associazione euratom-enea sulla fusione ----- culham 15-17 september 2003 riserva 11

Associazione Euratom-ENEA sulla Fusione -----

Culham 15-17 September 2003

RISERVA

11

ProtoSphera ParametersParameters 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, = 2.17Toroidal ST plasma current Ip = 180 kA

Safety factor of the ST at the edge q95 = 2.6

ST volume averaged electron density <ne> = 0.5•1020 m-3

ST volume averaged electron temperature <Te> = 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 ST S = 1.2•105

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 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 m

SP electron density nePinch = 0.15•1020 m-3

SP electron temperature TePinch = 36 eV



Magnetic line of forces

Field line in Bad Curvature region

Geodesic Curvature

Neoclassical transport

Micro-instability related to trapped particles

Conventional Tokamak

High

High

High

High

Spherical Torus

Low

Low

Low

Low

Why ULART ?



SpheromaksSpheromaks 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

Flux-Core-Spheromak obtained on the TS-3Filling 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 s), the configuration was sustained for 20 sec, i.e. 30•A

PROTO-SPHERA aims at sustaining the toroidal plasma through DC helicity injection

Higher MHD stability and high average total beta values:

T=20Vol/BT2(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



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



ST &

PROTO-SPHERA aims at a ST elongated ~2.3, to get q0~1 and q95~2.5÷3

In PROTO-SPHERA (Rsph=0.35 m) the structure of the fields has been

designed in order to be as far as possible from the pure Spheromak STRsph≤4.2



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 FRC’s 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



Formation TimeTS-3 took 80 s to reach Ip/Ie=1.2

Scaling up as S1/2A (Sweet-Parker reconnection)

and including all passive currents:

t= t0-100s t= t0+300s t= t0+600s t= 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

Stability

The ideal MHD stability limits to the ratio Ip/ Ie, depending upon ST=20ST/<B2>ST

With ST~30% Ip can reach a value of 1•Ie

With ST~20% Ip can reach a value of 2÷3•Ie

With ST~10% Ip can reach a value of 4•Ie (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

The pol=20Vol/Bpol2 marks

the distance from a force free-state ( jB =0).

In an ST( Bpol ~ BT )

A high (40%) plasmain 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)



ConclusionPROTO-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



PROTO-PINCH has produced Hydrogen and Helium arcs in the form of screw pinch discharges.

Pinch Length : 75 cm Stabilizing Field : 1.5 kGSafety Factor qPinch≥2

Ie = 670 AEmax = 6.7 A/cm2

Vpinch = 80 –120 VVcathode = 14.5 V

Image of PROTO-PINCH Hydrogen plasma with Ie=600 A, B=1 kG.



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, t = 2-5 sec

Cathode Treats , Recipes & Results



NoDamages after 1000 discharges

Material: W 95% Cu5%.

Anode : Puffed Hollow

PWAnode = 2/3 (670 120) KW W ( module) Asurface= 1.8 10-3 m2

Dpw = PW/ AsurfaceMW/m2

anode arc anchoringwithCathode DC heated (a) No Anode anchoring with AC cathode heating (b)

a b

Anode



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



Cathode Layout

Material Plates: MolybdenumColumns:TantalumInsulator : AluminaCoils : Pure WModule Power = 8.4 KWModule Current = 670 AModule Voltage = 14.5 VWire Number = 4Wire Length = 40.0 cmWire Surface = 4X25 cm2

Wire Temp = 2600 CWire Em = 6.7 Amp/cm2

Wire Weight = 4X22 Gr.



HeliConical Coil

Null Field Optimize Temperature Distribution Optimize Weight Distibution Ie =167 A (each coil)



Max VonMisess Stress 0.16

Kg/mm2

Max Displacement

42.9 m

Coil Safety Factor = 5.3

Structural Analysis



Emissivity vs Temperature (1)



ConclusionThe 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

VisibleSpectroscopy

Spectral lines of filling gas (H2/He ) and impurities

1 eV < Te 3.0 eV - No HeII (4686 Å)

EnlargedÅ

Å

Å

He

Very few impurities OII & CIII at a count level

of the largest Helium line counts

H2 ZoomH2

ÅÅ ÅÅÅ

He zoomHe



Density Measurements

2mm microwave interferometer with 140 GHz oscillator :

B = 1.25 kG : ne = 1.4 1019 m-3 per fringe ne 6 1019

m-3

Density measurableIn Helium discharge up to Ie = 200 ALine-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

fringes

Ie

MODELING of PROTO-PINCH PLASMA

Spectroscopy 1<TePinch<3 eV

Ohmic input = electron flow convected flux TePinch =

2 eVInterferometry suggests plasma 50% ionized at Ie=600 A pH2=8•10-3 mbar gives: ne

Pinch = 2•1020 m-3

However estimated Ohmic input P= 4kW main loss in electrode plasma sheaths Pelectrodes= 46 kWpower injected near the electrodes gives: Te

electrodes = 0.4

eVconstant electron pressure gives: ne

electrodes = 5•1020 m-3



EXTRAPOLATION to Screw Pinch of PROTO-SPHERA Assuming same plasma near the electrodes at Ie=60 kATe

electrodes = 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 eV constant electron pressure:

nePinch = 1.5•1019 m-3

Ohmic input P = 5.4 MW

OHMIC P 5.4 MW +

SHEATHS Pelectrodes 4.6 MW +

Helicity Injection PHI 0.6 MW =

TOTAL POWER PPinch 10.6 MW



Panel Questions & Answers1. 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 VIth FP? OK

10

p() = pe=constant for <X inside the SP and

p() = pe + Cp(X)1.1 for ≥X inside the ST

for < X inside the SP and

for ≥X inside the ST

Idia2 Ie

2 X

Idia2 Ie

2 CI2 X 1.1

• For every Equilibrium calculation the poloidal beta of the Spherical Torus is an input parameter as well as the total toroidal current Ip inside the ST and Ie inside SP

11

Ie Screw Pinch longitudinal current, pe is the pressure inside the SP and

X is the poloidal flux function at the separatrix the exponent

in the SP is =2 Idia()Screw Pinch is

force-free relaxation parameter . Constant inside Pinch

Idia2

=0 • /B2

j

B

Equilibria Computation Before Panel



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 varyingh and c=X+h(max-X)

X ≤≤c

>c

dFd

= Ie

X

1 - sin( - X )

2(c - X )

dFd

= Ie

X

1 -

dia 0

dIdia

d0CI

dFd

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:

Idia2 Ie

2 X



• 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.



Stability: CASE n=016

• 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).

T T T

e

B B T

B



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 Contract Tender Construction Check Assembly Final check

Guarantee

Tender Orders Work Final check

Tender Orders Assembly Final check

Design Contract Tender Construction Check Assembly Final check

Guarantee

Design Contract Tender Construction Check Assembly Check Final check

Guarantee

Design Tender Construction Check Check Assembly Final check

Guarantee

Design Tender Work Final check

Guarantee



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



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 =

B

B

Coincidence of zero of and of fixes =x1,4/2x1,3=2.026..., so that at

R=0, Z=x1,3/x1,4=0.775... the zeroes coincide



CKFThe superposition of the two force-free fields is: For ≥0.402..., in a simply connected region, toroidal current density j has the same sign:

r =1CK +

F

Chandrasekhar-Kendall-Furth force-free field (CKF)



CKF •CKF force-free-fields (p=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 StabilityCKF, 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=20ST/<B2

>ST≈1/3

Trend of MHD stability with IST/Ie:

same as in PROTO-SPHERA

CKF Stability

Even in free boundary up to ST=20ST/<B2>ST ≈1

Trend of MHD stability with :

same as in PROTO-SPHERA

IMPORTANCE of high for a reactor: reduces cost and size

Pfusion~2B4 therefore higher lower B

nTE~/ {a2B2} therefore higher lower a at same

associazione euratom-enea sulla fusione ----- culham 15-17 september 2003 riserva 11

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