Density peaking and fusion power H. Weisen

1. Density peaking and fusion powerH. Weisen • Role of fuel pressure profile • Effect of He dilution ITPA CDBM 7-10.5 2007

2. Motivation • Peaked fuel profiles have potential to improve fusion power for given average density and pressure . • Fixed Ti profile shape, combined with whole range of ne profiles encountered in JET, scaled to ITER inductive scenario (fixed b,ne) • Assumes impurity and He profiles and concentrations stay unchanged compared to flat reference case.

3. Fuel pressure profile governs fusion performance • Fusion power sv n2= svp2/T2 • For fixed p maximized fordlnsv/dlnTi=2(around 10keV) •  pfus p2, Pfus   p2dV =p2Vwhere p is fuel pressure •  Pressure profile merit factorp2/p2 • Density profile contribution to merit factor is JET ITER ITER • p2/p2in H-mode (&hybrid)increases towards lower collisionalities • This is due to density profiles merit factor • Effect of density peaking not cancelled by temperature flattening  • No systematic changes of temperature profile peaking with collisionality

4. Performance increase by density peaking depends on Ti profile • Each sample in JET database treated as model for ITER: scaled to bN=1.8, NGR=0.86, V=831m3. • Dilution adjusted to get 400MW for Ti profile like ITER inductive reference (Polevoi 2003 ) and for flat nD,T. • Peaked Ti profiles (small pedestals) lead to strongest increase with peaked density profiles. • Most temperature profiles in JET are broader than in ITER inductive scenario. • This means less fusion power for given bN, NGR, and ne0/ne ITER

5. Does an inward pinch lead to excessive Core He enrichment in reactor? • Performance increases with fuel pressure, depends on dilution by impurities and He from fusion reactions • How is He concentration affected by inward pinch? • Large experimental database on He transport constituted in current campaigns, but still being analysed.

6. Core He enrichment in reactor? With peaked ne and more He pumping for more Pfus nHe With peaked ne and same He pumping Compare peaked and flat ne = ne(a) nHe(a) must remain same for same pumping First RHS term larger if ne peaked because ne(a) <ne Second term smaller in core if ne peaked, compensates partly in core, where it matters.  Modest increas in dilution in core when transport coefficients and source same This will change if VHeVe, DHeDe With flat ne r a

7. Fusion power with peaked fuel and He profiles (nominal pumping) Each JET shot scaled up to ITER conditions Game rules: W=const. as in ITER inductive ref. scenario, Ptot=const (120MW) ne=1.011020m-3, same Ti profile shape as ITER simulation (A.P). 80MW of alpha power requires nHe(0.95)= 11018m-3, (nominal pumping) DHe=De=0.667ceffVHe/Ve=v*He= 0,1 & 2 (Reference has Pfus =400MW) ITER

8. Fusion power with peaked fuel and He profiles (case of lousy pumping) Each JET shot scaled up to ITER conditions Game rules: W=const. as in ITER inductive ref. scenario, Ptot=const (120MW) ne=1.011020m-3, same Ti profile shape as ITER simulation (A.P) 80MW of alpha power requires nHe(0.95)= 41018m-3, (lousy pumping) DHe=De=0.667ceffVHe/Ve=v*He= 0,1 & 2(Reference would have Pfus =350MW)

9. Core He enrichment in reactor? • 3 important parameters for steady-state He profile: • Effect of peaking ‘neutral’ on He concentration if VHeDe/(VeDHe)1 • Effect of peaking on fusion performance positive even if VHeDe/(VeDHe)>1, provided nHe(a) not excessive. pumping peaking Eagerly awaiting JET results for He transport!