# LNF:Plasma Physics

Plasma Physics is the main research topic at the Laboratorio Nacional de Fusión, home of the TJ-II stellarator.

## Research areas

The CIEMAT / TJ-II scientific programme is organized around four high priority research areas:

1. Transport physics and 3-D effects, for improving confidence in transport predictions
2. Power exhaust: physics and technology of Liquid metals
3. Plasma stability and control
4. Fast particle physics

### Transport physics and 3-D effects, for improving confidence in transport predictions

#### Impurity transport

Challenges and opportunities

Impurity accumulation is recognized as a potential showstopper for the development of fusion energy. Particle and impurity transport are usually described empirically in terms of diffusive and convective terms, driven by neoclassical and turbulence mechanisms. In the framework of neoclassical mechanisms in tokamaks, the main ion density gradient (inwards) and the ion temperature gradient (outwards / temperature screening) are responsible for opposite convective fluxes. In non-axisymmetric devices the sign of the radial electric field plays a dominant role over the other thermodynamical forces, associated with temperature and density gradients in the convection of impurities.

A key point in stellarator/heliotron plasmas is the empirical observation of increasing impurity confinement at high densities.[1] Nevertheless, effective impurity control is possible in some regimes like the HDH-mode in W7AS[2] and impurity hole in LHD.[3]

The measurement and modelling of impurity density asymmetries are considerably simpler than those of radial impurity transport and yet can provide an indirect validation of the model prediction on radial transport. The localization of impurities in a flux surface depends on the location of magnetic and electrostatic wells as well as on ion-impurity friction.[4] Centrifugal effects become important in strongly rotating plasmas.[5] Recently, the impact of potential variation on neoclassical impurity transport has been addressed in the Large Helical Device (LHD) stellarator. The first direct observations of plasma potential asymmetries on a flux surface, consistent with MC calculations, have been reported in the TJ-II stellarator.[6]

Significant progress has been reported regarding the physics understanding of empirical actuators, like ECRH heating, to avoid core impurity accumulation. A reduction of the background density gradient leading to a reduction of the inward (neoclassical) convection of impurities as well as the amplification of core temperature screening can prevent core impurity accumulation has been reported in tokamaks. Furthermore, ECRH induced turbulent transport and development of plasma potential flux surface asymmetries[7] can play a role on impurity transport. Clarifying the role of ECRH on turbulent transport, plasma potential asymmetries and neoclassical effects remains as an open question.

Research plan in TJ-II and related research programmes (W7-X)

The TJ-II particle / impurity research is aimed at advancing the scientific understanding of plasma impurity transport and applying that understanding to help a reliable operation steady-state operation for ITER and particularly in W7-X research programme. To these ends, key elements of the TJ-II research programme on particle / impurity transport include:

• Quantifying the importance of variations in plasma potential on magnetic flux surfaces, radiation asymmetries and scaling with plasma scenarios. Comparative studies of TJ-II experimental results and simulations, including potential asymmetries, inertia and main ion distribution functions.
• Influence of Z on impurity dynamics.
• Physics of empirical actuators (ECRH) to control impurity accumulation
• Development of strategies for avoidance of impurity accumulation in fusion plasmas (3-D equilibrium studies and particularly W7-X).

#### Momentum transport: intrinsic rotation and 3-D effects on the L-H transition physics

Challenges and opportunities

In large scale devices like ITER rotation driven by external forces will be limited. Therefore, it is important to quantify the role of alternative driving mechanisms of plasma rotation, like those related with plasma turbulence and fast particles.

ITER will likely operate with Resonant Magnetic Perturbations (RMP) for ELM control. This raises the question of how magnetic perturbations will affect plasma rotation, radial electric field and H-mode access. 3-D effects are obviously crucial in stellarators, in particular for island divertor configurations such as W7-X.

Research plan in TJ-II and related devices

Experiments and model development to improve the physics basis for momentum transport to address the physics of confinement transition and intrinsic rotation will continue to be a major element of LNF´s research plan. The TJ-II has provided cause-effect evidence that three-dimensional magnetic structures convey significant alterations of the plasma rotation without harming the overall confinement. The device is in position of planning and executing further relevant experiments, including:

• Influence of 3-D magnetic topology on confinement and stability control. Development of comprehensive transport and 3-D equilibrium tools to interpret experimental data.
• Development of experimental schemes to assess the effect of edge island formation on the plasma conditions required for H-mode access and sustainment in view of W7-X: role of electromagnetic effects.
• Understanding the long-wavelength (neoclassical) radial electric field of stellerators in H-mode. Local neoclassical calculations usually underestimate the value and shear of the mean electric field in the H-mode of stellarators. One possible explanation that will be explored is that higher order corrections of the theory have to be included. This will be done with the code FORTEC-3D, which includes non-local effects in ion transport.
• It has been proven[8] that the calculation of radial transport of toroidal angular momentum in tokamaks, and equivalently the rotation profile and long-wavelength radial electric field, requires to deal with microturbulence. In particular, high-order gyrokinetic equations are needed. During the last few years, we have worked out the electrostatic gyrokinetic equations to the required order in arbitrary magnetic geometry and applied them to the tokamak. Concretely, we have given the equations that allow to compute the long-wavelength pieces entering the formula that determines the rotation profile.[9] However, the equations for the short-wavelength pieces are still lacking. We will compute them, and give all the explicit expressions that have to be implemented in a code aiming to determine tokamak intrinsic rotation. The expressions for flux-tube formulations of gyrokinetics will also be provided. A mid-term objective involves the actual code implementation and testing of the theory.

#### Isotope effect

Challenges and opportunities

There is clear experimental evidence that at comparable plasma discharge parameters deuterium (D) discharges have improved confinement properties as compared with hydrogen (H) ones. The isotope effect has been observed in many tokamaks. Interestingly, the isotope effect seems to be weaker in stellarators than in tokamaks. Understanding the physics of the isotope effect in plasma transport and confinement remains as a fundamental open question confronting the fusion community since more than 30 years of intense research with direct impact in the confinement properties of fusion D-T reactors.

Furthermore, considering the present ITER power capabilities, a reduction of the L-H power threshold ($P_{L-H}$) with ion mass (D vs. H) would have great impact on ITER plasma operation scenarios. The $P_{L-H}$ power threshold deduced from empirical scaling laws is sufficient to define the minimum power required for ITER operation. Experimental studies have shown a reduction of the L-H power threshold by about 50% when using Deuterium and He instead of Hydrogen.[10] Based on present ITPA scaling laws, H-mode operation is expected to be marginally feasible in H but likely in He.[11] Thus, better understanding of the dependence of the L-H power threshold on isotope mass is urgently needed to improve our confidence in ITER scenarios.

Studies in the TEXTOR tokamak[12] and the TJ-II stellarator[13] have reported the properties of local turbulence and Long-Range-Correlations (LRC) in Hydrogen and Deuterium plasmas concluding that there is a systematic increase in the LRC amplitude during the transition from H to D dominated plasmas in the TEXTOR tokamak but not in the TJ-II stellarator. These results suggest the role of the ion mass and viscosity on the amplitude of zonal flows and thus provide the first direct experimental evidence of the importance of multi-scale physics for unravelling the physics of the isotope effect on transport and confinement in fusion plasmas.

Research plan in TJ-II and related research programme supporting the W7-X programme

The goal of this work is to provide a bridge between properties of large-scale flows, local turbulence and isotope physics during the transition from D to H dominated plasmas in tokamaks and stellarators:

• Role of the working gas (H / D / He) in the development of the zonal flows (ZFs) and turbulence. Investigations of the role of edge magnetic topology on the damping/driving of Zonal flows and L-H transition.
• Gyrokinetic simulations of micro-instabilities and zonal flows

#### Neoclassical transport in stellarators and design optimization

Challenges and opportunities

Stellarator inter-machine studies[14] have shown that ion energy transport significantly affects energy confinement at medium-to-high densities ($n_e > 4 \cdot 10^{19} m^{-3}$). Since neoclassical transport in three-dimensional devices shows unfavourable temperature scaling, it becomes more important as the temperature is increased and the validation of local neoclassical theory at reactor-relevant conditions is needed. Extensions of the standard neoclassical theory are necessary for a better assessment of collisional transport in a stellarator reactor concept, such as non-local and non flux-surface based calculations.

Research plan in TJ-II and related research programme (W7-X)

Validation of neoclassical theory at reactor-relevant conditions as a step for the prediction of stellarator reactor scenarios:

• For high ion temperatures and steep plasma gradients, non-local effects such as (i) finite radial width of trapped particle orbits and direct particle loss near the last closed ux surface and (ii) the validity of the monoenergetic approximation, used in standard local neoclassics, needs to be understood. FORTEC 3D and DKES simulations and comparison of experiment with local and non-local neoclassical theories.
• The coupling of neoclassical energy and particle transport may lead to hollow density profiles in high temperature (ion and electron) plasmas, which is highly relevant for the discharge scenario development for large stellarator devices and reactor operation scenarios. The issue of particle transport therefore requires further documentation and deeper understanding in view of density control or impurity transport in three-dimensional devices. Experimental and theoretical studies of central fuelling (e.g. by pellets in operation in TJ-II) are under development supporting W7-X.
• The singular alteration of transport at magnetically resonant plasma regions. TJ-II offers the only experimental benchmark in Europe to evaluate their impact in deviating neoclassical predictions[15] and these studies can now be pursued with improved capabilities (e.g., higher NBI power and additional microwave heating in conditions of high plasma density).
• An important open question is the relation between neoclassical and turbulence optimization. It is seen that in LHD stellarator[16] both actions go together. Effort is being carried out to understand the zonal flow relaxation in stellarator geometry and the role of neoclassical radial electric fields on ZFs and confinement.

Optimized stellarator studies:

• Closeness to quasisymmetry. We know that perfect quasisymmetric stellarators do not exist and very few results in the literature indicate how close one can get. Also, if quasisymmetry has necessarily to be violated, are there ways to violate it still having an optimized stellarator? The answers have the potential to impact stellarator optimization approaches and, consequently, the design of new devices.
• If the stellarator is quasisymmetric, the requirements for the calculation of the long- wavelength radial electric field and rotation in the direction of symmetry are analogous to those for the tokamak. We plan to quantify how stellarators close to quasisymmetry behave in this respect.

#### Particle and energy confinement: physics of decoupling mechanisms for understanding ELM mitigated regimes

Challenges and opportunities

The demonstration of the ITER baseline scenario (i.e. H-mode operation with integrated Edge Localized Modes (ELMs) control) is progressing but its feasibility for DEMO will require full control of ELMs, which is very challenging. The full understanding needed for a confident extrapolation of new regimes as an alternative to type I ELMs to burning plasmas is not yet available, though several routes look promising. Clarifying the physics behind uncoupled (heat and particle) transport channels is one of the main scientific conundrums for understanding ELM control techniques (e.g. using magnetic perturbations[17]), as part of the ITER base-line scenario, and the development of plasma scenarios without ELMs (e.g. the I-mode[18]).

Research plan in TJ-II and related devices:

Theoretical studies:

• Gyro-kinetic simulations to characterize the changes in the cross-phase as a function of the relative dominance of ITG/ETG turbulence and to study differential effects between density and heat channels as the spectral content of zonal flows is varied.

Experimental studies:

• Characterization of plasma regimes with decoupled particle and energy transport channels in tokamaks.
• Assess the role of large-scale flows on the amplitude and phase relation between fluctuating fields in plasma regimes with decoupled turbulent transport channels.
• Assess amplitude and cross-phase between density, potential and temperature before and prior transport channel decoupling.

### Power exhaust: physics and technology of Liquid metals

Challenges and opportunities

Power exhaust is one of the most critical, complex and challenging issues for the development of a fusion power reactor, at present without a solution neither from the physics nor technological perspectives. Solid materials are the standard solution for fusion divertors, but are known to be prone to erosion, material damage, dust formation and neutron-induced permanent damage in configurations with high power flux.

Thus, in the framework of the design and assessment of innovative plasma configurations (snowflake, Super-X), liquid metals (Li, Ga, Sn) are also being considered. Lithium Capillary-Pore-system (CPS) limiters with a closed circulation loop are under development at the T11M tokamak.[19] CPS experiments are also in progress at the FTU tokamak[20] and the TJ-II stellarator.[21] CPS is a promising solution in the search for a candidate material (Li/Sn/Ga) that offers all the required properties.

Research plan in TJ-II

TJ-II will keep in the period 2016-2019 an active programme for the assessment of novel solutions for plasma facing components using liquid metals (Li and Li/Sn alloys). The TJ-II programme on liquid metals will address fundamental issues, like the self-screening effect of liquid lithium driven by evaporation to protect plasma-facing components against huge heat loads and tritium inventory control, using the Li-liquid limiters installed in 2012. The research on liquid metals as a possible PFC for a future Fusion Reactor is an important element of the EU Fusion Roadmap, involving the collaboration of different European groups [Portugal, Spain, Italy (ENEA), Latvia and Germany (FZJ, Julich)]. Concerning liquid Li technology, the possibility of adding first wall modules of liquid lithium, already designed by the Red Star team, will be investigated. This will allow the operation of TJ-II as a full liquid lithium device.

### Plasma stability and control

Challenges and opportunities

TJ-II experimental results have shown that stellarator stability is better than predicted by linear stability analyses.[22] This result strongly suggests that stability calculations, as those presently used in the optimization criteria of stellarators, might miss some stabilization mechanisms,[23] which could be explained by self-organization mechanisms between transport and gradients.[24] Understanding this discrepancy might help to relax some optimization criteria in 3-D magnetic configurations with possible impact in engineering design and reactor technology.

Research plan in TJ-II

Exploring the impact of relaxing some optimization criteria (e.g. stability via magnetic well) on reactor technology is an important element in the TJ-II research programme including theory/simulation and potential experimental validation. Relaxing those criteria can allow one to design coils easier to build and maintain. Moreover, those experiments can throw some light on the role of triangularity and elongation on tokamak stability:

• Reaching the highest heating capability including NBI, EBWH and ECRH, to achieve the highest beta: Influence of magnetic well on plasma profiles, fluctuations and confinement.
• Explore the role of magnetic topology and rational surfaces on the confinement and stability of TJ-II, especially owing to their capabilities as practical tools for real-time confinement control.

### Fast particle physics

Challenges and opportunity

Alpha-particle driven Alfvénic instabilities constitute a source of major uncertainty for predicting alpha-particle transport, alpha heating profile, and He ash accumulation in burning plasmas. Moreover, Alfvén Eigenmodes (AEs) can have strong influence on the confinement of fast ions, thus making less efficient NBI heating. The observed mitigation effect of ECRH on NBI beam-driven Alfvén eigenmodes (AEs) first reported in DIII-D and later in TJ-II[25] has opened an attractive avenue for a possible control of the AEs though the physics behind this effect is yet to be understood. Fast particle can have also influence on the broadband turbulence and on several instabilities like ITGs.

Research plan in TJ-II

• Assessing the relevance of ECRH plasma heating on AEs in the TJ-II stellarator: relevance of the ECRH power per particle and power density and influence of resonance radial position. Comparative studies in tokamaks and stellarators.
• Exploring the circumstances in which Alfvén Eigenmodes degrade the fast ion confinement.
• Prepare useful scenarios for W7-X determining the experimental parameter ranges (density, collisionality, magnetic configuration) in which ECRH application can become a control tool to reduce the Alfvenic activity.
• Development of theory models to understand the roles of several Alvén destabiling and damping mechanisms, including electron trapping.

## References

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