September 26-29, 2005 Knoxville. Tennessee USA Progress and Plan of the PROTO-SPHERA Program Outline of the Talk 1)Introduction to PROTO-SPHERA 2)The PROTO-PINCH Experiment: results 3)Engineering Design and Analysis of PROTO-SPHERA 4)Multi-Pinch Experiment 5)Conclusions Stamos Papastergiou, Franco Alladio, Alessandro Mancuso, Paolo Micozzi, CR-ENEA, CP 65, Frascati (Roma), Italy 1

September 26-29, 2005 Knoxville. Tennessee USA Aims at producing a Spherical Torus (ST) with a Screw Pinch (SP) replacing the centerpost 2 Introduction to PROTO-SPHERA (Spherical Plasma for Helicity Relaxation Assessment) Ip: toroidal current Bt: toroidal field Bp: poloidal field Je: pinch current

September 26-29, 2005 Knoxville. Tennessee USA 3 Engineering Design of PROTO-SPHERA

September 26-29, 2005 Knoxville. Tennessee USA Physics Design I e, I p = 40, 50 kA pinch, ST currentsI e, I p = 60, 240 kA A=R/a = 1.6 aspect ratioA ≤ 1.3  pulse = 80  s~1/100  R pulse duration  pulse ≥ 70 ms ~ 1  R PROTO-SPHERA aims at sustaining the plasma for more than  R =  0 a 2 /  (resistive time) to verify the stability of the configuration 4 Tokyo University - Japan

September 26-29, 2005 Knoxville. Tennessee USA Main Breakdown features  Screw pinch (SP) formed by a hot cathode breakdown  Filling pressure p H ~10 -3 ÷10 -2 mbar  Cathode filaments heated to 2600 °C  V e ~100 V applied on the anode  Initial electrode arc current limited to I e ~8.5 kA 5

September 26-29, 2005 Knoxville. Tennessee USA Test of PROTO-SPHERA Cathode & Anode Arc breakdown in configuration similar to PROTO-SPHERA PROTO-PINCH Experiment - Results 6

September 26-29, 2005 Knoxville. Tennessee USA Anode  No  Damages after 1000 discharges  Material W 95% Cu5%.  Scheme : Puffed Hollow Anode  P WAnode = 2/3 ( ) kW » 56 kW (module)  A surface = m 2  D pw = P W / A surface » 30 MW/m 2  anode arc anchoring with Cathode DC heated (a)  No Anode anchoring with AC cathode heating (b) ab 7

September 26-29, 2005 Knoxville. Tennessee USA Cathode Emissivity  Coil Material : Pure W  Wire Diameter = 2 mm  Total Coils Number = 378  Single Coil emission = 167 A  Tot126 module 3 X 167 A  63 kA  Current Density  1 MA/m 2 Conical Wire Terminal & Clamp Cathode – Extrapolation to PROTO-SPHERA 8 W Mo

September 26-29, 2005 Knoxville. Tennessee USA Gas PuffIe TotSurf = 0.2 m 2 W Cu5% OFHC Hardened CU Gas Puff Anode Power Handling Capability: PROTO-SPHERA Tested = 9 MW PROTO-SPHERA Max = 6 MW Anode Extrapolation to PROTO-SPHERA 9

September 26-29, 2005 Knoxville. Tennessee USA Engineering Design and Analysis of PROTO-SPHERA 10

September 26-29, 2005 Knoxville. Tennessee USA Design Philosophy - Simplicity, symmetry, conservatism. - Viewing capability and good access to electrodes. - All connections to PF coils from top and bottom flanges, external to Machine. - Bellows in coil feedthroughts for adjustment. - Operation and disruption (but no fault) conditions considered. 11

September 26-29, 2005 Knoxville. Tennessee USA Design Aims Operationability, Reliability, Repairability of Machine. Reliability of electrodes and avoidance of arc anchoring. Minimise Technological Risks: provide appropriate elecrical insulation, avoid local coil overheating ( >100°C ) due to hot anode and cathode ( 12MJ, 2750°C ), minimise electromagnetic stresses. 12

September 26-29, 2005 Knoxville. Tennessee USA Operating conditions Temperature20°C Vacuum < mbar (predicted outgassing rate: ~ mbarl/s) Baking 80-90°C Adequate for water removal, coil protection; compatible to Viton O-rings Possibility of N 2 1 mbar contact gas 13

September 26-29, 2005 Knoxville. Tennessee USA Spherical Torus (ST) diameter0.7 m Longitudinal Screw Pinch current 60 kA Toroidal ST current 120÷240 kA Plasma pulse duration 1 s Minimum time between two pulses 5 min. Maximum heat loads on first wall componenets in divertor region 2 MW/m 2, for 1 ms Maximum current density on the plasma-electrode interface 1 MA/m 2 Machine Parameters 14

September 26-29, 2005 Knoxville. Tennessee USA Protection Componenets Thermal and Heat Transfer Behaviour Philosophy: collect the heat ( via radiation ) from the electrodes to black resilient componenet and then conduct or radiate it away in a controlled way. Hot spots are thus avoided. Danger: the hot cathode (less the anode) radiates at 2750°C with ~3.5MW/m 2 and even a 10mm thick coil casing cannot sustain this power density for more than 0.5s without exceeding 100°C 15

September 26-29, 2005 Knoxville. Tennessee USA Thermal and Heat transfer Behaviour Solution: 1.Use of Cu componenets to collect or reflect the heat away from coils. Conservative (but symmetrical) design for the anode. Thermal mass of these components results in operational temperature < 100°C. Water cooled! Optimum design; i.e. one circuit/component, D~10-15mm, v~2m/s, duty cycle 5min. 2.Use of thermocouples in no water cooled componenets to avoid >100°C with frequent pulses; every 5min. 3.Use of 10mm divertor AISI 304LN plates. 2 MW/m 2 for 1 s gives  T~120°C,  th ~320MPa 4.Use of optical diagnostics to view the electrodes which are relatively easily accessible. 16

September 26-29, 2005 Knoxville. Tennessee USA Electromagnetic Forces and Stresses B=50T/sB=500Gt≤1 ms Subdivision of Cu and 304LN plates to achieve ≤100 MPa of eddy current stresses. Unaccounted skin effects (t ~1ms) significant. Conservative prediction of eddy current effects. Electromagnetic forces in the coils (~x10 weight) require to be sustained. 17

September 26-29, 2005 Knoxville. Tennessee USA Predicted and Permitted Stresses ASME III-NB 3221 Stainles Steel Allowable S m for 304LN 100°C : 200MPa [304L S m ~150MPa] Predicted e-m stresses : 320MPa Thermal (divertor) local stresses : 320 MPa (2 MW/m 2,1s) Criterion 1 : 100 <200x1.5 Criterion 2 : 100  320 <200x3 Copper Minimum required allowable at 100°C> 70 MPa (100MPa<70x1.5) Thus  y >115 Mpa and  u > 230 MPa at 20°C i.e. Cu hardenned 18

September 26-29, 2005 Knoxville. Tennessee USA 19 Provisional single anode Cathode ~ START Dimension Simplified PF coils system fed with 600 A, 120 V Does not need cooling Aim: verification of the arc formation & stability Multi-Pinch Experiment START Vacuum Vessel

September 26-29, 2005 Knoxville. Tennessee USA 20 START in Frascati Arrival Disassembling May 2004 October 2004

September 26-29, 2005 Knoxville. Tennessee USA 21 Multi-Pinch Experiment: Phase I & Phase II

September 26-29, 2005 Knoxville. Tennessee USA CONCLUSIONS There is high degree of confidence that the design, construction and reliable operation of PROTO-SPHERA are feasible The Multi-Pinch experiment is being constructed to prove the formation and stability of the PROTO-SPHERA initial arc PROTO-SPHERA can be obtained simply adding components to Multi-Pinch Adequate technological basis, particularly in relation to electrodes, for extrapolation to a bigger device is expected 22