Showing papers by "Princeton Plasma Physics Laboratory published in 2002"

Size scaling of turbulent transport in magnetically confined plasmas.

Abstract: Transport scaling with respect to device size in magnetically confined plasmas is critically examined for electrostatic ion-temperature-gradient turbulence using global gyrokinetic particle simulations. It is found, by varying device size normalized by ion gyroradius while keeping other dimensionless plasma parameters fixed, that fluctuation scale length is microscopic in the presence of zonal flows. The local transport coefficient exhibits a gradual transition from a Bohm-like scaling for device sizes corresponding to present-day experiments to a gyro-Bohm scaling for future larger devices.

The Magnetorotational Instability in a Collisionless Plasma

Abstract: We consider the linear axisymmetric stability of a differentially rotating collisionless plasma in the presence of a weak magnetic field; we restrict our analysis to wavelengths much larger than the proton Larmor radius. This is the kinetic version of the magnetorotational instability explored extensively as a mechanism for magnetic field amplification and angular momentum transport in accretion disks. The kinetic calculation is appropriate for hot accretion flows onto compact objects and for the growth of very weak magnetic fields, where the collisional mean free path is larger than the wavelength of the unstable modes. We show that the kinetic instability criterion is the same as in MHD, namely that the angular velocity decrease outward. However, nearly every mode has a linear kinetic growth rate that differs from its MHD counterpart. The kinetic growth rates also depend explicitly on β, i.e., on the ratio of the gas pressure to the pressure of the seed magnetic field. For β ~ 1 the kinetic growth rates are similar to the MHD growth rates, while for β 1 they differ significantly. For β 1, the fastest growing mode has a growth rate ≈ Ω for a Keplerian disk, larger than its MHD counterpart; there are also many modes whose growth rates are negligible, β-1/2Ω Ω. We provide a detailed physical interpretation of these results and show that gas pressure forces, rather than just magnetic forces, are central to the behavior of the magnetorotational instability in a collisionless plasma. We also discuss the astrophysical implications of our analysis.

Double transport barrier plasmas in Alcator C-Mod

Abstract: Double transport barrier plasmas comprised of an edge enhanced D? (EDA) H-mode pedestal and an internal transport barrier (ITB) have been observed in Alcator C-Mod. The ITB can be routinely produced in ICRF heated plasmas by locating the wave resonance off-axis near |r/a|~0.5, provided the target plasma average density is above ~1.4?1020?m-3, and can develop spontaneously in some Ohmic H-mode discharges. The formation of the barrier appears in conjunction with a decrease or reversal in the central (impurity) toroidal rotation velocity. The ITB foot is located near r/a~0.5, regardless of how the barrier was produced. The ITBs can persist for ~15 energy confinement times (?E), but exhibit a continuous increase of the central electron density, up to values near 1?1021?m-3 (in the absence of an internal particle source), followed by collapse of the barrier. Application of additional on-axis ICRF heating arrests the density and impurity peaking, which occurs along with an increase (co-current) in the core rotation velocity. Quasi-steady state double barrier plasmas have been maintained for 10?E or longer, with a bootstrap fraction of 0.13 near the ITB foot.

The Magnetorotational Instability in a Collisionless Plasma

Abstract: We consider the linear axisymmetric stability of a differentially rotating collisionless plasma in the presence of a weak magnetic field; we restrict our analysis to wavelengths much larger than the proton Larmor radius. This is the kinetic version of the magnetorotational instability explored extensively as mechanism for magnetic field amplification and angular momentum transport in accretion disks. The kinetic calculation is appropriate for hot accretion flows onto compact objects and for the growth of very weak magnetic fields, where the collisional mean free path is larger than the wavelength of the unstable modes. We show that the kinetic instability criterion is the same as in MHD, namely that the angular velocity decrease outwards. However, nearly every mode has a linear kinetic growth rate that differs from its MHD counterpart. The kinetic growth rates also depend explicitly on beta, i.e., on the ratio of the gas pressure to the pressure of the seed magnetic field. For beta ~ 1 the kinetic growth rates are similar to the MHD growth rates while for beta >> 1 they differ significantly. For beta >> 1, the fastest growing mode has a growth rate of sqrt{3} Omega for a Keplerian disk, larger than its MHD counterpart; there are also many modes whose growth rates are negligible, < beta^{-1/2} Omega << Omega. We provide a detailed physical interpretation of these results and show that gas pressure forces, rather than just magnetic forces, are central to the behavior of the magnetorotational instability in a collisionless plasma. We also discuss the astrophysical implications of our analysis.

Stability analysis of improved confinement discharges: internal transport barriers in Tore Supra and radiative improved mode in TEXTOR

Abstract: Results of stability analysis are presented for two types of plasma with good confinement: internal transport barriers (ITBs) on Tore Supra and the radiative improved (RI) mode on TEXTOR. The stability analysis has been performed with an electrostatic linear gyrokinetic code, evaluating the growth rates of microinstabilities. The code developed, KINEZERO, is aimed at systematic microstability analysis. Therefore the trade-off between having perfect quantitative agreement and minimizing computation time is made in favour of the latter. In the plasmas analysed, it is found that the onset of the confinement improvement involves a trigger. For the ITB discharges, negative magnetic shear is involved, whereas for the RI discharges, the triggering role is played by the increase of the impurity concentration. Once the improved confinement is triggered, the simultaneous increases of temperature and density gradients imply an increase in both the growth rate and the rotation shearing rate. The rotation shear is found to be high enough to maintain an improved confinement through the stabilization of the large scale modes.

Influence of the q-profile shape on plasma performance in JET

Abstract: The fusion performance of JET plasmas can be enhanced by the generation of internal transport barriers The influence of the q-profile shape in the local and global plasma performance has been investigated in cases where the core magnetic shear ranges from small and positive to large and negative Internal barriers extending to large plasma radii can be effective in raising the global performance of the plasma It is found that such barriers tend to be generated more easily if the q-profile contains a region of negative magnetic shear The formation is favoured by neutral beam injection compared with ion cyclotron resonance heating in scenarios where the two systems are used together The minimum power level required to observe a local transport reduction is significantly lower than the value at which very steep pressure gradients can be achieved This results in a practical threshold in the power to access a regime of high plasma performance that is sensitive to the q-profile shape

Force-freeness of solar magnetic fields in the photosphere

Abstract: It is widely believed that solar magnetic fields are force-free in the solar corona but not in the solar photosphere at all. In order to examine the force-freeness of active region magnetic fields at the photospheric level, we have calculated the integrated magnetic forces for 12 vector magnetograms of three flare-productive active regions. The magnetic field vectors are derived from simultaneous Stokes profiles of the Fe I doublet λλ6301.5 and 6302.5 obtained by the Haleakala Stokes Polarimeter of Mees Solar Observatory, with a nonlinear least-squares method adopted for field calibration. The resulting vertical Lorentz force normalized to the total magnetic pressure force |Fz/Fp| ranges from 0.06 to 0.32 with a median value of 0.13, which is smaller than the values (~0.4) obtained by Metcalf et al., who applied a weak field derivative method to the Stokes profiles of Na I λ5896. Our results indicate that the photospheric magnetic fields are not so far from force-free as conventionally regarded. As a good example of a linear force-free field, NOAA Active Region 5747 is examined. By applying three different methods (a most probable value method, a least-squares fitting method, and comparison with linear force-free solutions), we have derived relatively consistent linear force-free coefficients for NOAA AR 5747. It is found that the scaled downward Lorentz force (|Fz/Fp|) in the solar photosphere decreases with increasing |α|. Our results also show that the force-freeness of photospheric magnetic fields depends not only on the character of the active region but also on its evolutionary status.

Effects of segmented electrode in Hall current plasma thrusters

Abstract: Segmented electrodes placed along a ceramic channel in a Hall thruster are shown to influence significantly the plasma potential distribution. Both the radial potential and the axial acceleration region are sensitive to the location of the segmented electrodes. The measured and theoretical potential profiles appear to be affected in detail by the electrode material ~graphite! having lower secondary electron emission than the ceramic channel walls. The measured plasma potential profile is shown as well to correlate with the observed and desirable narrowing of the plasma plume emanating from the thruster. © 2002 American Institute of Physics. @DOI: 10.1063/1.1510556#

Recent progress of Alfvén eigenmode experiments using N-NB in JT-60U tokamak

Abstract: Bursting modes in the frequency range of the toroidicity induced Alfv?n eigenmode are observed in the plasma to which the negative-ion-based neutral beam (N-NB) is injected. A bursting mode changes its frequency by 10-20?kHz in 1-5?ms and is called fast frequency sweeping (fast FS) mode. Another bursting mode evolves explosively in ?400??s and is called an abrupt large-amplitude event. The dependence of their saturation level was compared?with the experimentally observed growth rate and damping rate. The mode amplitude increases with the observed growth rate for fast FS modes. The modes with large amplitude and a large enhanced transport were observed when a large neutron emission rate was observed. The burst period increases as the drop ratio of the neutron emission increases.

Simulations of beam ion transport during tearing modes in the DIII-D tokamak

Abstract: Large coherent MHD modes are observed to reduce the neutral beam current drive efficiency and 2.5 ,MeV neutron emission in DIII-D by as much as ~65%. These modes result in large (width w20 cm for minor radius a≈60 cm), stationary, single helicity magnetic islands, which might cause anomalous deuterium beam ion losses through orbit stochasticity. An analytic estimate predicts that co-going, passing deuterons with E40 keV become stochastic at island widths comparable to those in the experiment. A Hamiltonian guiding centre code is used to follow energetic particle trajectories with the tearing mode modelled as a radially extended, single helicity perturbation. In the simulations, the lost neutral beam current drive and neutron emission are 35% and 40%, respectively, which is consistent with the measured reductions of 40±14% and 40±10%. Several features of the lost particle distribution indicate that orbit stochasticity is the loss mechanism in the simulations and strongly suggest that the same mechanism is responsible for the losses observed in the experiment.

Stabilization of the resistive wall mode in DIII-D by plasma rotation and magnetic feedback

Abstract: Suppression of the resistive wall mode (RWM) has been successfully demonstrated in the DIII–D tokamak by using rotational stabilization in conjunction with a close-fitting vacuum vessel wall. The duration of the high-pressure discharge was extended to hundreds of times the wall skin time. Frequently, the plasma pressure reached the ideal-wall magnetohydrodynamic (MHD) kink limit. The confined pressure is up to twice as high as the no-wall ideal MHD kink limit. Near its marginal stability point, the RWM is found to resonate with residual non-axisymmetric fields (e.g. components of the error field). A magnetic feedback system has been used to identify and compensate for the residual non-axisymmetric fields. This is to the best of our knowledge, the first demonstration of the sustainment of a stable plasma with pressure at levels well above the no-wall pressure limit. This technique is expected to be applicable to other toroidal devices.

Radio frequency energy effects on microorganisms in foods

Abstract: Liquids containing microorganisms were exposed to radio frequency (RF) energy to study non-thermal inactivation. RF energy was applied to the liquids while heat was simultaneously removed to control temperature. Turbulent flow was maintained to minimize localized heating. An 18 MHz RF processor applied an approximately 0.5 kV/cm electric field strength to the liquids. It was capable of pasteurizing the liquids provided that cooling was minimized. There were no non-thermal effects of RF energy detected on Escherichia coli K-12, Listeria innocua, or yeast in apple cider, beer, deionized water, liquid whole egg, and tomato juice; nor were there any synergistic effects of RF energy with heat. The low temperature effects of RF energy at 18 MHz and 0.5 kV/cm were due to heat.

Neutral beam stabilization of sawtooth oscillations in JET

Abstract: Recent experiments in the Joint European Torus (JET) have provided evidence of sawtooth stabilization by fast ions arising from deuterium neutral beam injection (NBI). A possible theoretical basis for the interpretation of the observed sawtooth period behaviour is investigated and predictions are compared with experimental results, using a sawtooth period model developed to predict the sawtooth period in the International Thermonuclear Experimental Reactor (ITER). Unlike the case of ion cyclotron resonance heating, a detailed comparison between theory and experiment for NBI has not yet been made. In the model employed in this paper, a beam ion contribution to the internal kink potential energy has been incorporated, using a simple analytical expression valid in the limit of isotropic fast particles. This analytical expression has been found to compare well with detailed calculations performed with a hybrid kinetic/MHD code NOVA-K, using fast particle distribution functions computed with a plasma analysis code TRANSP. The beam ion contribution has been implemented in a transport code PRETOR and a few representative JET discharges have been analysed and modelled. The beam ion term is found to be sufficiently stabilizing to produce simulated sawtooth periods in agreement with the experimental results. Sawtooth periods computed without taking this term into account are much shorter than the measured periods. The model indicates that sawteeth are triggered in these JET discharges by excitation of the internal kink in the semi-collisional ion-kinetic regime: this was found by previous authors to be the sawtooth trigger most likely to be relevant to ITER. The role of beam ions in determining the sawtooth period in JET is thus found to be similar to the predicted role of α-particles in ITER.

Computational grids in action: the national fusion collaboratory

Abstract: The National Fusion Collaboratory (NFC) project was created to advance scientific understanding and innovation in magnetic fusion research by enabling more efficient use of existing experimental facilities through more effective integration of experiment, theory, and modeling. To achieve this objective, NFC introduced the concept of "network services", which build on top of computational Grids, and provide Fusion codes, together with their maintenance and hardware resources as a service to the community. This mode of operation requires the development of new authorization and enforcement capabilities. In addition, the nature of Fusion experiments places strident quality of service requirements on codes run during the experimental cycle. In this paper, we describe Grid computing requirements of the Fusion community, and present our first experiments in meeting those requirements.

A Virtual Test Facility for the Simulation of Dynamic Response in Materials

Abstract: The Center for Simulating Dynamic Response of Materials at the California Institute of Technology is constructing a virtual shock physics facility for studying the response of various target materials to very strong shocks. The Virtual Test Facility (VTF) is an end-to-end, fully three-dimensional simulation of the detonation of high explosives (HE), shock wave propagation, solid material response to pressure loading, and compressible turbulence. The VTF largely consists of a parallel fluid solver and a parallel solid mechanics package that are coupled together by the exchange of boundary data. The Eulerian fluid code and Lagrangian solid mechanics model interact via a novel approach based on level sets. The two main computational packages are integrated through the use of Pyre, a problem solving environment written in the Python scripting language. Pyre allows application developers to interchange various computational models and solver packages without recompiling code, and it provides standardized access to several data visualization engines and data input mechanisms. In this paper, we outline the main components of the VTF, discuss their integration via Pyre, and describe some recent accomplishments in large-scale simulation using the VTF.

Storing, retrieving, and processing optical information by Raman backscattering in plasmas.

Abstract: By employing stimulated Raman backscattering in a plasma, information carried by a laser pulse can be captured in the form of a very slowly propagating plasma wave that persists for a time long compared with the pulse duration. If the plasma is then probed with a short laser pulse, the information stored in the plasma wave can be retrieved in a second scattered electromagnetic wave. The recording and retrieving processes can conserve robustly the pulse shape, thus enabling the recording and retrieving with fidelity of information stored in optical signals.

Confinement properties of high density impurity seeded ELMy H-mode discharges at low and high triangularity on JET

Abstract: The design value for ITER is based on operation at n/nGW = 0.85, ?n = 1.8 and H98(y,2) = 1. These values have been routinely achieved in JET in argon seeded ELMy H-mode discharges in different divertor configurations and with different triangularities. Two main scenarios are emerging from the experiments. First, low triangularity (?u = 0.19) in septum configuration. In this case large D2 fuelling rates lead to confinement degradation towards L-mode. The seeding of Ar during the D2 fuelling phase gives rise to a density close to the Greenwald value. After the switch-off of the D2 gas fuelling (`afterpuff' phase), the confinement recovers to H-mode quality whereas the density stays near the value reached at the end of the main fuelling phase and Zeff stays close to or below 2. Acting on the refuelling of Ar and D2 in the `afterpuff' phase allows us to improve the stationarity of the high performance phase while maintaining up to the end of the heating phase the good confinement, density and radiation level. Second, high triangularity (?u = 0.45) in vertical target configuration. In this case large fuelling rates do not lead to strong confinement degradation and the D2 fuelling is applied continuously throughout the discharge. A radiated power fraction of up to 70 %, H98(y,2) = 0.9 at ?n = 2.1 and n = 1.15nGW - together with the formation of a radiating mantle and moderate Zeff - are achieved in this scenario. Furthermore, there are indications of significantly reduced heat load on the divertor target plates.

Radiation pattern and impurity transport in argon seeded ELMy H-mode discharges in JET

Abstract: This paper addresses the issues of impurity behaviour during the argon seeding experiments in JET, in which argon and D2 have been simultaneously puffed in ELMy H-mode discharges to reach high density regimes, maintaining good confinement properties throughout the plasma discharge. The analysis is based mainly on a 1-D impurity diffusion model, which evaluates the impurity transport coefficients in three experimental scenarios. The available experimental data include, as a function of the time, the brightnesses of the soft x-rays and of several impurity lines, the effective charge Zeff, the profiles of the radiated power and the fully stripped Ar and C ion densities from charge-exchange recombination. From the simulation of these data it is possible to characterize the considered scenarios in terms of high-Z ion increase in the plasma centre and high radiation from the edge. In particular, it has been found that the configuration at high triangularity with continuous D2 puffing, characterized by an outward impurity pinch velocity, features flat or slightly hollow impurity ion profiles and high radiation from a narrow region at the edge, possibly associated to the presence of a significant neutral density. In this case an edge radiative mantle is established without high radiation from the plasma centre.

Progress towards increased understanding and control of internal transport barriers in DIII-D

Abstract: Substantial progress has been made towards both understanding and control of internal transport barriers (ITBs) on DIII-D, resulting in the discovery of a new sustained high performance operating mode termed the quiescent double barrier (QDB) regime. The QDB regime combines core transport barriers with a quiescent ELM-free H mode edge (termed QH mode), giving rise to separate (double) core and edge transport barriers. The core and edge barriers are mutually compatible and do not merge, resulting in broad core profiles with an edge pedestal. The QH mode edge is characterized by ELM-free behaviour with continuous multiharmonic MHD activity in the pedestal region and has provided density and radiated power control for longer than 3.5 s (25τE) with divertor pumping. QDB plasmas are long pulse high performance candidates, having maintained a βNH89 product of 7 for five energy confinement times (Ti≤16 keV, βN≤2.9, H89≤2.4, τE≤150 ms, DD neutron rate Sn≤4×1015 s-1). The QDB regime has only been obtained in counter-NBI discharges (injection antiparallel to the plasma current) with divertor pumping. Other results include successful expansion of the ITB radius using (separately) both impurity injection and counter-NBI, and the formation of ITBs in the electron thermal channel using both ECH and strong negative central shear (NCS) at high power. These results are interpreted within a theoretical framework in which turbulence suppression is the key to ITB formation and control, and a decrease in core turbulence is observed in all cases of ITB formation.

Predictive capability of mhd stability limits in high performance diii-d discharges

Abstract: Results from an array of theoretical and computational tools developed to treat the instabilities of most interest for high performance tokamak discharges are described. The theory and experimental diagnostic capabilities have now been developed to the point where detailed predictions can be productively tested so that competing effects can be isolated and either eliminated or confirmed. The linear MHD stability predictions using high quality discharge equilibrium reconstructions are tested against the observations for the principal limiting phenomena in DIII-D: L mode negative central shear (NCS) disruptions, H mode NCS edge instabilities, and tearing and resistive wall modes (RWMs) in long pulse discharges. In the case of predominantly ideal plasma MHD instabilities, agreement between the code predictions and experimentally observed stability limits and thresholds can now be obtained to within several per cent, and the predicted fluctuations and growth rates to within the estimated experimental errors. Edge instabilities can be explained by a new model for edge localized modes as predominantly ideal instabilities with low to intermediate toroidal mode number. Accurate ideal calculations are critical to demonstrating RWM stabilization by plasma rotation, and the ideal eigenfunctions provide a good representation of the RWM structure when the plasma rotation slows. Ideal eigenfunctions can then be used to predict stabilization using active feedback. For non-ideal modes, the agreement in some cases is promising. Δ' calculations, for example, indicate that some discharges are linearly unstable to classical tearing modes, consistent with the observed growth of islands in those discharges. Nevertheless, there is still a great deal of improvement required before the non-ideal predictive capability can routinely approach levels similar to those for the ideal comparisons.

Magnetic Field Generation through Angular Momentum Exchange between Circularly Polarized Radiation and Charged Particles

Abstract: The interaction between circularly polarized radiation and charged particles can lead to generation of magnetic field through an inverse Faraday effect. The spin of the circularly polarized electromagnetic wave can be converted into the angular momentum of the charged particles so long as there is dissipation. We demonstrate this by considering two mechanisms of angular momentum absorption relevant for laser-plasma interactions: electron-ion collisions and ionization. The precise dissipative mechanism, however, plays a role in determining the efficiency of the magnetic-field generation.

Theoretical modelling of the feedback stabilization of external MHD modes in toroidal geometry

Abstract: A theoretical framework for understanding the feedback mechanism for stabilization of external MHD modes has been formulated. Efficient computational tools - the GATO stability code coupled with a substantially modified VACUUM code - have been developed to effectively design viable feedback systems against these modes. The analysis assumed a thin resistive shell and a feedback coil structure accurately modelled in θ and , albeit with only a single harmonic variation in . Time constants and induced currents in the enclosing resistive shell are calculated. An optimized configuration based on an idealized model has been computed for the DIII-D device. Up to 90% of the effectiveness of an ideal wall can be achieved.

Characteristics of the first H-mode discharges in the national spherical torus experiment.

Abstract: Summary. —In summary, we have induced H -modedischarges in NSTX, in which the energy confinementtime increased transiently by between 100% 200%.These H modes had energy confinement well above ELM-free H -mode scaling laws, and had a significantly greaterthreshold power than predicted. Thus H modes in NSTXwill eventually help extend the confinement and thresholdpower scalings to low aspect ratio. Finally, H modes havebroader pressure profiles than L modes (e.g., the pressurepeaking factor was reduced by 15%in NSTX), andbroad profiles generally have higher b limits in tokamaksdue to improved low- n kink stability, e.g., TFTR [20] andDIII-D [21]. Thus achievement of H modes is a potentialpath for achieving higher b in NSTX.This research was supported by the U.S. Departmentof Energy under Contract No. DE-AC05-00OR22725,No. DE-AC02-76CH03073, No. W-7405-ENG-36, andGrant No. DE-FG02-99ER54524. We gratefully acknowl-edge the contribution of the NSTX technical staff and neu-tral beam operations staff, as well as useful discussionswith R. Akers, M. Gryaznevich, and A. Sykes from theMAST team.

Eliminating Islands in High-Pressure Free-Boundary Stellarator Magnetohydrodynamic Equilibrium Solutions

Abstract: Magnetic islands in free-boundary stellarator equilibria are suppressed using a procedure that iterates the plasma equilibrium equations and, at each iteration, adjusts the coil geometry to cancel resonant fields produced by the plasma. The coils are constrained to satisfy certain measures of engineering acceptability and the plasma is constrained to ensure kink stability. As the iterations continue, the coil geometry and the plasma simultaneously converge to an equilibrium in which the island content is negligible. The method is applied with success to a candidate plasma and coil design for the National Compact Stellarator Experiment [Phys. Plasmas 8, 2083 (2001)].

Ion and electron acceleration in the field-reversed configuration with an odd-parity rotating magnetic field

Abstract: The method for accelerating ions and electrons in the field-reversed configuration using odd-parity rotating magnetic fields (RMFs) in the ion-cyclotron range-of-frequencies (ICRF) is studied The approach is based on long, accurate numerical integration of Hamilton’s equations for single-particle orbits Rapid ion heating to thermonuclear conditions occurs in <01 ms in a modest-sized FRC Strong variation of the magnetic-field strength over the confinement region prevents a true cyclotron resonance, resulting in stochastic though effective heating Lyapunov exponents are computed to demonstrate chaotic orbits Electrons are also effectively heated in this frequency range, primarily by a mechanism involving trapping in the wells of the azimuthal electric field Odd-parity RMF promotes oppositely directed ion and electron motion near the minor axis, appropriate for supporting the plasma current

A Plan for the Development of Fusion Energy

Abstract: This is the final report of a panel set up by the U.S. Department of Energy (DOE) Fusion Energy Sciences Advisory Committee (FESAC) in response to a charge letter dated September 10, 2002 from Dr. Ray Orbach, Director of the DOE's Office of Science. In that letter, Dr. Orbach asked FESAC to develop a plan with the end goal of the start of operation of a demonstration power plant in approximately 35 years. This report, submitted March 5, 2003, presents such a plan, leading to commercial application of fusion energy by mid-century. The plan is derived from the necessary features of a demonstration fusion power plant and from the time scale defined by President Bush. It identifies critical milestones, key decision points, needed major facilities and required budgets. The report also responds to a request from DOE to FESAC to describe what new or upgraded fusion facilities will “best serve our purposes” over a time frame of the next twenty years.