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  • FSC

    Magneto-Inertial Implosion Experiments on the OMEGA Laser

    O. V. Gotchev et al.Laboratory for Laser EnergeticsUniversity of Rochester

    Innovative Confinement Concepts WorkshopFebruary 12-14, 2007

    College Park, Maryland

  • FSC

    Novel implosion experiments, using magnetic flux compression are underway on OMEGA

    A magnetized cylindrical target is imploded by OMEGA to compress a pre-seeded magnetic flux to high values.

    A ~0.1 MG seed magnetic field is generated with a double coil driven by a portable capacitive discharge system (MIFEDS)1.

    Proton radiography technique is used for detection of the compressed magnetic fields.

    Laser-driven flux compression will be used for thermal insulation of an ICF hot spot, laboratory astrophysics experiments and HEDP physics.

    Summary

    1 MIFEDS Magneto-Inertial Fusion Electrical Discharge System

  • FSCCollaborators

    N. W. JangJ. P. KnauerM. D. Barbero

    D. D. MeyerhoferR. Betti

    R. D. PetrassoC. K. Li

    Laboratory for Laser EnergeticsUniversity of Rochester

    Plasma Science and Fusion CenterMassachusetts Institute of Technology

  • FSC

    Magneto-inertial fusion: ICF assisted by a magnetic field

    Ignition requirements and gain limits of conventional ICF

    Hot spot insulation with strong magnetic fields

    Feasibility of laser driven magnetic flux compression

    A seed magnetic field generator for OMEGA

    Design of laser-driven flux compression (LDFC) experiments

    Initial experiments and discussion

    Future directions and applications

    Outline

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    Assuming ignition,

    Low implosion velocities Vi result in cold hot-spot:

    Conventional ICF designs have only moderate gain.

    A solution:

    Provide MGauss magnetic insulation to reduce thermal conductionlosses in the forming hot spot.

    An added benefit is increased hydrodynamic stability.

    Massive ICF targets, imploded with low velocity provide higher gains*

    1.3iLaser

    Fusion

    V1~

    EEG =

    * R. Betti and C. Zhou, Phys. Plasmas, 12 110702 (2005)

    1.4ihs V~T

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    Effects of a strong magnetic field on ICF target hot-spot parameters

    At 10 MG compressed field: At 100 MG:

    0.01|| for cee12

    r=27 m -particles magnetically trapped: r/rhs 0.5

    4104 0.2|| for cee1.2 4102

    r=270 m r/rhs > 5

    LILAC simulation of NIF 1.5 MJ, direct-drive point design* hs 30g/cc, Ths 7keV (before ignition), rhs 50m.

    Braginskii conductivity used, anomalous effects not considered.

    *P. W. McKenty, et al., Phys. Plasmas 8, 2315 (2001)

    Tens of MG magnetic field is needed for effective reduction of the hot-spot thermal losses through magnetic insulation.

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    High magnetic fields can be generated with compression of a seed field

    A seed field can be inserted in a spherical target via an exploding wire

    D2

    An azimuthal seed field is created by the current in an exploding lithium or carbon wire. Red lines show current path after gas ionization.

    I0

    B0

    Or in a cylindrical target using Helmholtz coils.

    The axial seed field is created in a Helmholtz coil outside the target

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    The imploding shell compresses the ionized gas fill that has trapped the magnetic field

    The magnetic Reynolds number Rem in OMEGA cylindrical implosions is high, due to the high implosion velocity and low plasma resistivity in the ionized gas fill.

    An average value of Rem~100 is obtained from simulations.

    D2D2

    Laser

    )1/Re2(1

    min

    00maxz

    m

    RRBB

    =

    Shock

  • FSC

    The seed field is initially compressed by the ionizing front of the shock moving ahead of the CH shell.

    LILAC-MHD simulation of 5-atm, D2-fill cylindrical implosion. B0=0.1 MGOMEGA pulse is 16 kJ, 1-ns square.

    20

    0 )()(

    tRRBtB

    Time evolution from t=1.8 ns to t=2.3 ns Time evolution from t=2.3 ns to t=3.0 ns

  • FSC

    The shell further compresses the ionized gas, amplifying the field, up to the time of stagnation

    D2 gas fill

    Magnetic field levels at which r/rhs ~1 are reached in the D2 hot spot.

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    The reduction of (Braginskii) thermal conductivity leads to significant increase of the hot-spot temperature in D2

    (/|| )ions

    (/|| )e-

    B0=0

    Ths i

    Ths e

    After effective B-field compression, the ion temperature in the hot-spot increases 6-fold.

    Anomalous effects are not included.

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    The seed magnetic field is generated in a double coil configuration suitable for OMEGA implosions

    1Radia Simulation (O. Chubar, P. Elleaume, J. Chavanne, J. Synchrotron Rad. (1998). 5, 481-484)

    Coil dimensions:

    d=4.4 mm and R=2 mm

    Coil parameters:

    L~25 nH and R~0.1

    d=4.4 mm

    R=d

    Simulated B-field along the coil axis1

    1.8 m

    0.3 m

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    A TIM-based, fast pulser delivers energy efficiently, while reducing the transmission distance and EMI issues.

    Capacitive discharge system that can safely store up to 130 Joules of energy (at 36 kV).

    Low-impedance (

  • FSC

    -0.2

    0

    0.2

    0.4

    0.6

    0.8

    1

    1.2

    -1.00E-06 0.00E+00 1.00E-06 2.00E-06 3.00E-06

    Time(s)

    Rel

    ativ

    e In

    tens

    ity30 kV 07/24/0630 kV 07/06/06'30kV 07/13/06

    We have measured 0.1 to 0.15 MG seed magnetic field with the prototype system charged to 25 - 30 kV

    MGmm10radV,1mmd,(t)dBV(t)

    (t))(cosI(t)I

    z

    zzrot

    rot2

    0DET

    ===

    =

    Oscilloscope

    MIFEDS prototype

    Diagnostic TIM

    0.15MGB MAXz_

    MIFEDS in DTIM

    200 nF, 40kV max

    Probe laserPolarizer

    Polarizer Detector

    Transmission lineHelmholtz coil

    Vacuum chamber

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    The target is compressed by 40 OMEGA beams (~16 kJ) while 20 (8 kJ) are used for proton radiography

    1C. K. Li et al., Phys. Rev. Lett. 97, 135003 (2006)

    Cylindrical implosion target860 m diam. and 1.5 mm long

    Cylindrical target stalk

    Backlighter target stalk

    Compression and fields are measured with proton radiography1.

    14.7 MeV D+3He fusion protons are produced by imploding a D3He filled glass microballoon.

    MIFEDS coil4 mm diam. 500 m wide

    Distance from backlighter to target is 9 mm.

    Distance to the CR-39 detector is ~10.5 cm.

    Backlighter target

  • FSC

    Proton radiograph of an imploded cylindrical D2-filled shell without fields shows imploded core.

    No seed field present in this shot.

    The dense core slows-down the 14.7-MeV proton below detection threshold

    1.5 mm

    CR-39 track density map Track diameter mapStalk

  • FSC

    One cylindrical implosion with the seed field on, was performed. The protons through the core are not detectable with current set up.

    For our June 2007 experiments, the filter thickness in front of the CR-39 has been reduced to record the protons traveling through the core.

    1.5 mm

    Track density map Track diameter map

    Stalk

  • FSC

    Low-adiabat, low-velocity implosions of magnetized ICF (MIF) targets will be pursued in the future.

    Magnetic field compression in cylindrical and spherical geometry in the context of inertial fusion.

    Guiding fields for hot electrons in fast ignition.

    Generation of positron-electron plasma in the laboratory1.

    Propagation of plasma jets in large scale magnetic field.

    B

    Petawatt Beam

    Compressed field

    OMEGA EPbeam

    OMEGA beams

    1500 m

    500 m

    Wire targete+e-

    1J. Myatt et al., Bull. Am. Phys. Soc. 51 (7), 25 (2006)

  • FSC

    Novel implosion experiments, using magnetic flux compression are underway on OMEGA

    A magnetized cylindrical target is imploded by OMEGA to compress a pre-seeded magnetic flux to high values.

    A ~0.1 MG seed magnetic field is generated with a double coil driven by a portable capacitive discharge system (MIFEDS)1.

    Proton radiography technique is used for detection of the compressed magnetic fields.

    Laser-driven flux compression will be used for thermal insulation of an ICF hot spot, laboratory astrophysics experiments and HEDP physics.

    Summary

    1 MIFEDS Magneto-Inertial Fusion Electrical Discharge System

  • FSC

    Magnetic flux ratio and conservation:

    where

    - effective flux compression for Rem >> 1

    Controlled megagauss magnetic fields have been generated with magnetic flux compression

    ),Re

    1(1R(t)

    v2R(t)

    )v2(vdtdB

    B1

    m

    ifi =

    =

    2

    min

    00

    0

    2

    min

    00max R

    RB

    RRBB

    =

    =

    )/Re12(1

    min

    00max

    m

    RRBB

    =

    ==

    ===

    =

    Lv

    vvRe

    ,v2R

    2RL

    RL

    dtdln

    0i

    f

    im

    f

    0

    sh

    sh1

    Supplemental

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    Small interaction volume at TCC requires low mass, single turn coils. (Low inductance system ~ tens of nH).

    The current rise must be fast in order to minimize the action integral that determines the lifetime of the small coil (due to Joule heating).

    Need high voltages to maximize energy (Estored=CV2/2) density.

    The energy must be delivered via low-impedance transmission line through a fast switch.

    Macroscopic seed field changing little on the timescale of an Omega shot is needed.

    t)(EtIL2

    tt2

    I T)R(B,t)(E m2max

    2max

    j