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

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

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Fuel pressure profile governs fusion performance Fusion power v n 2 = v p 2 /T 2 For fixed p maximized for dln v /dlnT i =2 (around 10keV) p fus p 2, P fus p 2 dV = p 2 V where p is fuel pressure Pressure profile merit factor p 2 / p 2 Density profile contribution to merit factor is p 2 / p 2 in 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 JET ITER

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Performance increase by density peaking depends on Ti profile Each sample in JET database treated as model for ITER: scaled to N =1.8, N GR =0.86, V=831m 3. Dilution adjusted to get 400MW for Ti profile like ITER inductive reference (Polevoi 2003 ) and for flat n D,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 N, N GR, and n e0 / n e ITER

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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.

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Core He enrichment in reactor? ar Compare peaked and flat n e = n e (a) n He (a) must remain same for same pumping First RHS term larger if n e peaked because n e (a) < n e Second term smaller in core if n e peaked, compensates partly in core, where it matters. Modest increas in dilution in core when transport coefficients and source same This will change if V He V e, D He D e With flat n e With peaked n e and same He pumping n He With peaked n e and more He pumping for more P fus

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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, P tot =const (120MW) n e =1.01 10 20 m -3, same Ti profile shape as ITER simulation (A.P). 80MW of alpha power requires n He (0.95)= 1 10 18 m -3, (nominal pumping) D He =D e =0.667 eff V He /V e =v* He = 0,1 & 2 (Reference has P fus =400MW) ITER

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Each JET shot scaled up to ITER conditions Game rules: W=const. as in ITER inductive ref. scenario, P tot =const (120MW) n e =1.01 10 20 m -3, same Ti profile shape as ITER simulation (A.P) 80MW of alpha power requires n He (0.95)= 4 10 18 m -3, (lousy pumping) D He =D e =0.667 eff V He /V e =v* He = 0,1 & 2 (Reference would have P fus =350MW) Fusion power with peaked fuel and He profiles (case of lousy pumping)

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Core He enrichment in reactor? 3 important parameters for steady-state He profile : Effect of peaking ‘neutral’ on He concentration if V He D e /(V e D He ) 1 Effect of peaking on fusion performance positive even if V He D e /(V e D He )>1, provided n He (a) not excessive. pumpingpeaking Eagerly awaiting JET results for He transport!

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