablationwill take place in the plasma edge causing plasma bumps with positive and negative den-sity gradient regions [21]. Eventually particles could be transported radially inwards byturbulence. Fluid and gyrokinetic simulations have been used to investigate the levelof inward turbulent particle transport in the inverted density gradient region [22] butcomparison of simulations with experimental fluctuation levels and fluxes is still missing.First 2-D poloidal contour plots of plasma potential and density fluctuations have beenmeasured with the TJ-II HIBP diagnostic [23], which have allowed to characterise turbu-lence in positive and negative gradient regions: density fluctuations appear both at thepositive and negative gradient regions, their amplitude being minimum in the zero densitygradient zone while is stronger in the negative than in the positive gradient region [24].Gyrokinetic simulations (EUTERPE) of ion and electron-driven modes in ECRH plasmasyield results consistent with the experiment. In the electron root scenario (positive radialelectric field) Ion Temperature Gradient (ITG) modes are found to be stable due to thenearly flat ion temperature profiles. Linear and collisionless Trapped Electron Modes

7 OV/4-3

(TEM) simulations (kinetic ions and electrons) show that the most unstable modes arelocated in the negative density gradient region [25].

4. INTERPLAY BETWEEN RADIAL ELECTRIC FIELDS, TURBULENCE AND

ATOMIC PHYSICS

FIG. 4: a) Biasing voltage waveform; b),c) and d) Radial profiles of floating poten-tial, ion saturation current and turbulentflux, respectively, during dynamical bias-ing in the plasma boundary region. r0 de-notes the location of the LCFS.

A number of research areas related to plasma tur-bulence in TJ-II, mainly of experimental nature sofar, are providing additional valuable results.

• Impact of the edge neutral density fluctua-tions on the observed turbulent structures.

An almost unexplored area of research, the im-pact of neutral fluctuations on the observed turbu-lent structures has been investigated in TJ-II, show-ing that thermal neutrals react to low frequencyplasma fluctuations [26], becoming also turbulent.Hence, neutral-induced non-linearities would be ex-pected in plasma turbulence. The non-linear be-haviour of plasma turbulence in the temperaturerange where ionization strongly depends on electrontemperature is under investigation [27].

• L-H transition triggered by turbulence stabi-lization due to both zero and low-frequencyvarying large scale flows.

Mean radial electric fields as well as low fre-quency ZF-like global oscillations have been iden-tified during the Low to High (L-H) transition inhydrogen and deuterium plasmas. No evidence ofisotope effect has been observed in the L-H transi-tion dynamics [28]. These observations emphasisethe critical role of both zero and low frequency vary-ing large-scale flows for stabilising turbulence duringthe triggering of the L-H transition in magneticallyconfined toroidal plasmas.

• Impact of the radial electric fields on turbulence spreading in the edge and Scrape-Off Layer.

Understanding filamentary and blob transport across the Scrape-Off-Layer (SOL) is akey issue for the design and operation of future devices as it is crucial in determining thepower load to the divertor and first wall of the machine. Consequently, clarifying whetherthe SOL width is set by local effects inside the SOL region or/and by transport arrivingfrom the plasma edge, and the role played by Er ×B sheared flows in edge-SOL couplingare important issues. The radial transport (spreading) of turbulent energy is a conse-quence of the redistribution of fluctuation energy from unstable regions with dominant

8 OV/4-3

free energy sources to adjacent regions without or with weaker free energy sources. Theinfluence of radial electric fields on turbulence spreading has been studied experimentallyin TJ-II co-NBI heated plasmas (PNBI ≈ 500kW ) [29]. A 2-D Langmuir probe array[30] was used to measure the floating potential, ion saturation current and turbulentdriven particle transport, neglecting the influence of electron temperature fluctuations.The radial electric field has been modified with the aid of a biasing electrode insertedinto the plasma edge. The influence of edge biasing on plasma profiles is shown in Fig. 4.During the negative biasing phase [-300 V], the floating potential becomes more negative(Fig. 4b) and the perpendicular phase velocity increases up to 9 km/s. The latter directlyreflects changes in radial electric fields induced by edge biasing. Also in this phase, theedge floating potential (Fig. 4b) and ion saturation current (Fig. 4c) profiles get steeper,while the turbulent particle flux (Fig. 4d) is reduced in the region between the las closedflux surface (LCFS) and the far SOL, concomitant with a reduction in the magnitude ofthe shoulder in the ion saturation current. Hence, the shearing rate of edge radial elec-tric fields can be an important tool to suppress turbulence and decouple edge and SOLregions.

5. EFFECT OF MAGNETIC CONFIGURATION ON TRANSPORT

Transport studies in TJ-II , as in many other devices, require the consideration of mag-netic configurations where magnetic islands can be present. These are often detected ascoherent modes in magnetic signals where, owing to the low poloidal mode numbers ofthe main low-order rationals of the rotational transform, the helicity can be resolved withthe help of a poloidal array of magnetic coils or other diagnostics with some degree ofpoloidal/toroidal resolution. Since 1D transport codes are well-developed tools for trans-port analyses, a practical transformation of coordinates has been developed [31] to adaptthe metric coefficients of the unperturbed configurations to a new one where the islands(or stochastic) region is excluded. Such regions can always be calculated separately iftransport is non-trivial in them.

In addition, it is often the case that the MHD modes associated to low order rationalsof the rotational transform do not respect the four-fold toroidal periodicity of TJ-II .Theoretical studies on the MHD equilibrium of a four-fold advanced helical device [32]have demonstrated that the energy of periodicity-breaking perturbed configurations canbe found in local minima with respect to the perturbation parameter. These minima liebelow the energy of the unperturbed configurations and provide a natural explanation forthe ease with which periodicity-breaking MHD modes can be found in real experiments.

• Investigation of the radial propagation of small, spontaneously generated, tempera-ture perturbations using Transfer Entropy.

Electron heat transport in fusion plasmas presents significant challenges, among whichone may consider extraordinary phenomena such as power degradation, rapid (non-local)transport, and profile stiffness. To study heat transport, we make use of a novel analysistechnique, Transfer Entropy (TE), which measures the causal relation or information flowbetween two time series. This technique has some remarkable properties that convertsit into a very sensitive tool to study the propagation of small perturbations in highlynon-linear systems, such as fusion plasmas. TE has been firstly validated by analysingthe propagation of heat waves in modulated ECRH TJ-II discharges and comparing withtraditional Fourier techniques [33].

9 OV/4-3

FIG. 5: Examples of Transfer Entropy, calcu-lated from ECE data, between a reference chan-nel and the others whose location is indicated bywhite circles at the left vertical axis. The ref-erence ECE channel is located at ρref ≈ −0.07.Results for two heating powers are shown: a)PECRH = 205kW and b) PECRH = 603kW .The color scale indicates the value of T. The ap-proximate location of some major rational sur-faces is indicated by horizontal dashed lines; theline labels specify the corresponding rotationaltransform, n/m.

Then, we have applied it to analysethe propagation of small, spontaneouslyarising temperature perturbations in non-modulated discharges [34]. We show thatheat transport in TJ-II is not a smoothand continuous (diffusive) process, but in-volves mini-transport barriers associatedwith low-order rational surfaces and rapidnon-local radial ‘jumps’. In addition,we find that the non-local contributionto transport becomes more prominent athigher input power, see Fig. 5. This re-markable finding might shed new light onthe nature of anomalous transport.

6. PLASMA CORE FUELLING. PEL-

LET PHYSICS AND MODELLING

Core fuelling is a critical issue on the path-way to the development of steady-state op-erational scenarios in magnetic confinedplasmas. Cryogenic pellet injection (PI)has become a prime candidate for this pur-pose. A compact pipe-gun type PI hasbeen operated on TJ-II since 2014. Fourdifferent pellets can be injected at veloc-ities between 800 and 1200 m/s into theplasma from its outer edge [35]. In dedi-cated experiments it was determined thatpellet fuelling efficiency (pellet particles deposited in the plasma/number of pellet parti-cles) in TJ-II can range from about ∼ 20% to ∼ 80%, this value depending strongly onthe pellet penetration depth [16]. Efficiencies tend to be < 40% for ECRH plasmas inwhich pellets do not penetrate beyond the magnetic axis. In contrast, for NBI heatedplasmas, in which the core electron temperature is lower (300-400 eV), the fuelling ef-ficiency is significantly higher. This tendency for increasing efficiency with penetrationdepth beyond the magnetic axis is considered to be due to inwards effective drifting ofthe partially ionized clouds that surround a pellet (plasmoids) created beyond that point.

This has been understood from pellet ablation and deposition simulations made usinga TJ-II adapted version of the Hydrogen Pellet Injection 2 code (HPI2) [36] (see figures19 and 20 therein). Moreover, another study carried on TJ-II has shown that even whenpellet particle density peaking is initially observed outside the core, this can be followedby an inward shift of the same [15].

With the expansion of the TJ-II pellet database it has been observed that pelletfuelling efficiency can benefit from the presence of a population of fast electrons in itsplasma core [37].

As discussed in section 2, the need to explore favourable operation scenarios capableof preventing, or avoiding, impurity accumulation is critical. One method for performingsuch studies is the Tracer-Encapsulated Solid-Pellet (TESPEL) technique in which a hol-

10 OV/4-3

low polystyrene shell, (−CH(C6H5)CH2−)n, filled with a known quantity of a suitabletracer element, is injected into a plasma [38]. In consequence, the impurity element is de-posited directly in the plasma core, thereby eliminating inward diffusion from the physics.

0

20

40

60

80

100

Effi

cien

cy (%

)

Pellet Penetration Depth ( ρ)1 0.5 0 0.5 1

Pellet DirectionOuter Inner

FIG. 6: Fuelling/deposition efficiency as afunction of pellet penetration depth. Cryo-genic hydrogen (filled blue diamonds) andpolystyrene (green triangles) pellets injectedinto ECRH plasmas and cryogenic hydro-gen pellets (red dots) injected into NBIplasmas. Cases for which a population offast electrons is present in the core dur-ing pellet injection are contained within theoval. Examples of pellets that are not fullyablated within the plasma are plotted on theright axis.

Recently, a TESPEL injector was temporar-ily piggy-backed onto the up-stream end of PI[39, 40], thereby making it a unique system asboth pellet types can be injected along adjoin-ing guide tubes into the same toroidal sectorof TJ-II, albeit not simultaneously. For bothpellet types, the ablated particles in the plas-moid should experience the same physical ef-fect(s) thus they should undergo similar pro-cesses, e.g., drift and diffusion. However, fewsystematic comparisons of these pellet typesexist due to the limited number of devicesequipped with both cryogenic and impurity pel-let injection systems.

In a comparative study, it was shown thatthe post-injection electron density increase, af-ter an injection in ECRH plasma, is signifi-cantly higher for carbon/hydrogen pellets thatfor solid hydrogen pellets (having similar elec-tron content), as shown in Fig. 6. This is due toa reduction in plasmoid outwards drift for theformer that is inversely proportional to particleatomic mass [41]. Although it was reportedin [42] that excess ablation was observed afterpenetration inside ρ = 0.4 for TESPEL, the en-hancement of deposition efficiency due to fastelectrons was not appreciated. However, whenadditional TESPEL data were considered, such an enhancement also occurs. Simulationof the TESPEL pellet injections using the TJ-II version of the HPI2 code will be com-plicated as the processes of polystyrene sublimation, carbon ionization, etc. need to beconsidered.

7. CONTROL OF FAST PARTICLE DRIVEN MHD MODES

Experiments have been carried out in TJ–II NBI plasmas using ECRH on-axis with andwithout ECCD, in order to study the influence of Electron Cyclotron Current Drive(ECCD) on the observed Alfven Eigenmodes (AEs) activity [43]. The influence of thedifferent contributions to the plasma current (NBCD, ECCD, Bootstrap) on the rota-tional transform profile has been calculated on the basis of the measured plasma profiles.Then, the resulting rotational transform profile is computed and used to obtain the ShearAlfven Spectrum (SAS) by means of the STELLGAP [44]. The result indicates that theexperimentally observed steady frequency mode could correspond to modes appearing inthe HAE2,1 gap. This gap becomes wider as the iota profile evolves from the pure NBIto the NBI+ECCD case, thus favouring the presence of the mode in the latter case.

11 OV/4-3

8. ALTERNATIVE PLASMA FACING COMPONENTS BASED ON LIQUID

METALS

Two liquid metals (LM), Li and LiSn (20:80 at), presently considered as alternative ma-terials for the divertor target of a fusion reactor, have been exposed to the plasma in acapillary porous system (CPS) arrangement in TJ-II [45] . A negligible perturbation ofthe plasma has been recorded in both cases. The surface temperature of the LM CPSsamples (made of a tungsten mesh impregnated in SnLi or Li) has been measured dur-ing the plasma pulse with ms resolution by pyrometry and the thermal balance duringheating and cooling has been used to obtain the thermal parameters of the SnLi andLi CPS arrangements. Temperatures as high as 1150 K during TJ-II plasma exposurewere observed for the LiSn solid case. Strong changes in the thermal conductivity of thealloy were recorded in the cooling phase at temperatures close to the nominal meltingpoint. The deduced values for the thermal conductivity of the LiSn alloy/CPS samplewere significantly lower than those predicted from their individual components.

With respect to the potential use of LiSn alloys as LM for an alternative target inDEMO, the results obtained in TJ-II agree with the expectations from laboratory results,namely a dominant Li emission from the alloy when exposed to the plasma, in spite of itsminority in its composition. Together with the very low H retention reported previously[46], the use of this alloy can be considered in a reactor. However, it must be pointed outthat the information about this alternative LM alloy is rather scarce. In particular, thelow thermal conductivity inferred from the T versus t data and the possible depletion ofLi from the alloy at long exposure times remain as potential showstoppers at present.

ACKNOWLEDGMENTS

This work has been carried out within the framework of the EUROfusion Consortium and has received

funding from the Euratom research and training programme 2014-2018 under grant agreement No 633053.

The views and opinions expressed herein do not necessarily reflect those of the European Commission.

It has been partially funded by the Ministerio de Ciencia, Inovacion y Universidades of Spain under

projects ENE2013-48109-P, ENE2015-70142-P and FIS2017-88892-P. It has also received funds from

the Spanish Government via mobility grant PRX17/00425. The authors thankfully acknowledge the

computer resources at MareNostrum and the technical support provided by the Barcelona S.C. It has

been supported as well by The Science and Technology Center in Ukraine (STCU), Project P-507F

References

[1] K. McCormick et al., Phys. Rev. Lett. 89 (2002) 015001.

[2] K. Ida et al., Phys. Plasmas 16 (2009) 056111.

[3] J. M. Garcıa-Regana et al., Nucl. Fusion 57, (2017) 056004.

[4] I. Calvo et al., Plasma Phys. Control. Fusion 59, 055014 (2017).

[5] I. Calvo et al., to appear in Journal of Plasma Physics. arXiv:1804.11104.

[6] J. L. Velasco et al., Plasma Phys. Control. Fusion 60, 074004 (2018).

[7] P. Helander et al., Phys. Rev. Lett. 118, 155002 (2017).

[8] I. Calvo et al., ”Stellarator impurity flux driven by electric fields tangent to magneticsurfaces”, arXiv:1803.05691.

[9] J. M. Garcıa-Regana et al., PPCF, accepted (2018): arXiv:1804.10424.

12 OV/4-3

[10] A. Mollen et al., Plasma Phys. Control. Fusion 60 084001 (2018).

[11] P. Monreal et al., Plasma Phys. Control. Fusion 5 9, 065005 (2017).

[12] A. Mishchenko, P. Helander, and A. Konies 2008 Phys. Plasmas 1¯5 72309

[13] F. Jenko et al. 2000Phys. Plasmas 7 1904

[14] G. Jost et al. Physics of Plasmas, 8(7), 3321 (2001).

[15] J.L. Velasco et al. 2016 Plasma Phys. Control. Fusion 5¯8 084004

[16] K. J. McCarthy et al., 2017 Nuclear Fusion 5¯7 056039

[17] J.A. Alonso et al. Phys. Rev. Lett. 118.18, p. 185002 (2017).

[18] E. Sanchez, Plasma Phys. Control. Fusion 60 (2018) 094003 .

[19] E. Sanchez et al., ”Validation of global gyrokinetic simulations in stellarator config-urations ” (this Conference, EX/P1-11).

[20] T. Estrada et al., ”Turbulence and radial electric field asymmetries measured atTJ-II plasmas” (this Conference, EX/P1-9).

[21] P. Vincenzi et al., Nuclear Fusion 55 (2015)113028

[22] C. Angioni et al., Nucl. Fusion 57 (2017) 116053

[23] R. Sharma et al., Proc. 45th EPS Conference on Plasma Physics, Prague, CzechRepublic 2018, Europhysics Conference Abstracts vol. 42A,P5.1061.

[24] R. Sharma, Ph. Khavanov et al., to be published.

[25] E. Sanchez et al., to be published.

[26] E. de la Cal, Nucl. Fusion 56 (2016) 106031.

[27] F. C. Prasanth, B. van Milligen et al., to be submitted.

[28] U. Losada et al., Plasma Phys. Control. Fusion 60 (2018) 074002.

[29] G. Grenfell at al., ”Measurement and control of turbulence spreading in the Scrape-Off Layer of TJ-II, submitted to Nuclear Fusion.

[30] A. Alonso et al., Nuclear Fusion 52 (2012) 063010

[31] D. Lopez-Bruna et al.,Nucl. Fusion 58 (2018) 106031

[32] A. W. Cooper et al., Nucl. Fusion 58 (to be published)

[33] B.Ph. van Milligen, et al., Nucl. Fusion 57, 5 (2017) 056028

[34] B. van Milligen et al., Phys. Plasmas 25 (2018) 062503.

[35] S. K. Combs et al., Fusion Sci Tech. 64 (2013) 513.

[36] N. Panadero et al., Nucl. Fusion 58 (2018) 026025.

[37] K. J. McCarthy et al. accepted by Plasma Phys. Control. Fusion (2018)

[38] S. Sudo et al., Rev. Sci. Instrum. 83 (2012) 023503

[39] N. Tamura et al., Rev. Sci. Instrum. 87 (2016) 11D619.

[40] K. J. McCarthy et al., Phys. Scripta 93 (2018) 035601.

[41] A. Matsuyama et al., Plasma Fusion Res: Lett. 7 (2012) 1303006.

[42] K. J. McCarthy et al., Europhys. Lett. 120 (2017) 25001.

[43] A. Cappa et al., Proc. 45th EPS Conference on Plasma Physics, Prague, CzechRepublic 2018, Europhysics Conference Abstracts vol. 42A, P4.1040.

[44] D. Spong, R.Sanchez and A.Weller, Phys.of Plamas 10, 3217 (2003)

[45] F.L. Tabares et al., 2017 Phys. Scr. 2017 014054

[46] F.L. Tabares F L et al 2016 Nucl. Mater. Energy 12 1368–73