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Studies of Requirements for ITER Disruption Mitigation Systems D.G. Whyte* 1), L.R. Baylor 2), D.J. Campbell 3), R. Granetz 1), M. Gluga 3), R. Hawryluk 4), R. Pearce 3), S. Maruyama 3), M. Sugihara 3), J.C. Wesley 5), and M. Wykes 3) for the ITER Organization, ITER Domestic Agencies, and ITER Collaborator 1) MIT Plasma Science and Fusion Center, Cambridge, Massachusetts 02139, USA, 2) Oak Ridge National Laboratory, Oak Ridge, Tennessee 37931, USA, 3) ITER-IO, Cadarache 13108 Saint Paul Lez Durance, France, 4) Plasma Physics Laboratory, Princeton University, Princeton, New Jersey, USA, 5) General Atomics, San Diego, California 92186-5608, USA *e-mail contact of main author: [email protected] 1. Introduction There are clear incentives in ITER to both avoid the occurrence of disruptions whenever possible and to reduce the deleterious effects of such disruptions that do occur. Disruption damage may limit the usable lifetime of the affected internal components, and time consuming reconditioning of plasma facing surfaces after disruptions will likely be required for subsequent discharges. Requirements for disruption mitigation fall into three categories: 1. Reduction of thermal loading on divertor and first-wall plasma facing component surfaces, and in particular avoiding material thermal limits (melting/ablation). 2. Reduction of electromagnetic forces associated with halo currents. 3. Mitigation of runaway electron conversion in the current quench phase of the disruption. Methods and actions that accomplish these categories of mitigation have been tested and demonstrated with a large degree of success and ITER relevancy in present tokamaks [1-3]. This paper summarizes recent work, conducted under the aegis of the US Burning Plasma Organization and in concert with the ITER Fusion Science and Technology Department, related to establishing recommendations for requirements and design concepts for ITER systems. The initial focus of the work has been on establishing requirements for a single massive gas injection (MGI) system sized to accomplish simultaneously, if possible, all three mitigation objectives. Goals a) To quantify the MGI requirements to the best of our present knowledge, b) Identify critical gaps in science and technology and c) Discuss strategies for the deployment and testing of disruption mitigation in ITER. 2. Physics Basis & Specifications for ITER MGI System Recent studies combining empirical observations [4,5] and numerical modeling [6,7] have provided better understanding of the mitigation phenomena of MGI in present devices. • Injected impurities strongly cool the boundary plasma • Progressive cooling produces unstable current profile with evolving and growing islands; cooling and island growth at q=2 finally invokes a rapid cooling of the entire plasma through both convection and impurity mixing. • The core energy is effectively dissipated by impurity radiation, • A cold, dense poorly conducting outer region results in a significant reduction of wall heating. • Poloidal halo currents are reduced by forcing a high resistivity, and therefore rapid resistive decay, of the core plasma which prevents the plasma [3,8] from strongly limiting on the wall, such as is the case in unmitigated vertical displacement induced disruptions. • Better runaway electron avoidance than with “killer pellets” Open research question: Exact toroidal and poloidal distribution of the radiated energy, since the Be wall of ITER may melt if the radiation pattern is too asymmetric. Thermal/electromagnetic (T/E) mitigation requirements: • n impurity ~ 5x10 21 m -3 (gas particles / plasma volume) introduced on a several millisecond timescale. For ITER ~ 5x10 24 ~ 20 kPa-m 3 • Further optimization is possible by mixing high-Z noble gases at low percentage with lighter (faster) carrier gases [10]. 2. Physics Basis & Specifications: Runaway Electrons Runaway electrons (RE) “beams” can be produced and sustained in the high electric fields, E (V/m), of the current quench. • If a substantial RE beam (> MA) is produced and eventually lost into the material structures the localized heating and relatively deep penetration of relativistic electrons could substantially damage first wall components. The specific concern in extrapolation of runaway generation to ITER is the effect of “knock-on’ avalanche amplification [11]. The RE rate equation: I seed is the seed RE population produced by the Dreicer or hot-tail [12] mechanism in the rapidly cooling plasma of the thermal quench, γ RA is the avalanche growth–rate, and γ loss is a generic loss rate due to RE transport out of the plasma. Rosenbluth found γ RA as a function of collisional slowing rate, normalized electric field E/E c and impurity nuclear charge Z [11] demonstrated in Fig. 1 Connor and Hastie [13] showed that if E falls below the critical electric field, E c (V/m) ~ 10 -21 n e , RE are not possible; a limit which was which was modified by Rosenbluth [11] such that n e includes all electrons (free + bound) that may be present during disruption mitigation. Therefore if the total electron density in the plasma volume can be raised to this Connor-Hastie-Rosenbluth (CHR) limit, E c ≥ E, then collisional suppression of RE is guaranteed (Figures 1-2). If E ≥ E c , γ RA is positive, exponential growth of I seed occurs. Integrating through the current quench duration τ CQ (s), I RE ~ I seed exp{(γ RA - γ loss ) τ CQ } ~ I seed exp{G}, G is the exponential gain factor. - At fixed resistivity, which we expect for mitigated disruptions, τ CQ ∝ S ~ κ a 2 plasma area. - Difficult extrapolating MGI RE avoidance from present devices, G is significantly larger for ITER due to its size, such that exp(G) becomes so large (G > 15, Fig. 2) that extremely small seed currents can lead to nearly full conversion of plasma current to RE [11], even though γ RA is of similar Fig. 2 Example 0-D model predictions of E/E c and exponential RE gain factor, G, for varying total gas particle injections (in plasma volume) and different neon-deuterium mixtures. 2. Physics Basis & Specifications: Runaway Suppression Assuming γ loss = 0 we use two methods used to calculateE and E c Method 1 • Empirical database [14] of normalized CQ times, τ CQ /S > 1.7 ms/m 2 up to ITER S=21 m 2 • Maximum E (∝ S/τ CQ ) ~ 60 V/m, τ CQ ~ 40 ms. • Table 1: “in-plasma” CHR limit - high-Z reduces N gas since more electrons per atom Method 2 • 0-D radiation energy balance [3,9] code KPRAD, self- consistentely calculates E. • Table 2, Fig. 2: Good agreement to empirical extrapolation for pure noble gases. • Possible tradeoffs in gas mixing for faster response time + RE suppression. • Within x2 of CHR limit RE avalanche (Fig. 2) amplification suppressed as G < 5. Collisional Suppression Calculations • 20% assimilation is based on average experimental measurements of free-electron increase in DIII-D MGI experiments [4] although there is evidence that different species may have higher assimilation factors. • Thermal/Electromagnetic mitigation requirement met. • This recommendation adopts a “worst-case scenario” for N gas such that: a) runaways are assumed to have perfect confinement (γ loss =0), b) the most pessimistic particle assimilation is taken, and c) pure low-Z gases are used for collisional suppression even though high-Z gases require lower N gas (Tables 1-2). • Optimal gas mixture and quantity undetermined: Recommend variety of gases, mixtures thereof and large dynamic range for N gas • The after-injection torus pressure rises to ~300 Pa. • For He injection, the torus and neutral beam cryo-pumps are likely to regenerate. Vacuum recovery time is presently estimated at ~3-4 hours with some system optimization, marginally acceptable for device operations. • Hydrogenic injection would surpass the current deflagration pressure limits for cryo-pump enclosures; Significantly lower levels (~ factor of 20) of hydrogenic injection could be acceptable. • Argon is not preferred due to activation issues. • Neon gas is considered the optimal choice for gas removal. Esimated that ≤ 100 kPa - m 3 injection (assuming 20% assimilation) of neon could be recovered with minimal impact to the ITER operational duty cycle. - A possible concern with pure high-Z injection is more seed electrons Overall, substantial variability in the recovery consequences as gas species is varied. At the same time, the disruption mitigation also depends on the gas species. Hence we suggest that the choice of the optimal gas species, or mixture of species, for ITER MGI should remain open, and that pure or mixed gas injection with significant low-Z (H, He) gases be anticipated. 3. Impact of Injected Gas or Particles 5. Conclusions • Recommended maximum massive gas injection (< 500 kpa-m 3 ) can have impact on ITER operations, esimated at 3-4 hour delay. • Conservativism in gas quanty is warranted since runaway suppression is sufficiently different and challenging than on present devices (G > 20!!). • MGI opimization possible during Hydrogen phase since runaway physics set by size and current, not thermal energy. • Strong recommendation is to assure flexibility in gas quantity, species and mixtures in order to emprically determine optimal disruption mitigation in ITER itself. • Disruption mitigation physics and technology development must be integrated into the ITER research plan. References [1] Chapter 3 ITER Physics Basis, Nucl. Fusion 39 2253 (1999). [2] Progress in the ITER Physics Basis, Nucl. Fusion 47 S128 (2007). [3] D.G. Whyte, et al., J. Nucl. Mater. 313-316 1239 (2003). [4] E.M. Hollmann, et al., Nucl. Fusion 45 1046 (2005). [5] R. Granetz, et al. Nucl. Fusion 26 1001 (2006). [6] V.A. Izzo, Nucl. Fusion 46 541 (2006). [7] V.A. Izzo, et al., Phys. Plasmas 15 056109-1-1 (2008). [8] D.G. Whyte, et al J. Nucl. Mater. 363-365 1160 (2007). [9] M. Bakhtiari, D.G. Whyte, Phys. Plasmas 13 112511 (2006). [10] M. Bakhtiari, et al. Bull. Am. Phys. Soc. 48th Annual Meeting of the Division of Plasma Physics, 2006, Philadelphia PA, J01.00006 (2006). [11] M.N. Rosenbluth, et al., Nucl. Fusion 37 955 (1997). [12] R.W. Harvey et al. Phys. Plasmas 7 4590 (2000). [13] J.W. Connor, R.J. Hastie Nucl. Fusion 15 415 (1975). [14] J.C. Wesley, et al., “Disruption Characterization and Database Activities for ITER,” Proc. 21st IAEA Fusion Energy Conf., Chengdu, China, 2006, IT/P1-21 (2006). [15] R. Granetz, et al. "Massive D2 killer pellet injection experiments on Alcator C-Mod," U.S. Disruption Physics R&D Report for FY 1997," Report ITER/US/97/PH-14 (1997). [16] H. Tamai, et al. Nucl. Fusion 42 290 (2002). [17] M. Lehnen, et al. Phys. Rev. Lett. 100 255003 (2008). Acknowledgments This work was supported in part by the US Department of Energy under DE-AC05-00OR22725, DE-AC02-76CH03073, and DE-FC02-04ER54698. This report was prepared as an account of work by or for the ITER Organization. The Mem- bers of the Organization are the People's Republic of China, the European Atomic Energy Community, the Republic of India, Japan, the Republic of Korea, the Russian Federation, and the United States of America. The views and opinions ex- pressed herein do not necessarily reflect those of the Mem- bers or any agency thereof. Dissemination of the information in this paper is governed by the applicable terms of the ITER Joint Implementation Agreement Fig.1 Avalanche growth rate, γ RA vs. total electron density (free+bound) in ITER when E=65 V/m. C-H-R limit for RE suppression is reached at 5x10 22 m-3. 0 5 10 15 0 1 2 3 Injected gas density in plasma volume (particles / m 3 ) 10 21 10 22 10 23 E / E c Exponential gain factor G Complete RE suppression C-H-R limit Neon fraction to D 2 (%) 5 10 25 100 Massive Gas Injection Recommendations • Motivated to minimize N gas since this drives recovery time - Examine “double-layer” of conservatism behind recommendations, which results in extrapolation ~25x times large n gas than present MGI. Assimilation of Injected Particles: • 20% based on free density increase: may expect neutrals in cold CQ plasmas. Better diagnosis & modeling of neutral assimilation required. • Alternate CQ injection concepts to increase assimilation: pellets, liquids, condensable materials? - Concern is not penetration, but stopping of material in cold CQ plasma - Need laboratory and tokamak validation Runaway electron transport : • If γ loss > γ RA then G < 0 and RE suppression occurs • Harvey et al [12]: stochastic transport losses set by δB/B ~ 0.1% were sufficient to effectively suppress RE amplification - magnetic fluctuation partially explain present RE avoidance below n CHR . - Recent MHD modeling suggests stochastic field development during MGI [7], - external application of stochastic fields using coils was found to suppress RE on JT-60U [16] and recently on TEXTOR [17]. Use RMP on ITER? • Therefore there is a strong probability that a combination of intrinsic and extrinsic stochastic RE losses will substantially reduce the N gas requirements. - Need model and tokamak validation 3. Optimization and Alternate Injection Concepts
