Experimental study of strong shocks driven by compact pulsed power

J. Larour1, J. Matarranz1, C. Stehlé2, N. Champion2, A. Ciardi2

1 Laboratoire de Physique de Plasmas LPP, UMR 7648, École Polytechnique, UPMC, CNRS, 91228 Palaiseau, France2 LERMA, UMR 8112, Observatoire de Paris, UPMC, CNRS, 5 Place J. Janssen, 92195 Meudon, France

P.We_79

LERMA

Strong shocks (M>>1), in gases achieved high T : Tshock ~ matom vshock

2

T v

unshocked

shocked Radiative absorption => heating / ionization

hnhn

Precursor

Optically thick

hnStrong Absorption

T

THEY RADIATE !MODIFICATION OF THE STRUCTURE OF THE SHOCK i.e. radiative precursor

RADIATIVE SHOCKS

ASTROPYSICAL CONTEXT

YOUNG STARS

accretion from the circumstellar disk to the photosphere;High velocity (free fall) : u ~  (2 GM/R)0.5 400 km/s

Shock : Highly supersonic : M >>1 Temperature Tshock ~ 3 m u2/(16 k) ~ several 106K

The accretion shocks are not resolved : radiative signatures (for instance in X rays) & models => accretion rate

Young Star with his disk and accretion columns.

Artist view taken from Brickhouse et al ApJ 2010

Where is the shock forming? Photosphere or chromosphere ? different regimes for the radiation transport (optically thin or not )

The laser is focalized on a foil, which converts the laser energy into mechanical energy.

PALS iodine Laser in Prague (1 kJ, 1.3 mm, 0.3 ns)

STANDARD CONDITION 60 km/s Xe P≤ 0.3 bar

Principle of Radiative Shock generation with a laser

Objectives Defining and testing : a compact electrical driver (1 kJ) capable to launch quasi 1-D shocks in low pressure

gases suitable to test diagnostics before laser experiments providing a benchmark for codes easy to handle for training

First tasks Simulate shocks in a low pressure device optimize the geometry build a device make tests

Laser pulsed power

Flux 1014 W.cm-2

Characteristic time <nsEnergy > 50JTube length ~ mmTube diam 400µm to 1 mmPressure 0.1 – 0.3 barShock speed > 60km/s

Flux 109 W.cm-2

Characteristic time ~ µsEnergy ~ kJTube length ~ cmTube diam 1 mmPressure ~ mbarShock speed 5-30km/s

Te prec = T0 Te prec = T0 to 3T0 Te prec = T0 to 3T0

No precursor emerging precursor immediate precursor

Electron temperatureNumerical simulation of 1D shocks

Hydro-rad MULTI code Lagrangian description of the shock Macroscopic approach of the plasma :• Density• Electron temperature• Mean charge

Sh

ock

spee

d k

m/s

Ma

ch nu

mb

er

Exp (Kondo 2006)Rad. Limit VradMULTI simulation

Shock front

Rad. precursor

P.We_79

Conclusion

Shocks of interest for astrophysics can be launched electrically

Gas pressure and electrode shaping can be optimized for getting high Mach number shocks and noticeable plasma temperature and gas ionisation.

A radial optical observation of the shock front with high spatial resolution and spectral capabilities is possible with a set of optical fibers.

Additional diagnostics are under implementation.

<Z> in shock

Mean charge Density

HV power supply 15kV

Current probe

Conical electrodes

Trigger

shock tube

Pumping & pressure regulation

Optical fiber

11 caps 0.6µF

70 kV Marx generator

switch

Voltage measurement

Air for the

switch

Ar Xe for shock tube

80 Pa air11 caps 0.6µF14kV

Speed 6,7 km/s(Kondo: v=15km/s 80Pa Xe

First results with a conical tube

Light (V)

Time (ns)

Shock tube by Kondo (IFSA 2005-2007)

shorter tube by X 0.5

longer tube by X 2Kondo’s tube is Optimized for 200 Pa XeA longer cone might be better at low P

Optimisation of conical electrodes

Summary of simulation results

Radiative regime at low pressure and high speed (P<100Pa, V> 10km/sExtended precursor in a low density gas >10cmPrecursor temperature not sufficient for pre-ionising At 12.5 Pa Tmax ~ 9eV and <Z> ~ 8

Experimental scheme Principle of a Mather plasma focus (PF)

A plasma sheath, initiated by a surface flashover, is lanched by the jxB magnetic pressure

caps few Torrs gas pinch

insulator electrodes

With

and x~ 1 the snowplow factor

Straight tube

Conical tube

Mass

Speed

Current

Charge

Position

Straight tube vs conical tube :No change on current (amplitude, risetime)Mass of the plasma sheath smaller because the tube cross section decreases)Speed roughly x 2

Influence of tube shape

Experimental study of strong shocks driven by compact pulsed power

J. Larour1, J. Matarranz1, C. Stehlé2, N. Champion2, A. Ciardi2

1 Laboratoire de Physique de Plasmas LPP, UMR 7648, École Polytechnique, UPMC, CNRS, 91228 Palaiseau, France2 LERMA, UMR 8112, Observatoire de Paris, UPMC, CNRS, 5 Place J. Janssen, 92195 Meudon, France

P.We_79

LERMA

Strong shocks (M>>1), in gases achieved high temperatures : Tshock ~ matom vshock

2

T v

unshocked

shocked Radiative absorption => heating / ionization

hnhn

Precursor

Optically thick

hnStrong Absorption

T

THEY RADIATE !MODIFICATION OF THE STRUCTURE OF THE SHOCK, i.e. radiative precursor

RADIATIVE SHOCKS

ASTROPYSICAL CONTEXT

YOUNG STARS

accretion from the circumstellar disk to the photosphere;High velocity (free fall) : u ~  (2 GM/R)0.5 400 km/s

Shock : Highly supersonic : M >>1 Temperature Tshock ~ 3 m u2/(16 k) ~ several 106K

The accretion shocks are not resolved : radiative signatures (for instance in X rays) & models => accretion rate

Young Star with his disk and accretion columns.

Artist view taken from Brickhouse et al ApJ 2010

Where is the shock forming? Photosphere or chromosphere ? different regimes for the radiation transport (optically thin or not )

The laser is focalized on a foil, which converts the laser energy into mechanical energy.

PALS iodine Laser in Prague (1 kJ, 1.3 mm, 0.3 ns)

STANDARD CONDITIONS : 60 km/s Xe P ≤ 0.3 bar

Principle of Radiative Shock generation with a laser

Conclusion

Shocks of interest for astrophysics can be launched electrically

Gas pressure and electrode shaping can be optimized for getting high Mach number shocks and noticeable plasma temperature and gas ionisation.

A radial optical observation of the shock front with high spatial resolution and spectral capabilities is possible with a set of optical fibers.

Additional diagnostics are under implementation.

80 Pa air11 caps 0.6µF14kV

Speed 6,7 km/s(Kondo: v=15km/s 80Pa Xe

First results with a conical tube

Light (V)

Time (ns)

Summary of simulation results

Hydro-rad MULTI, Lagrangian description of the shockRadiative regime at low pressure and high speed (P<100Pa, V> 10km/sExtended precursor in a low density gas >10cmPrecursor temperature not sufficient for pre-ionising At 12.5 Pa Tmax ~ 9eV and <Z> ~ 8

Objectives Defining and testing : a compact electrical driver (1 kJ) capable to launch quasi 1-D shocks in low pressure

gases suitable to test diagnostics before laser experiments providing a benchmark for codes easy to handle for training

First tasks Simulate shocks in a low pressure device optimize the geometry build a device make tests

Laser pulsed power

Flux 1014 W.cm-2

Characteristic time <nsEnergy > 50JTube length ~ mmTube diam 400µm to 1 mmPressure 0.1 – 0.3 barShock speed > 60km/s

Flux 109 W.cm-2

Characteristic time ~ µsEnergy ~ kJTube length ~ cmTube diam 1 mmPressure ~ mbarShock speed 5-30km/s

Experimental scheme Principle of a Mather plasma focus (PF)

A plasma sheath, initiated by a surface flashover, is lanched by the jxB magnetic pressure

caps few Torrs gas pinch

insulator electrodes