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Associazione Euratom-ENEA sulla Fusione ----- Culham 15-17 September 2003 RISERVA 11

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Associazione Euratom-ENEA sulla Fusione ----- Culham 15-17 September 2003 ProtoSphera Parameters Parameters of the spherical torus (ST): Equatorial, major, minor radius of the STR sph = 0.36 m, R = 0.20 m, a = 0.16 m Aspect ratio of the ST (R/a), ElongationA= 1.25, = 2.17 Toroidal ST plasma currentI p = 180 kA Safety factor of the ST at the edgeq 95 = 2.6 ST volume averaged electron density = 0.510 20 m -3 ST volume averaged electron temperature = 140 eV Energy confinement time of the ST E = 1.6 ms Resistive & Alfvén time of the ST R = 70 ms, A = 0.5 s Magnetic Lundquist number of the STS= 1.210 5 Total beta & poloidal beta of the ST T = 10÷30%, pol 0.15 Parameters of the screw pinch (SP): Equatorial radius of the SP Pinch (0) = 0.04 m Longitudinal current in the SP I e =60 kA...corresponding to a toroidal fieldB T0 = 0.05 T at R = 0.23 m...... including paramagnetismB T = 0.14 T at R = 0.23 m SP electron densityn e Pinch = 0.1510 20 m -3 SP electron temperatureT e Pinch = 36 eV

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Associazione Euratom-ENEA sulla Fusione ----- Culham 15-17 September 2003 Magnetic line of forces Field line in Bad Curvature region Geodesic Curvature Neoclassical transport Micro-instability related to trapped particles Conventional Tokamak High Spherical Torus Low Why ULART ?

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Associazione Euratom-ENEA sulla Fusione ----- Culham 15-17 September 2003 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 gun The 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

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Associazione Euratom-ENEA sulla Fusione ----- Culham 15-17 September 2003 Flux-Core-Spheromak obtained on the TS-3 Filling gas (p H ~210 -2 mbar); break-down (V e ~1 kV) using two plasma guns Screw pinch current increases: toroidal plasma, non-linear kink: q Pinch <1÷2 Compression coils pulsed: flux swing drive much of toroidal plasma current After formation (~60 s), the configuration was sustained for 20 sec, i.e. 30 A PROTO-SPHERA aims at sustaining the toroidal plasma through DC helicity injection

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Associazione Euratom-ENEA sulla Fusione ----- Culham 15-17 September 2003 Higher MHD stability and high average total beta values: T =2 0 Vol /B T 2 ( T =40% with axis =70% on START) High T START : relatively high energy confinement times and density limits with H-mode in NBI X-point discharges Some ULART Features

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Associazione Euratom-ENEA sulla Fusione ----- Culham 15-17 September 2003 Helicity Injection The plasma with open field lines (intersecting electrodes) has ~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

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Associazione Euratom-ENEA sulla Fusione ----- Culham 15-17 September 2003 ST & PROTO-SPHERA aims at a ST elongated ~2.3, to get q 0 ~1 and q 95 ~2.5÷3 In PROTO-SPHERA (R sph =0.35 m) the structure of the fields has been designed in order to be as far as possible from the pure Spheromak ST R sph 4.2

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Associazione Euratom-ENEA sulla Fusione ----- Culham 15-17 September 2003 ST, Spheromak, FRC The feasibility of simply connected, fusion relevant, magnetic configuration would strongly simplify the design of a fusion reactor The most investigated magnetic fusion configurations are not simply connected: a central post links the Plasma Torus Compact Tori yield simply connected plasma configurations: Spheromaks and FRCs They 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

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Associazione Euratom-ENEA sulla Fusione ----- Culham 15-17 September 2003 Formation Time TS-3 took 80 s to reach I p /I e =1.2 Scaling up as S 1/2 A (Sweet-Parker reconnection) and including all passive currents: t= t 0 -100 st= t 0 +300 st= t 0 +600 st= t 0 +1 ms I e =8.5 kAI e =45 kAI e =54 kAI e =60 kA I p =0 kAI p =30 kAI p =60 kAI p =120 kA

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Associazione Euratom-ENEA sulla Fusione ----- Culham 15-17 September 2003 Stability The ideal MHD stability limits to the ratio I p / I e, depending upon ST =2 0 ST / ST With ST ~30% I p can reach a value of 1I e With ST ~20% I p can reach a value of 2÷3I e With ST ~10% I p can reach a value of 4I e (design limit) 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

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Associazione Euratom-ENEA sulla Fusione ----- Culham 15-17 September 2003 The pol =2 0 Vol /B pol 2 marks the distance from a force free-state ( j B =0). In an ST( B pol ~ B T ) A high (40%) plasma in an ST is much nearer to a force-free configuration than a low (4%) plasma in a Tokamak 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 Reasons to push towards the Ultra Low Aspect Ratio Torus (ULART, A 1.3)

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Associazione Euratom-ENEA sulla Fusione ----- Culham 15-17 September 2003 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

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Associazione Euratom-ENEA sulla Fusione ----- Culham 15-17 September 2003 PROTO-PINCH has produced Hydrogen and Helium arcs in the form of screw pinch discharges. Pinch Length : 75 cm Stabilizing Field : 1.5 kG Safety Factor q Pinch 2 I e = 670 A E max = 6.7 A/cm 2 V pinch = 80 –120 V V cathode = 14.5 V Image of PROTO-PINCH Hydrogen plasma with I e =600 A, B=1 kG.

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Associazione Euratom-ENEA sulla Fusione ----- Culham 15-17 September 2003 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. I cath =550-590 A (rms.) at V cath =14.5 V (rms.) allows for I e =600-670 A of plasma current I e /I cath 1. P cath 8.5 kW allows for P e 50-70 kW into the Pinch No damages after 400 shots at I e = 600 A, t = 2-5 sec Cathode Treats, Recipes & Results

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Associazione Euratom-ENEA sulla Fusione ----- Culham 15-17 September 2003 No Damages after 1000 discharges Material: W 95% Cu5%. Anode : Puffed Hollow P WAnode = 2/3 (670 120) KW W ( module) A surface = 1.8 10 -3 m 2 D pw = P W / A surface MW/m 2 anode arc anchoring with Cathode DC heated (a) No Anode anchoring with AC cathode heating (b) ab Anode

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Associazione Euratom-ENEA sulla Fusione ----- Culham 15-17 September 2003 HeliConical Coil Test Test Results : Very Small Displacement after 2700 Sec at 2700 C PROTO-SPHERA Workshop - Frascati, 18- 19/03/2002 Associazione Euratom-ENEA sulla Fusione

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----- Culham 15-17 September 2003 Cathode Layout Material Plates: Molybdenum Columns:Tantalum Insulator : Alumina Coils : Pure W Module Power = 8.4 KW Module Current = 670 A Module Voltage = 14.5 V Wire Number = 4 Wire Length = 40.0 cm Wire Surface = 4X25 cm 2 Wire Temp = 2600 C Wire Em = 6.7 Amp/cm 2 Wire Weight = 4X22 Gr.

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Associazione Euratom-ENEA sulla Fusione ----- Culham 15-17 September 2003 HeliConical Coil Null Field Optimize Temperature Distribution Optimize Weight Distibution Ie =167 A (each coil)

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Associazione Euratom-ENEA sulla Fusione ----- Culham 15-17 September 2003 Max VonMisess Stress 0.16 Kg/mm 2 Max Displacement 42.9 m Coil Safety Factor = 5.3 Structural Analysis

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Associazione Euratom-ENEA sulla Fusione ----- Culham 15-17 September 2003 Emissivity vs Temperature (1)

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Associazione Euratom-ENEA sulla Fusione ----- Culham 15-17 September 2003 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 tori If 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

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Associazione Euratom-ENEA sulla Fusione ----- Culham 15-17 September 2003 Visible Spectroscopy Spectral lines of filling gas (H 2 /He ) and impurities 1 eV < Te 3.0 eV - No HeII (4686 Å) Enlarged Å Å Å He Very few impurities OII & CIII at a count level of the largest Helium line counts H 2 Zoom H2H2 Å Å Å Å Å He zoom HeHe

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Associazione Euratom-ENEA sulla Fusione ----- Culham 15-17 September 2003 Density Measurements 2mm microwave interferometer with 140 GHz oscillator : B = 1.25 kG : n e = 1.4 10 19 m -3 per fringe n e 6 10 19 m -3 Density measurable In Helium discharge up to I e = 200 A Line-averaged electron density increase linearly with current I e Helium ionization degree is about 16% at filling pressure of 4 10 -3 mbar & I e = 200 A fringes IeIe

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Associazione Euratom-ENEA sulla Fusione ----- Culham 15-17 September 2003 MODELING of PROTO-PINCH PLASMA Spectroscopy 1<T e Pinch <3 eV Ohmic input = electron flow convected flux T e Pinch = 2 eV Interferometry suggests plasma 50% ionized at I e =600 A p H2 =810 -3 mbar gives: n e Pinch = 210 20 m -3 However estimated Ohmic input P = 4kW main loss in electrode plasma sheaths P electrodes = 46 kW power injected near the electrodes gives: T e electrodes = 0.4 eV constant electron pressure gives: n e electrodes = 510 20 m -3

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Associazione Euratom-ENEA sulla Fusione ----- Culham 15-17 September 2003 EXTRAPOLATION to Screw Pinch of PROTO-SPHERA Assuming same plasma near the electrodes at I e =60 kA T e electrodes = 0.4 eV, n e electrodes = 510 20 m -3 Power into electrode sheaths P electrodes = 10046 kW = 4.6 MW In the main body of the discharge (far from electrode sheaths) Ohmic input = electron flow convected flux: T e Pinch = 36 eV constant electron pressure: n e Pinch = 1.510 19 m -3 Ohmic input P = 5.4 MW OHMICP 5.4 MW + SHEATHSP electrodes 4.6 MW + Helicity InjectionP HI 0.6 MW = TOTAL POWERP Pinch 10.6 MW

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Associazione Euratom-ENEA sulla Fusione ----- Culham 15-17 September 2003 Panel Questions & Answers 1.Is the physics basis for undertaking an experiment as proposed with PROTO-SPHERA adequate? OK, BUT We recommend that a wider range of operation scenarios of and pressure profiles be analysed... (equilibria & stability &n0 stability ) 2)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... 3) … In particular, is the proposed size adequate for the purposes of a Concept Exploration Experiment?.. OK 4)How likely is.... to advance the present state of science and technology substantially.. OK 5).. likely to produce new information that is adequate as basis to extrapolate to a SPHERA device that achieves fusion relevant parameters? OK 6)What diagnostics should be planned in order to properly measure the properties of the PROTO-SPHERA plasma? OK 7)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 VI th FP? OK 10

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Associazione Euratom-ENEA sulla Fusione ----- Culham 15-17 September 2003 p( ) = p e =constantfor < X inside the SP and p( ) = p e + C p ( X ) 1.1 for X inside the ST for < X inside the SP and for X inside the ST For every Equilibrium calculation the poloidal beta of the Spherical Torus is an input parameter as well as the total toroidal current I p inside the ST and I e inside SP 11 I e Screw Pinch longitudinal current, p e is the pressure inside the SP and X is the poloidal flux function at the separatrix the exponent in the SP is =2 I dia ( ) Screw Pinch is force-free relaxation parameter. Constant inside Pinch = 0 /B 2 Equilibria Computation Before Panel

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Associazione Euratom-ENEA sulla Fusione ----- Culham 15-17 September 2003 Panel Questions Concerning Scenarios..We recommend that a wider range of operation scenarios of and pressure profiles be analysed to engender greater confidence in the successful operation of the machine before considering moving towards any construction phase. A wider range of scenarios explored inside ST varying h and c = X +h ( max - X ) X c > c 12 Screw Pinch force-free (constant p( ) inside the SP) reasonable ( open magnetic field lines) Hypothesis that ( )=constant inside it could be questionable. An investigation has been performed by varying i.e. the current inside the SP:

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Associazione Euratom-ENEA sulla Fusione ----- Culham 15-17 September 2003 In PROTO-SPHERA resistive MHD instabilities are required to inject magnetic helicity from SP into ST The combined configuration must be MHD stable Stabilty 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 axis require a well suited (perturbed displacement) decomposition, to avoid perturbated potential energy divergence for R=0.

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Associazione Euratom-ENEA sulla Fusione ----- Culham 15-17 September 2003 Stability: CASE n=0 16 The perturbed displacement, has in STABLE code been decomposed in terms of the normal, binormal and parallel 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).

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Associazione Euratom-ENEA sulla Fusione ----- Culham 15-17 September 2003 TIME SCHEDULE Year 1 Year 2 Year 3 Year 4 LOAD ASSEMBLY ASSEMBLY WORK PUMP,GAS, CONTROL PF COILS ELECTRODE POWER SUPPLY ELECTRICAL WRK Design ContractTenderConstructionCheckAssembly Final check Guarantee TenderOrdersWork Final check TenderOrdersAssembly Final check DesignContractTenderConstruction CheckAssembly Final check Guarantee DesignContractTenderConstructionCheck AssemblyCheckFinal check Guarantee Design TenderConstructionCheck AssemblyFinal check Guarantee Design TenderWork Final check Guarantee

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Associazione Euratom-ENEA sulla Fusione ----- Culham 15-17 September 2003 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

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Associazione Euratom-ENEA sulla Fusione ----- Culham 15-17 September 2003 CKF Chandrasekhar-Kendall Force-free fields Furth square-toroids A simply connected magnetic confinement scheme is obtained superposing two axisymmetric homogeneous force-free fields, both having, with the same relaxation parameter = 0 /B 2 =14.066... in unitary sphere = Coincidence of zero of and of fixes =x 1,4 /2x 1,3 =2.026..., so that at R=0, Z=x 1,3 /x 1,4 =0.775... the zeroes coincide

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Associazione Euratom-ENEA sulla Fusione ----- Culham 15-17 September 2003 CKF The superposition of the two force-free fields is: For 0.402..., in a simply connected region, toroidal current density j has the same sign: Chandrasekhar-Kendall-Furth force-free field (CKF)

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Associazione Euratom-ENEA sulla Fusione ----- Culham 15-17 September 2003 CKF CKF force-free-fields ( p=0) contain a magnetic separatrix with ordinary X-points (B0) 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

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Associazione Euratom-ENEA sulla Fusione ----- Culham 15-17 September 2003 CKF Stability CKF, with this kind of and p profiles, are stable in free boundary to ideal MHD perturbations with low toroidal mode numbers (n=1, 2, 3), at ST =2 0 ST / ST 1/3 Trend of MHD stability with I ST /I e : same as in PROTO-SPHERA

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Associazione Euratom-ENEA sulla Fusione ----- Culham 15-17 September 2003 CKF Stability Even in free boundary up to ST =2 0 ST / ST 1 Trend of MHD stability with : same as in PROTO- SPHERA IMPORTANCE of high for a reactor: reduces cost and size P fusion ~ 2 B 4 thereforehigher lower B nT E ~ / {a 2 B 2 }thereforehigher lower a at same

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