Plasma Fueling Research Articles

Fusing together an outline design for sustained fuelling and tritium self-sufficiency.

Ensuring tritium fuel self-sufficiency while maintaining continuous and high-specification fuel flow to the tokamak via a low tritium inventory and controllable fuel cycle is a significant challenge to the STEP plant design. Effective and high-quality fuelling and exhaust design is required to sustain and control a stable plasma, whereas fuel sufficiency is required to prevent depletion of available tritium supply. Concerns regarding the lack of tritium availability preventing continuous tritium import are countered by breeding, where highly energetic neutrons from the core fusion reactions interact with lithium atoms suspended in the surrounding breeder blanket to produce tritium. The compact nature of STEP prohibits the integration of inboard breeder blankets posing a significant challenge for the design team looking to ensure more tritium is bred and made available than consumed within the core plasma. This paper outlines how purposeful technology selection and system architecting has converged on the outline of a conceivable and tritium-capable fuel cycle and breeder blanket design. Before introducing the STEP fuel cycle design outline and summarizing the approach undertaken to address the challenges facing plasma fuelling, key aspects of fuel self-sufficiency are discussed. This includes discussing a proposed helium-cooled liquid lithium breeder blanket and possible technology options for tritium extraction from lithium. Lastly, there is a brief process modelling overview, which emphasizes the central contribution of various employed modelling methods. Reflections on the presented fuel cycle design outline conclude that substantial development work is still required to realize a continuous tritium fuel cycle design and overcome the major challenges posed by tritium and lithium handling. Reflections on the presented breeder blanket design proposal conclude that while many substantial risks and blockers remain to achieve fuel self-sufficiency, high breeding ratios are expected to be achievable with a compact spherical tokamak configuration. Nonetheless, it is recognized that further consideration is required to ensure that the selection of liquid lithium as a breeder medium provides the overall simplest route to a self-sufficient and realizable design.This article is part of the theme issue 'Delivering Fusion Energy - The Spherical Tokamak for Energy Production (STEP)'.

Study of the pellet ablation cloud using the tomography technique for two-directional simultaneous photography in GAMMA 10/PDX

The pellet ablation mechanism is an interesting subject for plasma fuelling in fusion plasmas. In GAMMA 10/PDX, pellet injection experiments for higher density plasma production are planned to conduct detached plasma experiments in the higher density plasma condition. We measured the pellet ablation cloud by using the two-directional simultaneous photography system in GAMMA 10/PDX. The tomography reconstruction technique was used for considering the pellet trajectory in the plasma and pellet ablation. The three-dimensional pellet trajectory and pellet ablation images in the plasma were clearly obtained for the first time, to the best of our knowledge.

Using rational surfaces to improve pellet fuelling in stellarators

Pellet injection is currently the primary candidate for achieving efficient plasma fuelling, one of the key issues for steady-state operation in large fusion devices. In this paper, pellet injection experiments are performed for several magnetic configurations of the TJ-II stellarator. The aim of this study is to increase the understanding of the role played by rational surfaces in plasmoid drift and deposition profiles in stellarators. The analysis of experimentally observed plasmoid drifts is supported by simulations of such cases made with the HPI2 code. Plasmoid drift is found to be significantly reduced, as in tokamaks, in the vicinity of rational surfaces. This is attributed to the fact that plasmoid external charge reconnection lengths are shorter near rational surfaces, resulting in a more effective damping of the plasmoid drift. Although the effect of plasmoid external currents on the drift is expected to be negligible in stellarators, compared with those caused by plasmoid internal currents, the effect observed in TJ-II is clearly measurable. In addition, simulations show that enhanced drift reductions near rational surfaces lead to significantly different deposition profiles for the magnetic configurations included in this study. This implies that it should be possible to select the magnetic configurations to obtain more efficient pellet fuelling.

Design and simulation of 1.5 T conduction-cooled superconducting magnet with flip-over capability

The Compact Torus Injection (CTI) system is a plasma physics device used to create and study compact torus plasmas, which are dense and highly energetic structures with potential applications in fusion energy research and space propulsion. The Experimental Advanced Superconducting Tokamak compact torus injection (EAST-CTI) system is a device used for plasma heating and fueling in fusion experiments. In this paper, a 1.5 T cryocooled superconducting magnet was developed for the EAST-CTI system, which requests the magnet system can work vertically or horizontally to meet different working conditions. The design of the superconducting magnet and cryogenic system is based on the analysis of the thermal and mechanical characteristics of the magnet system. Thermal and mechanical finite element simulation were used to support the design. The 1.5 T magnet superconducting magnet was manufactured according to the design and simulation works. After 35 h cooling, the superconducting magnet stabilized at 3.87 K, and successfully provided a horizontal magnetic field environment for the experiments on EAST-CTI system.

Correction of JET bolometric maximum likelihood tomography for local gas puffing

Tomography is applied daily to bolometric data in magnetically controlled nuclear fusion devices to infer important quantities of the studied plasmas, such as the emissivity profiles or the radiated power in different locations of the main chamber. Tomographic reconstructions are also crucial for power balance analysis and to estimate heat fluxes for turbulence studies. One of the issues, associated with any tomographic technique, resides in the ill-posed nature of the mathematical problem, meaning that more than one possible emissivity profile is compatible with the measurements within the estimated uncertainties. Therefore, a careful evaluation of the quality of the obtained reconstructions is an important step of the analysis. On Joint European Torus (JET), since the two cameras are located in different toroidal locations, axial symmetry of the plasma radiation must be assumed to perform traditional tomographic reconstructions. Such a specific hypothesis, however, cannot be guaranteed in the case of symmetry-breaking mechanisms, such as strong local gas puffing for fuelling. The present contribution addresses this problem of local plasma fuelling, focusing on the observed significant mismatch between measured and back-calculated projections in specific pulses, including the last JET DT campaign (DTE2) carried out in 2021. The analysis indicates that the observed discrepancy, due to the gas injected primarily from valves located in the same octant where one of the bolometer arrays is located, can be rectified acceptably well by a physically-based correction. The sensible improvements in the bolometric maximum likelihood tomography’s outputs are also documented, and the possible physical explanation for such a phenomenon is discussed. The study performed in this article is expected to have a significant impact on the analysis of JET data and on the physical interpretation of the results obtained during the recent DTE2 experiments, for which a proper evaluation of the radiated power estimates is of non-negligible relevance.

Electron density pedestal behaviour in strike-point sweeping experiment on JET

Strike-point sweeping, a technique often used to spread heat loads on divertor targets, was employed in JET experiments for the first time to generate an edge-localized modulated particle source for investigating plasma fuelling and particle transport in the edge region. This approach was motivated by the possibility of achieving higher modulation frequencies than those available from traditional gas puff modulation at JET. Higher frequencies would enable the collection of more edge-localized information from the electron density response to the modulated particle source. Various sweeping frequencies, up to 18.5 Hz, were commissioned and utilized in the experiments. Both strong and weak electron density responses were observed in H-mode plasmas, depending on the strike-point configuration and the distance the strike-points moved during the sweep cycle. The electron density response exhibited complex and unconventional behaviour (compared to gas puff modulation), which presented challenges for interpretation. In this study, we analyse one experiment in detail using an optimization framework in which transport and particle source parameters are determined by fitting our forward model parameters to the experimental electron density measurements. We demonstrate that a consistent picture emerges and that our approach can provide new insights into these complex data. However, we note that while strike-point sweeping generates the desired modulated edge-localized particle source, it also modifies the properties of the edge transport barrier. Therefore, the strike-point sweeping methodology is a promising but challenging way to study edge particle transport and edge fuelling properties, requiring very precise measurements.

A comparison of the influence of plasmoid-drift mechanisms on plasma fuelling by cryogenic pellets in ITER and Wendelstein 7-X

Pellet injection is the most promising technique to achieve efficient plasma core fuelling, key for attaining stationary scenarios in large magnetic confinement fusion devices. In this paper, the injection of pellets with different volumes and speeds into standard plasma scenarios in ITER (tokamak) and Wendelstein 7-X (stellarator) is studied by modeling the pellet ablation and particle deposition, focusing on the evaluation of the expected differences in pellet plasmoid drifts in tokamaks and stellarators. Since the efficiency of the damping-drift mechanisms is predicted to depend on the magnetic configuration, device-specific characteristics are expected for the temporal evolution of the plasmoid drift acceleration. For instance, plasmoid-internal Pfirsch–Schlüter currents dominate the drift damping process for stellarators, while plasmoid-external currents are more relevant for tokamaks. Also, relatively larger drifts are in principle expected for W7-X due to higher field gradients in relation to machine dimensions. However, shorter plasmoid-internal charge reconnection lengths result in the drift damping due to internal Pfirsch–Schlüter currents being more effective than in a tokamak. Therefore, the average relative drift displacement during the whole plasmoid homogenization may a priori be comparable in both magnetic configurations. Moreover, High Field Side (HFS) injection is expected to be highly advantageous to maximize pellet particle deposition in ITER, whereas it may only be beneficial in medium to high β environments in W7-X. Finally, there may be means for the optimization of pellet injection configurations in both ITER and W7-X for the considered plasma scenarios despite the sizeable differences in the relative importance of the mechanisms of plasmoid drift acceleration and deceleration in play.

First feedback-controlled divertor detachment in W7-X: Experience from TDU operation and prospects for operation with actively cooled divertor

In the last experimental campaign (OP1.2b) of the stellarator Wendelstein 7-X (W7-X), boronisation as a mean for first wall conditioning was applied for the first time which led to strongly reduced impurity fluxes from plasma-facing components. Thermal detachment at the uncooled target plates of the test divertor unit (TDU) was reached at higher plasma densities and was accompanied by high recycling of neutrals at the target plate (Feng et al., 2021), (Jakubowski et al., 2021) [1] . A feedback control system was established in W7-X to actively control the gas injection (actuator) for plasma fuelling and impurity seeding (Effenberg et al., 2019) [2] through the divertors. It allowed very successful stabilisation of the detached plasma condition state as well as mitigation of thermal overloads to some baffle tiles. Different routinely available diagnostic signals were used as input parameters (sensors). We describe the setup of the feedback control system, its performance and provide some example results with the main focus on the development of the control scheme which led to the detachment stabilisation over the entire longest (30 s) high-power discharge at W7-X so far. In view of the achieved very successful detachment stabilisation and the necessity to include simultaneous optimisation of the core performance in the future, the feedback control system is being upgraded for the upcoming campaign (OP2.1) in which the water cooled and hereby inherently steady-state capable divertor has been currently installed. The prospects and some experiment ideas for active detachment control are discussed.

Preliminary study of supersonic molecular beam injection in Thailand Tokamak-1 using 3D fluid model

Successful operation of a tokamak relies on effective and appropriate methods of plasma fueling. The development plan of Thailand Tokamak-1 (TT-1) will employ the supersonic molecular beam injection (SMBI) which delivers the fueling gas more effectively and deeper than the gas puffing method. In this work, we study the effect of SMBI on the plasma transport of TT-1 plasma by using 3D fluid simulation. The model includes the continuity equation, energy balance equation, momentum equation, continuity of fuel equation, and momentum of fuel. BOUT++ is then used to solve these equations by a finite difference method with the field-aligned coordinates in the edge region. The simulation shows that when hydrogen fuel gas is by the SMBI method from the low-field side into the plasma with a speed of 1000 m/s, the electron density in the edge locally rises as a result of the dissociation and ionization. The ion and electron temperatures then drop. After that, the density spread over the whole plasma volume within about 10 ms. When the injection speed increases, it results in a deeper penetration length of the fuel deposition.

First Observation of Pellet Ablation Clouds Using Two-Directional Simultaneous Photography in GAMMA 10/PDX

The study of pellet ablation mechanisms is an important subject for plasma fueling in fusion plasmas. In GAMMA 10/PDX, sub-millimeter hydrogen pellet injection experiments are conducted in the higher electron density plasma for detached plasma experiments. We observed the pellet ablation cloud by two-directional simultaneous photography during the GAMMA 10/PDX pellet injection experiments. The three-dimensional pellet ablation cloud and its trajectory were clearly obtained for the first time.

Experimental investigation of current jumps in linear geometry hot cathode ionization gauges in strong magnetic fields

In magnetic confinement nuclear fusion, neutral gas in the plasma edge region plays an important role for plasma fuelling and plasma wall interaction and thus influences the overall plasma behaviour. The ASDEX pressure gauge is a diagnostic instrument designed to provide local and temporally highly resolved measurements of the neutral gas flux density within the strong magnetic fields of fusion devices. It is a hot cathode ionization gauge with a linear geometry. Its function principle relies on the measurement of an ion current, caused by oscillating electrons in the gauge which ionize the neutral gas. Anomalous jumps in the ion current of up to 30% have been observed during gauge operation in the tokamak ASDEX Upgrade and the stellarator Wendelstein 7-X, which introduces an error in the measurement. This paper describes for the first time the characterization of these jumps over the broad operational space. Parameter dependencies on pressure, electron current, electrode potentials and magnetic field angle are discussed. The appearance of electrostatic oscillations with frequencies in the range of 160MHz to 350MHz which are associated with the current jumps has been discovered. The frequency of these oscillations was found to scale with the oscillation frequency of the electron motion inside the gauge. Potential physical mechanisms to explain the jumps are discussed.

Kinetic modeling of neutral transport for a continuum gyrokinetic code

We present the first-of-its-kind coupling of a continuum full-f gyrokinetic turbulence model with a 6D continuum model for kinetic neutrals, carried out using the Gkeyll code. Our objective is to improve the first-principle understanding of the role of neutrals in plasma fueling, detachment, and their interaction with edge plasma profiles and turbulence statistics. Our model includes only atomic hydrogen and incorporates electron-impact ionization, charge exchange, and wall recycling. These features have been successfully verified with analytical predictions and benchmarked with the DEGAS2 Monte Carlo neutral code. We carry out simulations for a scrape-off layer (SOL) with simplified geometry and National Spherical Torus Experiment parameters. We compare these results to a baseline simulation without neutrals and find that neutral interactions reduce the normalized density fluctuation levels and associated skewness and kurtosis, while increasing auto-correlation times. A flatter density profile is also observed, similar to the SOL density shoulder formation in experimental scenarios with high fueling.

Experimental study on the role of the target electron temperature as a key parameter linking recycling to plasma performance in JET-ILW* *This manuscript has been authored by UT-Battelle, LLC, under contract DE-AC05-00OR22725 with the US Department of Energy (DOE). The US government retains and the publisher, by accepting the article for publication, acknowledges that the US government retains a nonexclusive,

Changes in global and edge plasma parameters (H 98(y,2), dimensionless collisionality ν *, core density peaking, separatrix density n e,sep) with variations in the D2 fueling rate and divertor configuration are unified into a single trend when mapped to ⟨T e,ot⟩, the spatially averaged spectroscopically derived outer target electron temperature. Dedicated JET with the ITER-like wall (JET-ILW) experiments in combination with an extended JET-ILW database of unseeded low-triangularity H-mode plasmas spanning a wide range of D2 fueling rates, I p, B t and heating power have demonstrated the importance of ⟨T e,ot⟩ as a key physics parameter linking the recycling particle source and detachment with plasma performance. The remarkably robust H 98(y,2) trend with ⟨T e,ot⟩ is connected to a strong inverse correlation between ⟨T e,ot⟩, n e,sep and ν *, thus directly linking changes in the divertor recycling moderated by ⟨T e,ot⟩ with the previously established relationship between ν *, core density peaking and core pressure resulting in a degradation in core plasma performance with decreasing ⟨T e,ot⟩ (increasing ν *). A strong inverse correlation between the separatrix to pedestal density ratio, n e,sep/n e,ped, and ⟨T e,ot⟩ is also established, with the rise in n e,sep/n e,ped saturating at ⟨T e,ot⟩ > 10 eV. A strong reduction in H 98(y,2) is observed as ⟨T e,ot⟩ is driven from 30 to 10 eV via additional D2 gas fueling, while the divertor remains attached. Consequently, the pronounced performance degradation in attached divertor conditions has implications for impurity seeding radiative divertor scenarios, in which H 98(y,2) is already low (∼0.7) before impurities are injected into the plasma since moderate gas fueling rates are required to promote high divertor neutral pressure. A favorable pedestal pressure, p e,ped, dependence on I p has also been observed, with an overall increase in p e,ped at I p = 3.4 MA as ⟨T e,ot⟩ is driven down from attached to high-recycling divertor conditions. In contrast, p e,ped is reduced with decreasing ⟨T e,ot⟩ in the lower I p branches. Further work is needed to (i) clarify the potential role of edge opacity on the observed favorable pedestal pressure I p scaling; as well as to (ii) project the global and edge plasma performance trends with ⟨T e,ot⟩ to reactor-scale devices to improve predictive capability of the coupling between recycling and confined plasma fueling in what are foreseen to be more opaque edge plasma conditions.

Impact of the plasma operation on the technical requirements in EU-DEMO

The plasma design point during stationary phase operation of EU-DEMO has been described extensively in the past, and the assumptions employed for its definition have been supported by various investigations. However, a stationary snapshot is by far not sufficient to ensure the actual operability of the plasma scenario. On the contrary, both normal and off-normal transients have a strong impact on the machine design, providing the most challenging requirements to the actuators. This has to be accounted for in the design from the early phases on. In this paper, the main requirements originating from plasma operation, studied during the Pre-Concept Design Phase, are briefly reviewed. These encompass the so-called planned transients, e.g. plasma current ramps, access and termination of the burn phase or plasma fueling with pellets, but also the mitigation of off-normal events which may lead to a disruption, e.g. the divertor reattachment or an uncontrolled increase of radiation after a sudden W influx from the wall.

Overview of the TJ-II stellarator research programme towards model validation in fusion plasmas

TJ-II stellarator results on modelling and validation of plasma flow asymmetries due to on-surface potential variations, plasma fuelling physics, Alfvén eigenmodes (AEs) control and stability, the interplay between turbulence and neoclassical (NC) mechanisms and liquid metals are reported. Regarding the validation of the neoclassically predicted potential asymmetries, its impact on the radial electric field along the flux surface has been successfully validated against Doppler reflectometry measurements. Research on the physics and modelling of plasma core fuelling with pellets and tracer encapsulated solid pellet injection has shown that, although post-injection particle radial redistributions can be understood qualitatively from NC mechanisms, turbulence and fluctuations are strongly affected during the ablation process. Advanced analysis tools based on transfer entropy have shown that radial electric fields do not only affect the radial turbulence correlation length but are also capable of reducing the propagation of turbulence from the edge into the scrape-off layer. Direct experimental observation of long range correlated structures show that zonal flow structures are ubiquitous in the whole plasma cross-section in the TJ-II stellarator. Alfvénic activity control strategies using ECRH and ECCD as well as the relation between zonal structures and AEs are reported. Finally, the behaviour of liquid metals exposed to hot and cold plasmas in a capillary porous system container was investigated.

Progress of engineering design of CFETR vacuum systems

Vacuum systems for the China Fusion Engineering Test Reactor (CFETR), including pumping system, fueling system, wall conditioning, as well as their control systems, are required to achieve high vacuum, effective plasma fueling, and good wall condition for steady-state plasma operation and to facilitate remote flexible control with high reliability to ensure safe operation. In the last few years, based on the requirements of vacuum systems and subsystems of CFETR, their features and challenges have been analysed, basic parameters have been defined, and preliminary designs have been developed. Herein, nine cryopumps with pumping speed of 68 m³/s for D2 in a molecular flow regime are proposed for use in a torus vacuum chamber to achieve vacuum pressure <1.0 × 10−5 Pa before plasma operation. The throughput of the torus pumping system is designed to be >290 Pa•m³•s−1 to meet the requirement of steady-state operation of CFETR discharge. Pellet injectors and the gas injection system for seeding the fuel and impurity gas and wall conditionings, including baking, GDC, and ICRF-DC, for removal of hydrogen isotopes/impurities of the torus vacuum are also designed. Moreover, integrated design of vacuum systems and space layout of all subsystems are also investigated.

Numerical prediction of the operating point for the cryogenic twin-screw hydrogen extruder system

An extruder-type cryogenic pellet injector system for plasma fuelling is under research at Institute for Plasma Research (IPR), India. A twin-screw extruder is a most stable device in the form of closed volumes of hydrogen between screw threads and promising process due to continuous production of hydrogen pellet for plasma reactor. In the present investigation, a non-Newtonian, non-isothermal CFD model was developed which modeled the shear rate and temperature-dependent viscosity in the Herschel Bulkley model. Brinkman number (Br) varies from 0.15 to 0.46 at 5 and 15 rpm respectively. Due to increasing the screw rotation speed for a given throughput, the shear rate increases (therefore, also the Br) and results in larger viscous dissipation. Solid hydrogen temperature is predicted at the location of the flight gap and intermeshing zone for the different barrel temperatures under varying screw rotation speeds (5–15 rpm). Prediction of hydrogen temperature at the intermeshing zone of the extruder system plays a vital role to decide the barrel temperature. Due to more number of moving surfaces at the intermeshing zone of the extruder system, the temperature of solid hydrogen is higher and increases above the melting point temperature. The effect of barrel wall temperature and screw rotation speeds (5–15 rpm) on the viscous dissipation rate and pressure development at required throughput was also examined for an extruder system. It is important to estimate the barrel wall temperature to get an operating point and corresponding viscous dissipation rate, which determines the cooling load required for extruder-die set-up. The results show that as the die wall temperature increases, the viscous dissipation, as well as the pressure drop decreases in the die. The investigation showed that the die’s wall temperature and screw rotation speed affect the operating point of the extruder-die system.

Reduced-model scrape-off layer turbulence (nSOLT) simulations comparing three fueling scenarios

The 2D scrape-off-layer turbulence code (nSOLT) includes 1D Boltzmann neutral–plasma interactions, a model of divertor recycling (introduced here), and a fixed source of plasma concentrated at the core-side boundary. Three fueling methods are considered herein: (1) neutral injection in the far-SOL is accomplished by specifying the density of Franck–Condon distributed neutrals streaming in from the boundary. (2) Divertor recycling is modeled by injecting a fraction of the particle parallel flux in the scrape-off layer (SOL) back into the edge region as a source of plasma. (3) A constant source fuels the edge plasma from the core-side boundary to model pellet injection. For machine parameters (B, Rm, and L//) illustrative of the MAST-U device, and for a deuterium plasma, turbulent equilibria are obtained that share the same plasma fueling rate for each of the three fueling methods, with only one of the sources on in each case. In the presence of self-consistent turbulence, quasi-steady plasma and neutral (deuterium) profiles, fueling efficiencies, SOL transparencies, and heat flux widths are compared. Characteristics of the turbulent fluctuations, including skewness, cross-phases, and power spectra, are described. The calculated fueling efficiencies, SOL transparencies to neutral penetration, and many of the turbulent properties are remarkably similar for all three fueling methods despite significant differences in the plasma profiles. The nonlinear states of the three cases are dominated by separatrix-spanning vortex cells that control particle and heat losses into the SOL.

The interpretation of magnetic activity associated with pellet injections into plasmas created in the stellarator TJ-II

Pellet injection into magnetic confinement plasma is widely employed for plasma fuelling and impurity transport studies. While post-injection effects include changes in density and temperature, several short-lived effects have been identified during and immediately after pellet ablation, e.g. pre-cooling of the plasma core or a strong reduction in fluctuations. Here, transitory effects observed in magnetic signals during and immediately after the injection of fuel and tracer encapsulated solid pellets into microwave and neutral beam heated plasma in the stellarator TJ-II are reported. One effect is the excitation of a clear, short-lived, magnetic instability during the ablation phase of a cryogenic pellet. Another effect is a brief reduction in broadband magnetic fluctuations immediately after an injection while a third effect is frequency jumps of already excited Alfvén eigenmodes (AE) when neutral beam heating is employed. The interpretation of these effects is facilitated by signals from 2 recently installed toroidal arrays of magnetic coils that follow the helicity of this device along one of its quadrants. It is concluded that interactions between outward drifting pellet particles and a low-order rational surface lead to the magnetic instability, that pellet-induced plasma cooling induces a transitory reduction in broadband magnetic fluctuations and that plasma mass density changes dominate jumps in AE frequencies.

Feasibility of D-D start-up under realistic technological assumptions for EU-DEMO

One of the main issues in view of the realization of a DEMOnstration fusion reactor is the availability of a sufficient external supply of tritium (T) to start operation. T is an unstable nuclide, which is almost absent in nature and is currently available as by-product in e.g. CANDU, whose operation in the next decades (both in terms of life extension of existing reactors and construction of new ones) is at the moment under debate. During DEMO operation, T will be generated on-site by breeding blanket, employing the neutrons originating from D-T reaction. However, it is considered that a certain initial amount of T is needed to start operation, the so-called start-up inventory. An alternative approach consists of obtaining the start-up inventory exploiting reactions occurring in a D-D plasma, which generate T both directly in the plasma and via breeding in the breeding blanket. In the present paper, the conditions under which D-D start-up becomes a favorable option for a power plant are discussed. The analysis mainly focuses on the EU-DEMO reactor concept, for which design data are sufficient for a fairly quantitative evaluation of the relevant parameters. It is found that the unavoidable presence of elements requiring saturation before they are able to release significant amounts of T clamps the T production rate to the same order of magnitude as D-D reaction rate. Thus, under very optimistic assumptions, several hundreds of full-current D-D discharges are necessary for T to be available for plasma fueling, but more realistic estimates let this number raise up to several thousands.