Progress and plans on the

Maryland Centrifugal Experiment

R.F. Ellis, A.Case, R. Clary, A.B. Hassam, R. Lunsford,

C. Teodorescu, R. Elton, J. Ghosh,

University of Maryland, College Park MD

Highlights

• Supersonic rotation : MS = 1-4

• Steady discharges : τLIFE and τMOM >> τinterchange

• Measurements of velocity shear sufficient for

MHD stability

• Velocity exceeding Alfven ionization limit in HR

mode

• Hα emission much lower in HR mode (tokamak

like)

• Hints of centrifugal confinement in HR mode

Upgrades this year

• Capacitor bank voltage increased from 10 to 20

kV, energy increased from 176 to 708 kJ

• New azimuthal magnetic probe arrays

• New 5 chord Doppler spectroscopy system

• New Hα emission detectors

• 2 color interferometer almost completed

Basic idea : centrifugal confinement scheme requires

supersonic rotation and velocity shear

Major Goals• Supersonic rotation(yes-sonic Mach # 1 <MS <4)

• Velocity shear stabilization (maybe- new results this meeting)

• Centrifugal confinement(hints)

V applied

Schematics of MCX and discharge circuit

V

insulatorinsulator

Initial vacuum E·B => breakdown, V~ 10 kV

Metallic core

MCX Cutaway : Stainless steel core is high voltage

electrode, disc insulators terminate plasma region

Disc Insulators

Disc InsulatorStainless Core

4 m

B ~ 1T , V ~ 10 kV

Rm ~ 9 (Bmid = 0.2 - 0.3 T)

ap= 0.20 m ~ 102 ρρρρi , Lp = 1.40 m

Hydrogen, Static Fill, P0 = 5-10 mtorr

ni ~ few ×××× 1020 m-3 (fully ionized)

T ~ 20 – 60 eV

V applied 5-20 kV, pulse length 1-10 ms

vrot ≡≡≡≡ uExB ≥≥≥≥ 100 km/s

Plasma Parameters

MCX has two operating modes

O (ordinary mode)

Sonic Mach # Ms = vROT /vti ~ 1

HR (high rotation mode)

Ms ~ 3

3 x larger stored energy

3 x longer momentum confinement time

Alfven Mach # MA = vROT /vA ~ 1

Plasma voltage remains steady for 1000’s of

MHD instability times

• MHD instability growth time τMHD ~ 2 - 20µs.

• Measured momentum confinement time τmom ~ 200µs.

• MHD stable due to velocity shear?

• HR mode:

MS ~3

• O mode:

1< MS <2

MS= vrot/ cS

VP

IP

0 1 2 3 4 5 6-10

-8

-6

-4

-2

0

2

Load

Voltag

e (

kV

)

Shot #: MCX031111-12 ; Dated: 11/11/2003 03:05:53 PM

0 1 2 3 4 5 6-4

-2

0

2

4

6

8

Load

Cu

rren

t (k

A)

Time (ms)

HRO

tstart tcrowbar

τmomτMHD

Calculated timescales for comparison to MCX

discharge duration (> 5 ms) and momentum

confinement time( 200 µµµµs)

Axial Alfven time ~ LP/v

A 5µs

Period of rotation ~ (2πR/uφ) 10µs

Interchange growth time ~ [(aPL

P)/(T/M

p)]1/2 10µs

Axial electron heat conduction time ~ (LP/λ)2 τ

e30µs

Axial sonic time ~ LP/(T/M

p)1/2 30µs

Electron-ion heat exchange time ~ (Mp/m

e)τ

e40µs

Classical viscous damping time ~ (aP/ρ)2 τ

ii8000µs

( n = 2x1020 m-3, T = 30 eV, B = 0.2 T)

2

3

4

5

6

7

6 8 10 12 14 16

Bank Voltage (kV)

Maxim

um

P

lasm

a V

oltage (k

V) - H

R

Avera

ge P

lasm

a V

oltage (k

V) - O

- HR Mode

- O Mode

Plasma Voltage -vs- Bank Voltage

MCX Phase Map

0.5 1 1.5 2 3 4 5 6 7 8

5 kV

6 kV

7 kV

8 kV

9 kV

10 kV

11 kV

12 kV

13 kV

14 kV

15 kV

16 kV

17 kV

18 kV

Series Resistance (W)

Capacitor Bank

Voltage

Alfven Ionization Condition

A proposed limit on velocity with which an ion cloud can move thru a

neutral cloud without ionizing the neutrals

(½ miv

i2)

CRIT= eΦ

Ioniz

In MCX it applies at the boundary between the rotating plasma and the end insulators and maps to the midplane plasma

disc

insulator

z

rω constant on a field line

vins = vmid(rins/rmid)

vROT/vCRIT at the disc insulator

0

1

2

3

0 50 100 150 200

vi/v

c

O Mode

HR Mode

Distribution of Alfven Mach Numbers

0

20

40

60

0.1-0.3 0.3-0.5 0.5-0.7 0.7-0.9 >0.9

MA Range

Nu

mb

er O MODE

HR MODE

• Supersonic rotation

• Alfven ionization

limit exceeded in HR

mode

• Significant Alfven

Mach #

Shot Index

H alpha emission measured at various axial locations

employing fast photodiodes

HR-Mode shows much lower Hα emission than O mode :

similar to tokamaks

• Mid-Plane B-: 3 kG

• Bank Voltage: 8 kV

• Series Resistor: 2.0 Ω

Midplane Hα detector signal

HR mode

Discharge current and voltage

Axial variation of Hα emission in HR & O-Mode

• Mid-plane Hα much less in HR-mode for 3kG parameter

• Bank Voltage: 7 & 8 kV

• Series Resistor: 1.0 Ω, 1.5 Ω, & 2.0 Ω

• Error bars show standard deviatioin for 14 shots

3 kG 2 kG

Doppler shifts show rotation consistent with ExB

broadening gives approximate ion temperature

Midplane Midplane

Spectroscopy Spectroscopy

SetupSetup

C IV Spectra

Unshifted line

Single Chord Measurements

Multi-chord spectroscopic measurements of

Doppler shifts at midplane

spectrometer

Fiber optic cables

Spectrometerwith

ICCD camera

• High-resolution spectrometer:

0.182 nm/mm or 0.692 nm/mm.

• 5-chord measurements across

plasma column at midplane.

• Signal is a superposition of signals

from each 5 user-defined annular

zones:

∑<

+=ji

ijijijiiiiiiiii vISvISvIS ),,(),(),( i, j = 1,...,5

• Deconvolution yields intensity I(R) and velocity v(R) radial profiles.

12

34

5

Plasma angular velocity peaks in the

middle of the plasma column

• E×B drift is the dominant motion.

•MCX plasma shows no solid-body rotation

0

1

2

3

4

5

6

4 10 16 22 28

radial position R (cm)

ΩΩ ΩΩ (

x 1

05 r

ad

/s)

C3+

C2+

C+

H

R outR int

Neutrals and impurities are largely depleted

in the middle of the plasma

0

0.2

0.4

0.6

0.8

1

1.2

4 10 16 22 28

radial position R (cm)

I (a

rb.

un

its)

C3+

C2+

C+

H

• Neutral drag is minimal inside the plasma core.

Shear is large enough for mode stabilization

during HR mode

during O mode

• Velocity shear from C+ Doppler shifts shows stability threshold is exceeded!

[ ] stable)( '2/13/1

int ⇔>Ω µγ RlnR

New azimuthal arrays of magnetic probes

Two arrays located axially

at z = +/- 63cm

16 probes per array,

located radially at last

outer closed flux surface

Complements movable

single probes

Magnetic probes

• 20 Channel δδδδBz array installed and tested,

operating at 1MHz sample rate.

• 4 x,y,z, δδδδB triple probes installed and tested.

• δδδδBz/Bz ranges from 0.3% to 3% depending on

operating mode

• Distinct differences in δδδδBz activity between

operating modes.

• HR mode δδδδBz activity is much higher frequency

than in other modes

• All modes show significant activity below

100 kHz (~rotation frequency).

• All modes show negligible cross correlations

between δδδδBz probes below 35 kHz850 900 950 1000 1050

−6

−4

−2

0

2

4

6

Raw data, Window #1, probes 1,5,9,13

Vo

lts (

plu

s o

ffse

ts)

time, microseconds

0 2 4 6 8 10−8

−7

−6

−5

−4

−3

−2

−1

0

1

Window #1Window #1 Window#2Window #1 Window#2 Window#3

Voltage Trace, shot MCX050826−2

time, ms

volta

ge, k

V

Wobble-free rotation ?

• Axial view of MCX plasma

• Phantom 7.1 camera

• t = 5 ms after breakdown

• dtexposure= 2 µs

• 91,000 frames/s

Picture courtesy of

Ricardo Maqueda, PPPL

End view - 4 consecutive frames 2 µs exposure

Next Steps and status

• Complete diagnostics to study stability and relation to velocity shear : multichord spectrometer(done), magnetic probe arrays(done), two color interferometer system( near completion).

• New diagnostics to study centrifugal confinement. (interferometer chord at mirror throat, diamagnetic loop at the mirror throat, endloss analyzers at disk insulators).

• Expand parameter space, assess scaling. (Surface conditioning, higher voltage 20 kV capacitor bank(done), increased magnetic field Bmid ~ 1.0 T, Bmirr ~ 2.6 T ).

• Direct injection of momentum.(funded experiment pulsed plasma gun – HyperV Corp - see poster this meeting).

Blank slide