MEASUREMENT OF ION ENERGIZATION IN LABORATORY PLASMAS · MEASUREMENT OF ION ENERGIZATION IN LABORATORY PLASMAS Earl Scime* Division of Plasma Physics Mini-conference on Physics of

MEASUREMENT OF ION ENERGIZATION IN

LABORATORY PLASMAS

Earl Scime*

Division of Plasma Physics Mini-conference on

Physics of the Radiation Belts: Collaboration between

Laboratory, Theory and Satellite Observations

April 2016

*In collaboration with: Evan Aguirre, Drew Elliott, Tim Good, Miguel Henriquez, Amy Keesee,

Julianne McIllvain, John McKee, Zach Short, and Derek Thompson

MOTIVATION

Theory/Model validation is critically important to establish causal relationships

between proposed physical processes and particle energization:

(a) chorus energization of electrons in the radiation belts

(b) ion temperature anisotropy driven instabilities

(c) ion acceleration in double layers

(d) ion beam structures created only in magnetic reconnection exhausts?

(e) Alfvén wave heating of ions in the solar corona

(f) Ion flows along and across magnetic fields oblique to boundaries

The laboratory plasma experiment

Measurements of ion velocity distributions in laboratory plasmas

Insight gained from the laboratory measurements

CHORUS ENERGIZATION OF ELECTRONS IN THE BELTS

Models indicate chorus waves required to generate

fluxes of energetic electrons observed by Van Allen

Probes.

Can model/theory be tested in the laboratory?

[Thorne GRL 2010]

[Tu et al. GRL 2014]

ION TEMPERATURE ANISOTROPY DRIVEN INSTABILITIES

In the magnetosphere, it appears that the

isotropization of ions can be described by a

simple expression of the form (Anderson and

Fuselier 1993; Anderson et al. 1994; Phan et al.

1994; Tan et al. 1998)

p

i

p

i

iS

T

T

||||

1 2

|| ||8i i onkT B

Here Sp and p are dimensionless fitting

parameters. Theoretical investigations of the

stability of collisionless anisotropic plasmas

indicate that two instabilities are likely to grow in

the high beta, ~ 1, anisotropic, Ti > Ti||,

conditions of the magnetosheath: the mirror

mode, and the Alfvén Ion Cyclotron Instability

(also known as the anisotropic ion cyclotron

instability).

Are waves observed?

Do the waves limit the anisotropy?

Space data

ION ACCELERATION IN DOUBLE LAYERS

Electric field structures appear spontaneously

in auroral zone

How stable are these structures?

Does the ion streaming lead to instabilities?

FIG. 1 (a) Parallel and (b) perpendicular electric fields at two different

band widths: 256 (black) and 16 Hz (red). (c) Ion and (d) electron energy

flux, in units of logeV=cm2 s sr eV and (e) ion density. The vertical black

arrows in panel (a) indicate the IACB crossings. [Main et al., 2006]

ION BEAMS IN MAGNETIC RECONNECTION EXHAUSTS

• Bursty bulk flow observed by THEMIS ‘B’ on 26 Feb 2008. 6 minutes of data.

• THEMIS initially in stagnant pre-existing plasma, then at 11:11:50 there is a dipolarization in the field (increase in +Bz, start of fast flow in +vx, distinct change in ion and electron spectrogram characteristics). This corresponds to the onset of reconnection driving plasma Earthward.

• THEMIS observes –Bx throughout, indicating it is below the current sheet. The change in the strength of Bx is probably due to either changes in the current sheet thickness, or current sheet flapping.

z

x

+vx

Approx. Location of THEMIS

+Bx

-Bx

-vx


Ion distribution at 11:12:55

Coordinate system is in GSM – so vx is Earthward/tailward, vz is north/south

This is a cut through the distribution. There is no integration in the y direction (out of page)

Real (?)

Real beam – fastHall fields across exhaust?

Real beam –most dense, ‘bulk’

NOT Real

sou

th -

no

rth

tailward - earthward


Are the ion beams a unique

signature of magnetic reconnection?

How many ion beams should appear

in the exhaust?

ALFVÉN WAVE HEATING OF IONS IN THE SOLAR CORONA

Ions clearly not heated from hot

source below the corona

Macroscopic surface features

consistent with excitation of

Alfvén waves

Alfvén waves at moderate to

large perpendicular

wavenumber expected to

accelerate electrons

Are Alfvén waves with large

parallel wavenumber created

at strong density gradients in

corona-like conditions?

Can such Alfvén waves

interact with ions and heat

them?

THE LABORATORY PLASMA EXPERIMENT: HELIX-LEIA

600 500 400 300 200 100 0

-60

-40

-20

0

20

40

60 BL=14 Gauss

Rad

ial d

ista

nce

(cm

)

Axial distance (cm)

BL=70 Gauss

c)

Hot hELIcon eXperiment

Expansion Chamber

~ 0.01 at measurement location

0(1 / )c Lv k 2 2

1/ 2 0( /8ln 2)( / )Bk T mc

Doppler shiftedparticle absorption

distribution

Laserline

profile

MEASUREMENTS OF ION VELOCITY DISTRIBUTIONS IN LABORATORY

PLASMAS: A LASER INDUCED FLUORESCENCE PRIMER

Species for single photon

and 2-photon LIF:

Ar I (dye and diode laser)

Ar II (visible and IR diode laser)

Xe I (TALIF)

Xe II (dye laser)

He I (dye and diode laser)

Kr I (TALIF)

H I (TALIF)

3d4F7/2

4s4P3/2

4p4D5/2

668.61 nm442.72 nm

3d2G9/2

4s2D5/2

4p2F7/2

611.6616 nm461 nm

Ar II

diode

laser

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

-8.0 -6.0 -4.0 -2.0 0.0 2.0 4.0 6.0 8.0

Arg

on Io

n E

mis

sio

n

Frequency (o) (GHz)

TAr II

=.22 eV

TYPICALLY WE BEGIN OUR THREE-LEVEL LIF SCHEMES FOR LOW-TEMPERATURE

PLASMAS WITH EXCITATION FROM A LOW-LYING METASTABLE STATE

Ar II

dye

laser

AR II velocity Distribution

Stark broadening and natural linewidth are

ignorable. Zeeman splitting ignorable for

perpendicular injection. For parallel

measurements, single circular polarization used.

Ar I

diode

laser

OCCASIONALLY NON-METASTABLE AND 4-LEVEL SCHEMES ARE EMPLOYED FOR LIF ON

HE I AND AR I

He I

diode

laser

4s(2P

0

3/2)1

4s’(2P

0

1/2)1

4p’(2P

0

1/2)0

750.59 nm667.91 nm

4s’(2P

0

1/2)0

4s(2P

0

3/2)2

metastable

21P

21S

31P 3

1D

excitation

transfer

667.99 nm501.71 nm

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

-6.0 -4.0 -2.0 0.0 2.0 4.0 6.0

Heliu

m N

eutr

al E

mis

sio

n

Frequency (o) (GHz)

THe I

=.03 eV

0

0.1

0.2

0.3

0.4

0.5

-6.0 -4.0 -2.0 0.0 2.0 4.0 6.0

Arg

on N

eutr

al E

mis

sio

n

Frequency (o) (GHz)

TAr I

=.03 eV

MULTIPLEXED AR LIF ENABLES SIMULTANEOUS PARALLEL AND PERPENDICULAR ION

VELOCITY DISTRIBUTION MEASUREMENTS

Isotropic ion distribution for these source

parameters


0

5

10

15

20

0 0.004 0.008 0.012 0.016

An

iso

tro

py

(T/T

||)

i||

y = 1+m1/m0^m2

ErrorValue

0.217780.41835m1

0.0733240.37398m2

NA307.57Chisq

NA0.64211R

(T/T

||) = 1 + 0.4*

-0.4 Laboratory measurements of the ion

temperature anisotropy show clear evidence of

an inverse correlation with parallel ion beta.

Measurements are consistent with both

spacecraft measurements and theory.

What about collisions?


i||

g = 10-4 Wp

An

iso

trop

y (

T/T

||)

0.011

10

0.0001 0.001

1+.15/.5

Phan et al., 1994Anderson et al., 1994

Gary et al., 1994

Data for which ion thermalization and ion-neutral

collision frequencies are constant to within 10%.

Note: varies a factor of 6 and anisotropy a factor

of 3 for identical collision parameters!

Collisions alone would give a scaling of -.3 for the

measured relationships between density,

temperature and .

Data/Model Sp p

Anderson et al., 1994 0.85 0.48

Phan et al., 1994 0.63 0.50

Gary et al., 1994 g = 10-4Wp 0.35 0.42

Keiter et al. 1999 (LEIA) 0.15 0.50

p

i

p

i

iS

T

T

||||

1

[Sun et al., PRL 2005]

• The plasma potential tracks the magnetic field

strength - decreasing along z.

• The plasma potential measurements are

consistent with the LIF ion energy

measurements.

• The pre-sheath and sheath are clearly visible and

large enough for detailed study.

sheath

pre-sheath

FULL DL STRUCTURE, INCLUDING PRE-SHEATH REGION, MEASURED IN HELIX.

EXCELLENT AGREEMENT WITH MC-PIC MODEL

80 120 160 200

0

3050

6100

9150

12200

Ion

Velo

city (m

/s)

Position (cm)

IONS ACCELERATED TO ~ 10 KM/S DOWNSTREAM OF DOUBLE LAYER

f(v)

“weak” double layer

as EBeam

~ 3kTe

IONS ACCELERATED ALONG THE FIELD RESTRICTED TO INNER REGION OF PLASMA

Parallel IVDF shows distinct radial boundary for

beam region

Beam transient time from source normalized to

the gyroperiod:

H = (0.25 m/8x103 m/s)/(2.4 x 10-4 s) = 0.1,

so ions complete 1/10 of a single gyro-orbit before

reaching measurement location.

As field expands, ion beam remains confined

radially.

Beam structure much larger than ion gyro-

diameter, ~ 6.5 cm.

DOWNSTREAM PLASMA HAS A HOLLOW DENSITY PROFILE

Hollow ion density profile consistent with LIF

measured hollow metastable ion density profile

and consistent with recent ANU publication

[Zhang, Charles, and Boswell, 2016]. Enhanced

plasma production at edge, hotter electrons,

beam confined to core.

Floating potential structure follows magnetic

field line geometry.

ION ACCELERATION DEPENDS WEAKLY ON FIELD ANTI-MIRROR RATIO

Beam amplitude (in metastable ions) decays

with axial distance with no change in beam

velocity. Like a result of metastable quenching

and not charge-exchange loss of the beam.

Parallel IVDF shows radial boundary for beam

region.

EVIDENCE OF PARALLEL ION BEAM EVIDENT IN PERPENDICULAR IVDF

Field aligned parallel ion beam restricted to

region where beam seen in parallel IVDF data.

Closer to source, enormous ion heating

observed at beam/bulk boundary. Perpendicular

ion temperatures > 1.0 eV, yielding ion

temperature anisotropy ratios > 10.

TIME RESOLVED MEASUREMENTS INDICATE BEAM DOES NOT FORM INSTANTEOUSLY

More detailed study with 1 ms time resolution: the LIF-determined argon ion velocity distribution

function during a 100 ms plasma pulse surface plot showing fast (~ 7.1 km/s) and a slow (~ 0.4 km/s)

ion populations.

The DL typically requires 10’s of ms – no short

pulse rockets?


At very low neutral pressure and large RF powers,

multiple beams spontaneously appear downstream of

the expansion region in HELIX-LEIA.

LIF measured ivdf (circles) as a function of velocity in

the expansion chamber 38 cm downstream of the

plasma source. A three Maxwellian component fit

(solid line) yields identical ion temperatures of ~ 0.16

eV for all three components.

(b) Same data as (a) minus the fit to the stationary

background population. A very small third accelerated

population appears around 2,500 m/s.

Spontaneous appearance of multiple potential

drops in a region of strong density gradient

predicted in PIC simulations in the 1970’s.

[Mason, Phys. Fluids (1971)] “Computer

Simulation of Ion-Acoustic Shocks: The

Diaphragm Problem”

Density structure in model should map to

potential structure



Laboratory ivdfs look remarkably similar to

THEMIS 1D cuts.

(a) The ivdf for a bursty bulk flow event on 26

Feb 2008 at 11:12:52 and three seconds later

at (b) 11:12:55.

A large background signal in the measurement

at zero velocity due to photoemission and

spacecraft charging has been deleted from the

THEMIS data.


Simulation ivdfs look

remarkably similar to

THEMIS 1D cuts and

the laboratory

measurements.

2.5D PIC simulation with a large guide field . The computational

domain size is Lx x Ly = 40 di x 20 di where di = c/wpi. Periodic

boundary conditions in x and perfect electric conductor

boundaries at y = 0 and y = Ly. The simulation starts with a classic

Harris sheet of high density particles surrounded by background

particles with a density an order of magnitude lower. The

distribution is obtained 20 ion inertial lengths downstream of the

reconnection site.

RECAP

Laboratory experiments:

(a) ion temperature anisotropy driven instabilities – demonstrated inverse scaling consistent with space data and

theory. Now a topic of interest in heliospheric physics.

(c) ion acceleration in double layers – simple expanding field creates double layers.

(d) ion beam structures in magnetic reconnection exhausts – simple expanding field creates multiple beams that

look like predictions for magnetic reconnection but are clearly not reconnection.

Related talks and posters at this meeting (some have already occurred but we will make them available on our website)

JP10.00014 Short et al., Measurement of argon neutral velocity distribution functions near an absorbing boundary in a

plasma

GP10.00139 Aguirre et al., Two Dimensional LIF Measurements and Potential Structure of Ion Beam Formation in an

Argon Helicon Plasma

GP10.00138 Good et al., Optical Tagging of Ion Beams Accelerated by Double Layers in Laboratory Plasma

UO4.00014 Thompson et al., 3D ion flow measurements and simulations near a boundary at oblique incidence to a

magnetic field

TP10.00084 Henriquez et al., Comparison of 3D ion velocity distribution measurements and models in the vicinity of an

absorbing boundary oriented obliquely to a magnetic field


10-9

10-8

10-7

10-6

10-5

10-4

103

104

105

( B

/Bo)2

[Hz-1

]

Frequency (Hz)

Wci

Bz

Br

Transverse, low frequency waves are observed. Br and

Bz fluctuations are shown here. Wave power near Wci

is roughly 1% of Bo. Electrostatic fluctuations at same

parameters are very small.

Theory predicts kc/wp ~ 1 for ion cyclotron wave

(Alfvén Ion Cyclotron wave).

0 100

2 1015

-4-3-2-101234

Am

pli

tud

e (a

rb. u

nit

s)

Wavenumber (in unit of wp/c)

Davidson and Ogden AIC

theory

MEASUREMENT OF ION ENERGIZATION IN LABORATORY PLASMAS · MEASUREMENT OF ION ENERGIZATION IN LABORATORY PLASMAS Earl Scime* Division of Plasma Physics Mini-conference on Physics of

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