RISERVA

1. 11 RISERVA

2. ProtoSphera Parameters Parameters of the spherical torus (ST):Equatorial, major, minor radius of the ST Rsph= 0.36 m , R = 0.20 m, a = 0.16 mAspect ratio of the ST (R/a), Elongation A= 1.25, k = 2.17Toroidal ST plasma current Ip = 180 kASafety factor of the ST at the edge q95 = 2.6ST volume averaged electron density <ne> = 0.5•1020 m-3ST volume averaged electron temperature <Te> = 140 eVEnergy confinement time of the ST tE = 1.6 msResistive & Alfvén time of the ST tR = 70 ms, tA= 0.5 msMagnetic Lundquist number of the ST S = 1.2•105Total beta & poloidal beta of the ST bT = 10÷30%, bpol ≤ 0.15Parameters of the screw pinch (SP):Equatorial radius of the SP rPinch(0) = 0.04 mLongitudinal current in the SP Ie=60 kA...corresponding to a toroidal field BT0 = 0.05 T at R = 0.23 m... ... including paramagnetism BT = 0.14 T at R = 0.23 mSP electron density nePinch = 0.15•1020 m-3SP electron temperature TePinch = 36 eV

3. Why ULART ? Magnetic line of forces Spherical Torus Conventional Tokamak Field line in Bad Curvature region Geodesic Curvature Neoclassical transport Micro-instability related to trapped particles High Low Low High High Low High Low

4. Spheromaks Spheromaks are usually formed by magnetized coaxial plasma guns used as helicity injectors, in presence of a close conducting shell Breakdown in small spaces, with very high filling pressures and kV voltages • Big amount of neutrals and impurities are released from the gunThe Spheromak formated is accelerated and expanded into a flux conserver • Field errors already present in the gun are amplified PROTO-SPHERA will form instead at tokamak-like densities, with low voltages (~100 V) and will not undergo any expansion

5. Flux-Core-Spheromak obtained on the TS-3 • Filling gas (pH~2•10-2 mbar); break-down (Ve~1 kV) using two plasma guns • Screw pinch current increases: toroidal plasma, non-linear kink: qPinch<1÷2 • Compression coils pulsed: flux swing drive much of toroidal plasma current After formation (~60 ms), the configuration was sustained for 20 msec, i.e. 30•tA • PROTO-SPHERA aims at sustaining the toroidal plasma through DC helicity injection

6. Some ULART Features High T • Higher MHD stability and high average total beta values: bT=2m0<p>Vol/BT2(bT=40% with baxis=70% on START) START: relatively high energy confinement times and density limits with H-mode in NBI X-point discharges

7. Helicity Injection • The plasma with open field lines (intersecting electrodes) has b~0, therefore || • Because of the twist of the open field lines, the current between the electrodes also winds in the toroidal direction near the closed magnetic flux surfaces • Resistive MHD instabilities convert, through magnetic reconnections, open current/field lines into closed current/field lines, winding on the closed magnetic flux surfaces • Magnetic reconnections necessarily break, through helical perturbations, the axial symmetry, as per Cowling's anti-dynamo theorem

8. mST &k • PROTO-SPHERA aims at a ST elongated k~2.3, to get q0~1 and q95~2.5÷3 • In PROTO-SPHERA (Rsph=0.35 m) the structure of the fields has beendesigned in order to be as far as possible fromthe pure Spheromak mSTRsph≤4.2

9. ST, Spheromak, FRC The most investigated magnetic fusion configurations are not simply connected: a central post links the Plasma Torus The feasibility of simply connected, fusion relevant, magnetic configuration would strongly simplify the design of a fusion reactor Compact Tori yield simply connected plasma configurations: Spheromaks and FRC’sThey have up to now been less successful than ST as they rely more heavily upon plasma self-organization, both for their formation as well as for their sustainment.Although many formation schemes have produced in the last twenty years interesting Spheromaks and Field Reversed Configurations (FRC), at the present moment no sustainment has been soundly and fully demonstrated

10. Formation Time TS-3 took 80 ms to reach Ip/Ie=1.2 Scaling up as S1/2tA (Sweet-Parker reconnection) and including all passive currents: t= t0-100mst= t0+300ms t= t0+600mst= t0+1 ms Ie=8.5 kA Ie=45 kA Ie=54 kA Ie=60 kA Ip=0 kA Ip=30 kA Ip=60 kA Ip=120 kA

11. Although finite amplitude resistive MHD instabilities are required to inject helicity from the pinch to the ST, the combined configuration must be ideal MHD stable New finite element method ideal MHD stability codes have been developed in order to analyze the combined screw pinch + spherical torus configuration of PROTO-SPHERA Stability The ideal MHD stability limits to the ratio Ip/ Ie, depending upon bST=2m0<p>ST/<B2>ST With bST~30% Ip can reach a value of 1•Ie With bST~20% Ip can reach a value of 2÷3•Ie With bST~10% Ip can reach a value of 4•Ie (design limit)

12. Reasons to push towards the Ultra Low Aspect Ratio Torus (ULART, A ≤ 1.3) • the critical central conductor cannot be shielded • it is bombarded by neutrons (cannot be a superconductor) • it should be periodically replaced • But the ULART does not leave enough space for an ohmic transformer and requires noninductive current drive • The bpol=2m0<p>Vol/Bpol2 marks the distance from a force free-state ( jÙB =0). •  In an ST( Bpol ~ BT ) • A high bT (40%) plasmain an ST is much nearer to a force-free configuration than a low bT (4%) plasma in a Tokamak

13. Conclusion PROTO-SPHERA project is in the framework of Compact Tori (ST, Spheromak, FRC): Its particular goal is to form and to sustain a Flux-Core-Spheromak with a new technique and to show that DC helicity injection can sustain it on the resistive time-scale • Will advance the knowledge of DC helicity injection  The magnetic configuration of the experiment has been designed aiming at a safety factor profile that is similar to the ones obtained in spherical tori with metal centerpost • Will complement the ST experiments (START, MAST, NSTX,…)  The current density and power load on the electrodes (W) will advance the state of technology • Will be relevant to the design of divertors for the main tokamak line

14. Image of PROTO-PINCH Hydrogen plasma withIe=600 A, B=1 kG. PROTO-PINCH has produced Hydrogen and Helium arcs in the form of screw pinch discharges. Pinch Length : 75 cmStabilizing Field : 1.5 kGSafety Factor qPinch≥2Ie = 670 AEmax = 6.7 A/cm2Vpinch = 80 –120 VVcathode = 14.5 V

15. Cathode Treats , Recipes & Results • Filling Pressure 1 10-3 – 1 10-2 • AC current for heating the cathode, to spread the ion plasma current over the filaments. • Time required for heating the cathode circa 15 s. •   Icath=550-590 A (rms.) at Vcath=14.5 V (rms.) allows for Ie=600-670 A of plasma current Ie/Icath≈1. • Pcath≈ 8.5 kW allows for Pe≈50-70 kW into the Pinch • No damages after »400 shots at Ie=600 A, Dt = 2-5 sec

16. Anode a b ·PWAnode= 2/3 (670 120) KW » 56 KW ( module)·Asurface= 1.8 10-3 m2·Dpw= PW/ Asurface» 30 MW/m2·anode arcanchoringwithCathodeDC heated (a)·No Anode anchoring with ACcathode heating(b) • NoDamages after 1000 discharges • Material: W 95% Cu5%. • Anode : Puffed Hollow

17. Associazione Euratom-ENEA sulla Fusione HeliConical Coil Test Test Results : • Very Small Displacement after 2700 Sec at 2700 °C PROTO-SPHERA Workshop - Frascati, 18-19/03/2002

18. Cathode Layout Material Plates: MolybdenumColumns:TantalumInsulator : AluminaCoils : Pure W Module Power = 8.4 KWModule Current = 670 AModule Voltage = 14.5 VWire Number = 4Wire Length = 40.0 cmWire Surface = 4X25 cm2Wire Temp =2600 °CWire Em =6.7 Amp/cm2Wire Weight = 4X22 Gr.

19. HeliConical Coil • Null Field • Optimize Temperature Distribution • Optimize Weight Distibution • Ie =167 A (each coil)

20. Structural Analysis • Max VonMisess Stress 0.16 Kg/mm2 • Max Displacement 42.9 mm • Coil Safety Factor = 5.3

21. Emissivity vs Temperature (1)

22. Conclusion The major points that have to be demonstrated on PROTO-SPHERA are: •That the formation scheme is effective and reliable •That the configuration can be sustained in 'steady-state' by DC helicity injection •That the energy confinement is not worse than the one measured on spherical toriIf these objectives are met, PROTO-SPHERA could try the inductive formation of a CKF • PROTO-SPHERA could lead to a proof-of-principle CKF experiment

23. H2 H2 Zoom He zoom He Å Å Å Å Å VisibleSpectroscopy Spectral lines of filling gas (H2/He) and impurities Enlarged Å Å 1 eV <Te 3.0 eV - No HeII (4686 Å) He Very few impuritiesOII& CIIIat a count level» 10-2of the largest Helium line counts Å

24. Density Measurements • 2mm microwave interferometer with 140 GHz oscillator : • B = 1.25 kG :ne = 1.4 1019m-3per fringeÞne ~6·1019m-3 • Density measurable • In Helium discharge up to Ie = 200 A • Line-averaged electron density increase linearly with current Ie • Helium ionization degree is about 16% at filling pressure of 4 10-3 mbar & Ie= 200 A Ie fringes

25. MODELING of PROTO-PINCH PLASMA • Spectroscopy Þ1<TePinch<3 eV • Ohmic input = electron flow convected fluxÞTePinch = 2 eV • Interferometry suggests Þplasma 50% ionized at Ie=600 AÞpH2=8•10-3 mbar gives: nePinch = 2•1020 m-3 • However estimated Ohmic inputPW= 4kW • main loss in electrode plasma sheaths Pelectrodes= 46 kW • power injected near the electrodes gives: Teelectrodes= 0.4 eV • constant electron pressure gives: neelectrodes = 5•1020 m-3

26. EXTRAPOLATION to Screw Pinch ofPROTO-SPHERA • Assuming same plasma near the electrodes at Ie=60 kA • Teelectrodes= 0.4 eV, neelectrodes =5•1020 m-3 • Power into electrode sheaths Pelectrodes=100•46 kW = 4.6 MW •  In the main body of the discharge (far from electrode sheaths) Ohmic input = electron flow convected flux: TePinch =36 eVÞconstant electron pressure: nePinch =1.5•1019 m-3 • Ohmic inputPW = 5.4 MW • OHMIC PW 5.4 MW + • SHEATHS Pelectrodes 4.6 MW + • Helicity Injection PHI0.6 MW = • TOTAL POWER PPinch 10.6 MW

27. 10 Panel Questions & Answers • Is the physics basis for undertaking an experiment as proposed with PROTO-SPHERA adequate? OK, BUTWe recommend that a wider range of operation scenarios of m and pressure profiles be analysed... (equilibria & stability &n0 stability ) • Are the PROTO-PINCH electrode experiments a sufficient technical basis for a reliable electrode operation in PROTO-SPHERA? • OK, BUT… are not yet adequate for reliable multi-electrode operation... • … In particular, is the proposed size adequate for the purposes of a Concept Exploration Experiment? .. OK • How likely is .... to advance the present state of science and technology substantially.. OK • .. likely to produce new information that is adequate as basis to extrapolate to a SPHERA device that achieves fusion relevant parameters? OK • What diagnostics should be planned in order to properly measure the properties of the PROTO-SPHERA plasma?OK • What are the unique contributions of the proposed experiment to the world magnetic fusion programs, and in particular to the European Magnetic Fusion Program during the VIth FP?OK

28. 11 p(y) = pe=constant for y<yX inside the SP and p(y) = pe + Cp(y-yX)1.1for y≥yX inside the ST for y <yX inside the SP and for y ≥ yX inside the ST Ie Screw Pinch longitudinal current, peis the pressure inside the SP and yX is the poloidal flux function at the separatrix the exponent in the SP is a=2Idia(y)µyScrew Pinch is force-free relaxation parameter. Constant inside Pinch m=m0 • /B2 Equilibria Computation Before Panel • For every Equilibrium calculation the poloidal beta of the Spherical Torus is an input parameter as well as the total toroidal current Ipinside the STand Ieinside SP

29. 12 • Screw Pinch force-free (constant p(y) inside the SP) reasonable ( open magnetic field lines) • Hypothesis that m(y)=constant inside it could be questionable. • An investigation has been performed by varying ai.e. the current inside the SP: • A wider range of scenarios explored inside ST varyingh andeyc=yX+h×(ymax-yX) • yX ≤y≤yc • y>yc ..We recommend that a wider range of operation scenarios of m and pressure profiles be analysed to engender greater confidence in the successful operation of the machine before considering moving towards any construction phase. Panel Questions Concerning Scenarios

30. 14 • Features of MHD stability codes (STABLE) • Boozer coordinates on open field lines are joined to the closed field lines Boozer coordinates at the ST-SP interface • •Boundary conditions at the ST-SP interface • •Vacuum magnetic energy in presence of multiple plasma boundary • •2D finite element method for accounting the perturbed vacuum energy • •Plasma on the symmetry axisrequire a well suited (perturbed displacement) decomposition, to avoid perturbated potential energy divergence for R=0. r x • In PROTO-SPHERA resistive MHD instabilities arerequiredto inject magnetic helicity from SP into ST • The combined configuration must be MHD stable Stabilty

31. 16 • The perturbed displacement , has in STABLE code been decomposed in terms of the normal xy, binormal hyand parallel m components • For n=0 the displacements and must be zero because, the flow along field lines and the toroidal flow do not contribute to the perturbate plasma potential magnetic energy but they contribute to the perturbed kinetic energy, creating spurious eigenvectors and eigenvalues. A modified displacement decomposition has been adopted to solve this problem(new code:STABLEN0MU). r x Stability:CASE n=0

32. Year 1 Year 2 Year 3 Year 4 LOAD ASSEMBLY Design Contract Tender Construction Check Assembly Final check Guarantee ASSEMBLY WORK Tender Orders Work Final check PUMP,GAS,CONTROL Tender Orders Assembly Final check PF COILS Design Contract Tender Construction Check Assembly Final check Guarantee ELECTRODE Design Contract Tender Construction Check Assembly Check Final check Guarantee POWER SUPPLY Design Tender Construction Check Check Assembly Final check Guarantee ELECTRICAL WRK Design Tender Work Final check Guarantee TIME SCHEDULE

33. Some Steps • • Following a formal request by the Euratom-ENEA Steering Committee in December 1999, • • After the ENEA internal peer-review and CTS review system (March 2000-March 2001) assigned to the PROTO-SPHERA project the mark 45/54, • • The PROTO-SPHERA Workshop held on March 18-19, 2002 • Questions raised by panel

34. A simply connected magnetic confinement scheme is obtained superposing two axisymmetric homogeneous force-free fields, both having , with the same relaxation parameter m=m0•/B2=14.066... in unitary sphere Ù=m Coincidence of zero of and offixesl =x1,4p/2x1,3=2.026...,so that at R=0, Z=x1,3/x1,4=0.775... the zeroes coincide CKF Chandrasekhar-Kendall Force-free fields Furth square-toroids

35. The superposition of the two force-free fields is: For g≥0.402..., in a simply connected region, toroidal current density jf has the same sign: CKF Chandrasekhar-Kendall-Furth force-free field (CKF)

36. CKF force-free-fields (Ñp=0) contain a magnetic separatrix with ordinary X-points (B≠0) • A main spherical torus (ST), 2 secondary tori (SC) and a surrounding discharge (P) • Two degenerate X-points (B=0) are present (top/bottom) on the symmetry axis CKF

37. CKF Stability CKF, with this kind of <m> and p profiles, are stable in free boundary to ideal MHD perturbations with low toroidal mode numbers (n=1, 2, 3), at b ST=2m0<p>ST/<B2>ST≈1/3 Trend of MHD stability with IST/Ie: same as in PROTO-SPHERA

38. CKF Stability Even in free boundary up to b ST=2m0<p>ST/<B2>ST ≈1 Trend of MHD stability with b: same as in PROTO-SPHERA IMPORTANCE of high b for a reactor: reduces cost and size Pfusion~b2B4 therefore higher bÞ lower B nTtE~b/c {a2B2} therefore higher bÞ lower a at same c