9c - experiments in magnetic flux compression...

University of Alabama in Huntsville

Experiments in Magnetic Flux Compression

Using Plasma Armatures

Matt Turner

Propulsion Research CenterDepartment of Mechanical & Aerospace Engineering

University of Alabama at Huntsville

Academic Advisor: Dr. Clark W. Hawk, PRC/UAH

Research Mentor: Dr. Ron J. Litchford, NASA/MSFC

A Cooperative Research Project betweenUAH/PRC and NASA/MSFC

University of Alabama in Huntsville

Potential Space Thruster Applications

Pulsed Reactor• Microfusion • HEDM

Performance Attributes• improved flux compression • reduced seed field• collimated thrust• power generation

Design Attributes• smaller• more compact• lower weight

detonationtarget

Winterberg / Daedalus ClassMagnetic Compression Reaction Chamber

superconductorcompression wall

exhaust

inductiongenerator

coil

magneticfield

excitationfield coils

electricalpower

structuralshell

University of Alabama in Huntsville

Major Uncertainties

100

>>== uLB

BR i

m σµ

Magnetic Reynolds Number

⇒ low magnetic diffusivitym

m RD

11

0∼=

σµ

micro fusion detonation → Rm >> 1

pulsed plasma jet → Rm > 1

high explosive detonation → Rm > 1

• achieving sufficiently highelectrical conductivity

• suppression of Rayleigh-Taylorsurface instabilities

conductivity → relatively lowtemperature → very highvelocity → very high

Plasma Jet Characteristics

Magnetic Flux Compression with Plasmas

University of Alabama in Huntsville

Fundamental Compression Experiments

Mark 1 Device

… measurement of plasma electricalconductivity is a critical need

Research goal is to investigate scientificand technological feasibility using non-nuclear detonations…

Radial-mode, magnetic flux compression reactor

•Opposing HE charges

•Radially expanding plasmaarmature

•Moderate scale technology demonstrator

•Low cost expendable unit

Hz

r ua

plasmaarmature

shieldingshell

magnetcoil

generatorcoil

shaped chargeexplosive

University of Alabama in Huntsville

Measurement of Plasma Jet Conductivity

Inductive measurement technique provides a low costdirect measurement of plasma jet electrical conductivity …

Lin, S-C., Resler, E. L., and Kantrowitz, A., “Electrical Conductivity of Highly Ionized Argon Produced by Shock Waves,” J. App. Phys., Vol. 26, No. 1, Jan. 1955.

field coil

cavitycharge

jet

search coil

Lin CoilSystem

evacuated tube

time of flightcircuit

University of Alabama in Huntsville

Principle of Inductive Measurement

• magnetic field created by field coil• conductive plasma jet displaces magnetic field lines• magnetic field lines cut across search coil and induce an emf• degree of field displacement (emf) is correlated with electrical conductivity• apparatus calibrated by pre-firing a slug with known conductivity & velocity• electrical conductivity determined by inversion of Fredholm Eqn. of 1st kind

field coil

plasma jet

search coil

Lin CoilSystem

evacuatedshock tube

field lines

University of Alabama in Huntsville

Lin Coil Design

Field CoilField Coil√ 900-turn coil

√ No. 18 enameled wireSearch CoilSearch Coil

√ 50-turn coil

√ No. 28 enameled wire

Design Specifications•• 0.5 −− 1 Tesla excitation field

•• 1-inch diameter working volume

•• 60 A/ 300 V excitation power limit

0.1250

0.1250

0.1250

1.5000

0.1250

0.3125

0.4000

1.3000

1.4250

2.4250

3.8000

all measurements in inches

material: nylon

University of Alabama in Huntsville

Search Coil Parameters

• 50-turns / No. 28 enamelled wire

• L = 8 mm / D = 5.7 cm

• dual 25-turn coil construction

− standard option

− push-pull option

Previous experience using inductive conductivity probes with shock layers has shown thata localized electrostatic charge can generate a signal in a standard configuration searchcoil due to finite capacitance between the coil and the ionized gas inside the tube

PushPush--Pull ConfigurationPull Configuration√ cancels capacitive pick-up

√ Rd is critical damping resistance (≈ 1000 Ω)

Search Coil Design ConsiderationsL

D

standard configuration

L

D

RdRd

push-pull configuration

University of Alabama in Huntsville

Lin Coil Calibration Apparatus

Lin CoilSystem

solenoidactutaed

valve

laser

guide tubeplastic barrel

opticaldetector

breech

glass tubesection

compressedgas cylinder

pressuregage

pressureregulated

helium

manual valve

• light gas gun for slug acceleration (150 psig He)• 1-inch i.d. plastic barrel with 6-inch long glass test section• aluminum slug (D = 1-inch / L = 4-inch / ρ ρ = 3(10)-8 ΩΩ-m)• direct optical measurement of slug velocity• automated solenoid actuation with high speed data capture

University of Alabama in Huntsville

Lin Coil Calibration – Timing Verification

Current

Velocity

Response Function

Velocity

Current

Response Fnc (push)

Response Fnc (pull)

University of Alabama in Huntsville

distance (cm)

indu

ced

volt

age

0 1 2 3 4 5 6 7 80

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

datagaussian fit(σ = 1.8 cm)

distance (cm)

σ/σ

c

-1 0 1 2 3 4 5 60

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

predictedexact

constrained linear inversion

(H = I; γ = 10-3)

Probe Calibration – Algorithm Verification

Probe Response Function Algorithm Performance

University of Alabama in Huntsville

Shape Charge Plasma Jet Conductivity

field coil

cavitycharge

Plasma Jet Conductivity Measurement

jet

search coil

Lin CoilSystem

evacuated tube

time of flightcircuit

University of Alabama in Huntsville

Preliminary HE Testing

• Octol (75% HMX/25% TNT)

• Potassium-Carbonate Seeding- 0%, 1%, 2% by mass

• 15 gram shape charges- 1.25 inch diameter- 44° cone angle

Cast Shape Charges

Control / DAQ Test Article in Chamber

probe

shape charge

detonator

University of Alabama in Huntsville

Preliminary Test Results

Time-of-flight

Response Fnc (push)

Response Fnc (pull)

University of Alabama in Huntsville

HE Testing – Results

Time-of-Flight Output

time (µs)

resi

stor

volt

age

0 10 20 30 40 500

5

10

15

20

25

30test 2-7 (0% seed)BA

11 µµs

10 cm

1

6

109.1 / sec

11(10) sec

dx mu km

dt

−

−= = =Plasma velocity:

cavitycharge

jet

Lin CoilSystem

time of flightcircuit

University of Alabama in Huntsville

time (µs)

resp

onse

(V)

0 10 20 30 40 50-25

-20

-15

-10

-5

0

5

10

15

20

25test 2-10 (0% seed)

push coil

pull coil

time (µs)

resp

onse

(V)

0 10 20 30 40 50-100

-75

-50

-25

0

25

50

75

100test 2-12 (2% seed)

push coil

pull coil

HE Testing – Results

Search Coil Response Function

0% KCO3 2% KCO3

University of Alabama in Huntsville

HE Testing – Analysis

Step Function Gaussian Curve

σ ∗

Vp

Φ

2 2*

1 22 2

c c p c c

c cp c

u I V u I

U IV U I

σ

σ

Φ= Ψ = Ψ

Φ

For a Gaussian function:

1 2 1Ψ = Ψ =

University of Alabama in Huntsville

Integral Method Calculation:

2-122-62-102-4Test

2102284622

18193.61.9

24265.12.5

5658115.9

2200Seed

(%)

51 0c

σ

σ

∗−×

210c c

u

u

σ

σ

∗−×

( / )kS mσ ∗

10 ( )u mµ σ ∗ −

HE Testing – Analysis2*

22c c

c c

u I

U I

σ

σ

Φ= Ψ

Φ

University of Alabama in Huntsville

Fundamental Research on Rayleigh Taylor

Instabilities

u ~ 104 m/s Te ~ 10 eV

vacuumchamber

copperbarrier

HZ

pulseplasmagun

magnetic flux compression wall

excitationcoil

Csparkgap

Tapered Pulse Plasma Gun

Plasma Jet Characteristics

•High speed imaging

•Time-resolved interferometry

•Time-resolved spectroscopy

•dB/dt probes

Diagnostics:

University of Alabama in Huntsville

Development of Pulsed Plasma Experiment

Lin CoilSystem

triggeredsparkgap

C

optical windowpulse plasmagun

vacuumchamber

diffusionpump

high voltagefeedthrough

University of Alabama in Huntsville

Pulsed Plasma Gun Design

University of Alabama in Huntsville

Experimental Set-Up

Design

• four 2-µµF capacitors(ONR surplus)

• configurablepower bus (1, 2, 3, or 4 capacitors in parallel)

• capacitor bank and discharge circuitmounted on cart forportability

University of Alabama in Huntsville

Conclusions• An electrode-less inductive probe for measuring plasma jet

electrical conductivity was designed, calibrated, and tested• Calibrated with slug of known conductivity

Inversion algorithm marginally successfulAssumed conductivity distribution with known solutions

• Laboratory experiments using seeded and unseeded shape charge explosives

Average jet velocity = 9-10 km/secMeasured conductivities: 4 kS/m (unseeded)

26 kS/m (2% seed)• Magnetic Reynolds numbers > 1 (for fractions of a meter scale)

Mark 1 device has a good chance of being sucessful• Inductive probing technique represents a very successful and

reliable tool for investigating plasma jet characteristics• High explosive plasma sources capable of producing high Rem

University of Alabama in Huntsville

Future Plans1.Pulsed plasma gun / discharge

circuit completed- tapered plasma gun designed,

fabricated, and tested- high voltage discharge circuit

operational- vacuum chamber, pump,

feedthroughs assembled- high-purity aluminum magnet

on its way- testing to begin very soon

2.Mark 1 Device, radial-mode magnetic flux compression reactor using high explosive shaped charges

electricalconnector

LN2 Dewarvacuum liner

solenoid

stainless steel

phenolic