Bromberg, L.

Cohn, D.R.

Myer, R.C.

Schwartz, J.

Williams, J.E.

DSpace@MIT

DOE/ET-51013Prospects for a High Field ITER Device.L.Bromberg, R.C. Myer, D.R. Cohn,J. Schwartzt and J.E.C. WilliamsMIT Plasma Fusion Center*, Cambridge, Ma 02139February 17, 1989t Supported by U.S.D.O.E. Contract DE-AC02-78ET51013.f This work performed under appointment to the Magnetic Fusion Energy Fel-lowship which is administered for the U. S. Department of Energy by Oak RidgeAssociated Universities.1PFC/JA-89-13ABSTRACTScoping studies of a high-field ITER device have been performed. Possibleadvantages of high field operation include: reduced machine size and cost, de-creased fusion power and tritium consumption, lower current and higher densityoperation (which may increase the design window for the divertor targets). Theuse of high aspect ratio minimizes the need for increased field at the superconduc-tor by increasing the ratio between the field at the plasma and the peak field atthe coil. Higher aspect ratio also results in stresses in the magnet structure thatare within present ITER design limits. The current densities in the conductorsare consistent with the use of Nb 3 Sn. The impact of high field on steady statecurrent drive is evaluated.2IntroductionThe ITER tokamak activity is an international collaboration'. The missionof ITER is to advance both the physics and technology base necessary to build ademonstration reactor. The present ITER design is based on a set of parametersresulting from a compromise between different empirical scaling laws on the onehand and the engineering and cost considerations on the other. There is a perceivedneed for high plasma current. This approach to assure ignition in ITER requiresa large plasma current ( I-: 20 MA) resulting in relatively low magnetic field andaspect ratio (BT = 5 T, A = 3).From an engineering standpoint, the present ITER baseline design representsa conservative approach. However, the modest level of magnetic field and lowaspect ratio design lead to a relatively large machine size and cost. The impactof the ITER approach on the cost of more advanced reactors is also a concern. Inthis paper we discuss the use of high magnetic fields in machines with the ITERmission. Plasma size and current are traded off for higher fields and aspect ratio.In addition to the possible cost reduction resulting from high field opera-tion in ITER type devices, there are several additional benefits which may berealized2' 3 ,4',: (1) lower fusion power and reduced tritium consumption for thesame neutron wall loading, (2) increased plasma density at the plasma edge, withthe possible consequence of decreased the plasma temperature and target erosion,and (3) reduced plasma current. The reduced plasma current reduces the me-chanical load during disruptions and also increases the margin of safety againstdisruptions, hence improving prospects for CW testing of components.The purpose of this report is to summarize a study of the the possible ben-efits and risks of increasing the magnetic field above the current ITER baselinedesign. Although further substantial gains could be obtained by utilizing a lessconservative engineering approach 4 , in this paper the magnet engineering con-straints of stress and current density are held constant at the values used in thepresent ITER design.3Minimum Cost Performance Scaling With Aspect RatioTo perform this scoping study we have used the Tokamak Engineering TestReactor Analysis code6 (TETRA) developed by the Fusion Engineering DesignCenter. The code solves a set of nonlinear equations used to describe the sub-systems of a tokamak reactor and performs a local optimization of various figuresof merit (cost, ignition margin, etc.) within parameter limits specified by theuser. The cases discussed below were obtained by locally minimizing the plantdirect costs. We have assumed a set of parameters which follow the ITER designguidelines.In order to be consistent with the present ITER base case design, the neutronwall loading for the technology phase is held fixed at 1 MW/m 2 . The thicknessof the shield, the gaps and the scrapeoff is approximately 1 m. If the plasmais inductively driven, the burn time is 600 s. The plasma elongation (K) is 2.2at the separatrix and the triangularity is 0.4. We have considered aspect ratiosin the range of 3 - 5 (the lower end represents the latest ITER design). Theincreased aspect ratio allows the field on axis to be increased to about 7 - 8 Teslawhile keeping the coil stresses below the 600 MPa value used in the present ITERdesign.A comparison of the parameters for the two minimum cost machines at thelow and high ends in the range of aspect ratios is given in Table .' The illustrativehigh field design has a field of 7 T at the plasma and 12.7 at the Nb 3 Sn magnet.There is a 800 M$ reduction in the tokamak construction cost for the high field,high aspect ratio case. There can also be significant savings in the auxiliaryheating/current drive system for the high field, high aspect ratio design. As will bediscussed further on, the current drive power can be reduced. However, the costingused here assumes that the same current drive power is required independent ofplasma parameters (the direct cost of this system is assumed to be 180M$).As shown in Figure 1, a detailed breakdown of the plant costs show thatinitially there are savings in the cost of the nuclear island with increasing aspect4ratio because of the decreasing size of the tokamak. However, once the peak fieldat the toroidal field magnet reaches approximately 12 Tesla there is an increase inthe cost of the magnets, due to limitations of the critical current density, whichadversely affects the overall cost of the reactor. Further increases in field withoutincreasing the cost could be achieved by slightly modifying the assumptions of theTETRA code, using higher field in the ohmic heating transformer and reoptimizingthe TF magnet design (for example, by grading the conductors in the OH and theTF magnets). However, for the purpose of this paper, the engineering details ofthe design are held constant.A more detailed breakdown of costs and the masses and volumes is given inTable 2. Clearly, the increased cost of the magnets is outweighed by a 40% savingsin the reactor system (first wall, blanket/shield and structure) and a 50% savingsin the vacuum systems. These cost reductions are reflected in the weight of themagnet and shield and the area of the first wall.For the assumptions made in this study (i.e., same engineering of the magnetsand constant ignition margin according to the Kaye-Goldston scaling relation) wefind the minimum cost to occur in the range of aspect ratios between A = 4.0and A = 4.5. This minimum in cost is relatively broad so that the benefits ofoperating at aspect ratios up to 5 may outweigh the slight increase in the cost ofthe magnets. Figure 1 shows the variation of plasma current, fusion power, andtritium consumption as a function of aspect ratio. In the range A = [3,5], we finda 50% reduction in the plasma current and a 40% decrease in the fusion powerand tritium consumption. The reduced fusion power contributes to savings in thecost of the nuclear island, heat removal, and fuel handling systems.The greatest uncertainty by far lies in the scaling of the confinement time7 ,.For most of the scaling laws we find that the plasma current may be traded-offfor the aspect ratio (or field) to give comparable performance at reduced cost orincreased margins at the same cost. The figures of merit (ignition margins) scaleas I(A') where a ~ 1.2 - 1.5. The cases presented in this paper have an ignitionmargin of unity based on Kaye-Goldston H-mode scaling. For neo-Alcator scaling5all of the cases presented here are overignited with an ignition margin in excessof three. Although the figure of merit 2 B 2 a increases modestly, the neo-Alcatorscaling decreases slightly since the fraction of the Murakami limit decreases tomaintain the fixed wall loading. Other scaling laws such as the JAERI scalinglaw do not show an improvement with increasing aspect ratio because of theirstrong dependence on the plasma current and minor radius, both of which decreasesubstantially in these cases. These scaling laws obviously push the design towardthe low aspect ratio, high current approach. Further experimental work needs tobe done in order to sort out the large differences resulting from the present dayscaling relations.Additional Advantages of High Field OperationAs mentioned before, for fixed ignition margin, several scaling laws allow atrade off of plasma current for magnetic field. Decreased plasma current has severaladvantages. The consequences of plasma disruptions are minimized (loads andenergy content of the plasma). As the power required for current drive decreases,steady state operation becomes more attractive (the decrease in the plasma currentand major radius more than balance the increase in the plasma density, resulting inlower values of PSS drive - nIR). For similar current drive efficiency, the currentdrive power requirement for the high field case is 25% lower than the present ITERdesign.Furthermore, at lower values of 3 (higher field) the accessibility of low nilower hybrid waves improves, making more efficient current drive possible9 .Operation at high magnetic field/high aspect ratio is also attractive due tolower fusion power and tritium requirements. For constant wall loading, the totalfusion power is directly proportional to the first wall area. The first wall area isminimized by high field operation (the plasma major radius decreases) and highaspect ratio (the plasma minor radius is also decreased). The tritium requirementsare shown in Table 3 for both the present ITER and the high field design. Sub-6stantial savings in both tritium consumption and inventory are achieved. Reducedtritium requirements could be important in terms of tritium availability, cost andenvironmental impact.The increased magnetic field allows for operation at higher density which canresult in reduced temperature at the divertor target. In high recycling divertor,the edge temperature scales as10 ,11((1 - frad)P) 2 0/9n28/9n,where fad is the fraction of the power radiated, P is the total plasma heatingpower (due to alpha, auxiliary and ohmic heating) and n, is the plasma densityat the edge. To avoid excessive erosion and impurity influx, the edge plasma tem-perature must be limited. With a high-Z divertor plate, the maximum acceptableplasma temperature at the divertor target is ~ 40 eV. Furthermore, the peakpower density at the target is((1 - fad)P)14 ,9qtarget 7/9n,Although not as limiting as the plasma temperature, reduction in the heat load isalso useful.ConclusionsA scoping study has shown that high field, low current operation could pro-vide advantages for the capital cost and operating cost of an ITER-type device.The same rules employed in the present ITER design could be used with the ex-ception of the JAERI confinement scaling and a higher magnetic field at the coil.The machine size and the plasma current have been decreased substantially; thefusion power and the tritium inventory could be reduced by a factor of two; andthe design of the divertor target simplified. More detailed design investigationsare needed to more fully establish these advantages. Finally, further potential ad-vantages of using even larger magnetic fields and reoptimizing the magnet designshould be explored.7AcknowledgmentsThe authors would like to thank Patrick Cooke (Culham) for invaluablediscussions on the plasma edge physics.8Table 1Comparison of Low and High Aspect Ratio ITER DesignsPresent High FieldITER approachAspect ratio 3.0 4.5Major radius (m) 5.80 5.10Toroidal field (T) 4.90 7Plasma current (MA) 18.4 9.9Plant direct cost (M$) 2200 1700Construction cost (M$) 3830 3010Field at TF coil (T) 10.7 12.7Burn time (s) 600. 600.Neutron wall load (MW/m 2 ) 1. 1.Magnet stresses (MPa) 600 600Figure of MeritB 2 a(T 2 m) 46 57Ignition MarginNeo-Alcator 3.3 3Kaye-Goldston (L) 0.7 0.7Kaye-Goldston (H) 1 1Goldston-H 1.25 1.2JAERI 1 0.69Table 2Estimated Costs and Volume ComparisonPresent High FieldITER approachMajor radius (m) 5.8 5.1Reactor system (M$) 500. 290.Magnets (M$) 540. 525.Vacuum systems (M$) 144. 69.Power conditioning (M$) 176. 129.Total direct cost (M$) 2198 1730Construction cost (M$) 3800 3020TF Magnet weight (tons) 1220. 1340.Shield weight (tons) 7000. 3800.First wall area (m2 ) 800. 430.10Table 3Tritium ConsiderationsPresentITER approachHigh FieldMajor radius (m)Tritium initial load (g)Tritium burnup (g/cycle)Tritium burnup (g/day)Tritium exhaust (g/day)Total tritium inventory (g)5.8 5.10.151.461458510000.050.67684048011REFERENCES'K. Tomabechi, Twelfth International Conference on Plasma Physics and Con-trolled Nuclear Fusion Research, Nice, France (October 1988).2 D.R. Cohn and L. Bromberg, J. of Fusion Energy, 5 (3), p. 161 (1986)3 D.R. Cohn, Journal of Fusion Energy, 6 (3), p. 281, (1987)"J. Schwartz, D.R. Cohn, L. Bromberg and J.E.C. Williams, Proceed-ings of the Twelfth Symposium on Fusion Engineering, Volume 2, p. 870(1987).5 R. Myer, L. Bromberg, D.R. Cohn, J. Schultz, J. Schwartz and J.E.C.Williams, Bull. Am. Phys. Soc. 33 no 9, p 1972 (1988)'R.L. Reid et al., ORNL/FEDC-87/7, (1988).7 D. Post, Twelfth International Conference on Plasma Physics and ControlledNuclear Fusion Research, Nice, France (October 1988).8N. Uckan, presented at the Eighth Topical Meeting of Fusion Energy, SaltLake City, Utah (Oct 1988).* S.Y. Yuen, D. Kaplan and D.R. Cohn, Nucl Fusion 20 159 (1980)0 Cooke, P., Culham, England, private communication"Harbour, P.J., Nucl Fusion 24 (1984) p. 1211.12aA000 ---0- -Plasma CurrentA-A-A -Busion Power+-+-+ -Tritium Consumptionx-x-x --Construction costFigure 1. Variation of plasma current, fusion power, tritium consumption, and costwith aspect ratio.13

Prospects for a high field ITER device

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Prospects for a high field ITER device

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