A Well-Confined Field-Reversed Configuration Plasma Formed by Dynamic Merging of Two Colliding Compact Toroids in C-2 Hiroshi Gota, Michl W. Binderbauer, Houyang Y. Guo, Michel Tuszewski, Dan Barnes, Leigh Sevier, and the TAE Team Tri Alpha Energy, Inc., Rancho Santa Margarita, CA 92688 Innovative Confinement Concepts (ICC) & US-Japan Compact Torus Plasma (CT) Workshops Seattle, WA, August 16, 2011

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Outline 

C-2 Experiment

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Highlights of Scientific Achievements:  Demonstration of long-lived FRCs by dynamic merging Compact Toroids (CTs)  Active control of n=2 mode rotational instability by (1) quadrupole fields and (2) electrode biasing  Reduction of background neutrals by wall conditioning (Titanium or Lithium getterings)  Improvement in flux confinement / transport properties

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Summary Presenter: H. Gota

ICC/US-Japan CT Workshops, Seattle, WA, Aug 16, 2011

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Schematic of the C-2 Device 1. Form FRC 2. Translate

3. Colliding CTs

 A field-reversed configuration (FRC) plasma is a highly elongated

compact toroid (CT) which has a closed poloidal field with zero or small self-generated toroidal field, an axisymmetric structure, and a high beta value.

 A high temperature FRC has been produced in the newly built and world’s largest compact toroid device, C-2, by dynamically merging two oppositely directed, highly supersonic CTs. Presenter: H. Gota

ICC/US-Japan CT Workshops, Seattle, WA, Aug 16, 2011

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Key Diagnostics in C-2 Thomson Scattering • Electron temperature at 9 spatial points with multi-pulse

Vacuum vessel

CO2/He-Ne Interferometer • Electron density profile using 6 chordal measurements Total temperature

• Magnetic Probes • External field and excluded-flux

radius measurements (Ext. probe) Magnetic field profiles (Int. probe)

• Spectrometers • Ion temperature and flow measurements Impurity monitor

C-2 midplane Presenter: H. Gota

• Bolometers • Tomography • FRC position • Radiated power

ICC/US-Japan CT Workshops, Seattle, WA, Aug 16, 2011

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C-2 Developed a Novel Technique to Form Hot FRCs by Merging Two Colliding θ-Pinch CTs

Diamagnetic signal achieved record lifetime exceeding 2ms

Presenter: H. Gota

ICC/US-Japan CT Workshops, Seattle, WA, Aug 16, 2011

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Key Approaches to Record Lifetime Dynamic Merging

• Dynamic formation • Dynamic fueling

Active Stabilization

Wall Conditioning

• External saddle coils

• Titanium gettering

• Electrode biasing

• Lithium gettering

Presenter: H. Gota

ICC/US-Japan CT Workshops, Seattle, WA, Aug 16, 2011

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Dynamic Colliding/Merging FRCs vZ ~250 km/s

 Time evolution of r∆φ ∼ rs during

translation & collisional merging. Presenter: H. Gota

 Simulation by a new 2D resistive

MHD code, LamyRidge.

ICC/US-Japan CT Workshops, Seattle, WA, Aug 16, 2011

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Density Profile from CO2 Interferometer Initial phase: Colliding two CTs (t = 20 – 50µs)

 Plasma exhibits a

“hollow” density profile as expected from the usual Rigid Rotor profile for an FRC.

 This provides an

important verification for the final FRC state resulting from colliding CTs.

Quiescent phase (t = 0.1 – 1.2ms)

Presenter: H. Gota

ICC/US-Japan CT Workshops, Seattle, WA, Aug 16, 2011

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Field Profiles from Internal Magnetic Probes r∆φ

 Internal probing corroborates the state of field-reversed

configuration with small toroidal field remaining after merging.. Presenter: H. Gota

ICC/US-Japan CT Workshops, Seattle, WA, Aug 16, 2011

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Rethermalization Occurs during Merging, Predominantly into the Ion Channel (300μs)

 Before merging: Te ≤ 50 eV

After merging: Te ≥ 100 eV (obtained from 9-chord TS) Presenter: H. Gota

 Strong ion heating occurs with

Ti ~ 4.5 Te, as in counter-helicity spheromak merging.

ICC/US-Japan CT Workshops, Seattle, WA, Aug 16, 2011 10

Merged FRCs Exhibit ~ 10x Improvement in Confinement over θ-Pinch/Translated FRCs  Flux confinement time, τφ,

is dramatically improved over θ-Pinch FRC scaling: τ φLSX = 6.52 ×10 −5 ρ L−1.07 xs0.5 rs2.14

 Diffusivity for best merged

FRCs approaches classical: D⊥ ≡ η ⊥ µ 0 = a 2 τ φ D⊥cl = 2 D//cl ≈ 0.45Z eff Tt [keV]−3 2

Presenter: H. Gota

ICC/US-Japan CT Workshops, Seattle, WA, Aug 16, 2011 11

Active Control of N=2 Instability: (1) Quadrupole Fields

 Applying quadrupole fields improves FRC stability, extends initial higher performance phase.  The n=2 mode frequency is of similar magnitude to ion diamagnetic frequency: fn=2 ~ Ω*/(2π) ~ 10 kHz Presenter: H. Gota

ICC/US-Japan CT Workshops, Seattle, WA, Aug 16, 2011 12

Active Control of N=2 Instability: (2) Electrode Biasing No Quad (#18409)

N=1 mode

N=2

Ring-electrode

 Applying inward/outward radial electric field may create a velocity shear (Er x Bz) near the separatrix.  N=2 rotational mode suppressed and n=1 mode appears by applying +500V on the electrode. Presenter: H. Gota

ICC/US-Japan CT Workshops, Seattle, WA, Aug 16, 2011 13

Active Control of N=2 Instability: (2) Electrode Biasing Mirnov Probes: 8 azimuthal locations

No Quad (#18409)

NB000 NB045

NB315

NB090

NB270

NB135

NB225 NB180

 FRC spins to an ion diamagnetic direction, but n=1 wobble motion

appears to be an electron diamagnetic direction with positivelybiased electrode. Presenter: H. Gota

ICC/US-Japan CT Workshops, Seattle, WA, Aug 16, 2011 14

Active Control of N=2 Instability: (2) Electrode Biasing Bolometers: AXUV100 – 16ch arrays

t=0.1ms

Trajectory of FRC center (t=0.1–0.9 ms)

 Bolometers also show the n=2 mode suppression and the n=1 wobble motion which indicates the electron diamagnetic direction.

Viewing chords Presenter: H. Gota

ICC/US-Japan CT Workshops, Seattle, WA, Aug 16, 2011 15

Wall Conditioning by Titanium or Lithium Gettering Titanium sublimators

Divertor

Lithium evaporators and deposition direction

Confinement chamber

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Ti / Li depositions covered over the confinement and the divertor stainless-steel chamber walls.

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Wall conditioning is effective at reducing both neutral recycling and impurity influx. Presenter: H. Gota

ICC/US-Japan CT Workshops, Seattle, WA, Aug 16, 2011 16

Wall Conditioning by Titanium Gettering Dα Emission

Impurity Concentration

Ti gettering

W/o Ti

Ti gettering

 Reduced significantly

Deuterium recycling (a factor of 4), as expected. Presenter: H. Gota

W/o Ti

 Also reduced Oxygen and

Carbon, as found in other fusion experiments.

ICC/US-Japan CT Workshops, Seattle, WA, Aug 16, 2011 17

Wall Conditioning by Titanium Gettering Dα Emission

Neutral Density Profile

separatrix

Measurement

DEGAS2 (Equilibrium code)

Separatrix

Dα peak

 “Fresh” Ti gettering significantly decreases neutral density outside the separatrix (e.g., decreases by a factor of 4 at r~65cm). Presenter: H. Gota

ICC/US-Japan CT Workshops, Seattle, WA, Aug 16, 2011 18

Summary 

Hot stable FRCs have been produced in C-2 using the novel merging FRC technique, and plasma lifetime achieved record exceeding 2 ms.

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The merged FRC state exhibits following key properties:  

Strong conversion from kinetic energy into thermal energy, predominantly into the ion channel. Significantly improved confinement over the original θ-pinch FRCs with flux transport rates approaching classical values.

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N=2 rotational mode has been controlled by quadrupole fields as well as by positively-biased electrode.

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Wall conditioning is effective at reducing both neutral recycling and impurity influx. Presenter: H. Gota

ICC/US-Japan CT Workshops, Seattle, WA, Aug 16, 2011 19