About the Next Step in the Development of a Tokamak Fusion Reactor *

1. About the Next Step in the Development of a Tokamak Fusion Reactor* Ernesto Mazzucato Princeton Plasma Physics Laboratory 6th Symposium on Current Trends in International Fusion Research Washington, D.C., March 7-11, 2005 *Opinions and comments are the author's and not necessarily shared by PPPL, but they should be.

2. Introduction Fm= The Holy Grail Fm=1 (Q= ) US Plan for Development of Tokamak Fusion Reactors Tomorrow +30 years ITER Fm=0.6 (Q=10) Tomorrow +35 years DEMO Fm=1.0 (Q= ) Past Results and Future Goal • In 1997, JET achieved Fm=0.1 (Q=0.6). • Since then, the general consensus is that the next step towards a tokamak fusion reactor must be a series of DT burning plasma experiments for the exploration of -dominated plasmas.

3. Outline The Present Plan is: SlowITER for 30 years – 10 years for construction, 20 years for operation & decommissioning. Modest The U.S. will play a minor role in ITER, on a par with China, South Korea and probably India – another type of outsourcing? Unwise Present understanding of tokamaks was the synergistic contribution of many experiments. ITER will put an end to this type of collaboration. RiskyThe promise of a DEMO in 35 years is totally devoid of any credibility. • The plan must be redirected towards a true synergistic international collaboration without relying on a single experiment for addressing the physics of burning plasmas. • ITER must be downsized without compromising any of its scientific objectives. Outline How to achieve the objectives of ITER with a smaller long-pulse tokamak

4. ITER • What makes ITER large and expensive is the goal of addressing both scientific and technical issues in an integrated fashion. This is why ITER will use reactor relevant superconducting coils producing a field of 5.3 T at the plasma center. • ITER can be downsized by increasing the magnetic field, i.e., using resistive coils. • Large superconducting magnets – needed for fusion reactors – could be developed in parallel and independently of physics experiments possibility of testing different magnet designs with B>5.3 T. • Another technical objective of ITER is to provide a high fluence neutron source for testing fusion materials and components – this could be done with smaller tokamaks as well. • ITER was designed to achieve Q=10 in the ELMy H-mode, whose global energy confinement time was assumed to follow the empirical scaling law (with tE[s], Ip[MA], B [T], n [1020 m-3], R [m], P=Pa+Paux[MW]). • This scaling law can be used for finding the parameters of a smaller burning plasma experiment capable of achieving all scientific objectives of ITER.

5. Scale-down of ITER • In the operation of tokamaks, two parameters of crucial importance are the normalized plasma density nN=n/nG with nG=Ip/pa2,and normalized plasma beta • bN=b/(Ip/Ba) Experiments indicate that plasma confinement degrades very quickly as the value of nN approaches unity (ITER assumes nN≈0.85). For constant nN and q, we get • Experiments indicate that ideal MHD instabilities (kink and/or ballooning) and neo-classical tearing modes limit bN to values of ~2 (ITER assumes bN≈1.8). For constant bN, q and plasma temperature we get • From the empirical scaling law, then, for a self similar scaling of ITER at constant Q, q and plasma temperature we get for either constant nN or constant bN. This is similar to the prediction of the Gyro-Bohm scaling r*-3 (where wc is the cyclotron frequency and r*= ri /a is the normalized ion Larmor radius).

6. Scale-down of ITER • In terms of non-dimensional variables, the empirical scaling law can be written as • showing a strong degradation in confinement with plasma b. • Recent experiments on both DIII-D and JET have confirmed the dependence on r* but disagree with that on b and n*, showing virtually no dependence on b and a dependence on n* of the form n*-0.3. This triggered a reexamination of the ELMy H-mode database producing the new scaling • where S=(pa2k) is the plasma poloidal cross section. • Repeating the previous exercise, we get • for a constant nN, and • for a constant bN.

7. Minor Radius vs. Toroidal Field @ constant nN Minor radius vs. toroidal magnetic field for self similar scaling of ITER at constant Q, q, T and nN. Solid lines are for the two empirical scaling laws, dashed line for Gyro-Bohm scaling. Triangles represent ITER, FIRE and IGNITOR, circle a smaller version of ITER (Tokamak Fusion Reactor Experiment).

8. Minor Radius vs. Toroidal Field @ constant bN Same at constant bN

9. An alternative to ITER Tokamak Fusion Reactor Experiment B=8.0 T a=1.2 m A=3.6 k=1.8 q95=3.2 Ip=12 MA Poloidal cross section of (from left) IGNITOR, FIRE, TFRX and ITER

10. Comparison Illustration of (from left) IGNITOR, FIRE, TFRX (rescaled Iter) and ITER

11. Q ~30 with both Old and New Scaling Law New Scaling Law Old Scaling Law • ITER assumptions: Zeff =1.65, tHe /tE=5, no/<n>=1.1 • Q ~30 & PF =400 MW (red dots) with: nN =0.85, bN =1.3, bT=1.7%, bp=0.5, <n>=2.1x1020 m-3, <T>=7 keV • Reaches ignition withZeff =1.4 • Increasing q95 to 3.5 lowers the value of Q to 15 (5 for ITER) Can investigate a wide range of plasma conditions with Q>10 bN=0.8÷2.6, bp=0.3÷1.0, PF=150÷800 MW (impossible with ITER)

12. TFRX vs. FIRE • Since TFRX and FIRE have identical A and k, as an exercise we assume that both TF magnet and OH transformer of TFRX are scaled from those of FIRE. TF Magnet TFRX has: • 8 times larger volume • 4 times longer time constant • 36% lower magnetic stress • 30% larger dissipated ohmic power • 6 times smaller dissipated ohmic power density • 4 times larger flux swing of OH transformer • 5 times longer inductively driven plasma flattop (~100 s)

13. TFRX vs. ITER TFRX has: • 4 times smaller volume • larger safety factor • lower normalized plasma beta • lower poloidal plasma beta • same fusion power • 3 times larger Q • only 40% lower neutron wall loading per pulse same capability of ITER for testing fusion materials

14. Conclusion Final Message: The present plan for development of tokamak fusion reactors must be redirected towards a true synergistic international collaboration without relying on a single experiment for addressing the physics of burning plasmas. • Experimental results on global energy confinement in tokamaks suggest that all scientific objectives of ITER could be achieved with smaller and less costly tokamaks using resistive coils. • We discussed one of these options with (compared to ITER): • 1/4 volume • 1/4 inductively driven flattop • 3 times larger energy gain • same fusion power • much wider range of operation • same test bed for fusion materials • Must be considered fully equivalent – if not superior – to ITER. • Compared to existing projects using copper alloys magnets, has better engineering reliability and flexibility and superior physics performance, albeit at a higher cost. • However, in the spirit of this presentation, both IGNITOR and FIRE should also be considered possible options for the investigation of burning plasmas.