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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 BEAMS IN MAGNETIC RECONNECTION EXHAUSTS
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
ION BEAMS IN MAGNETIC RECONNECTION EXHAUSTS
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
ION TEMPERATURE ANISOTROPY DRIVEN INSTABILITIES
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?
ION TEMPERATURE ANISOTROPY DRIVEN INSTABILITIES
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?
ION BEAMS IN MAGNETIC RECONNECTION EXHAUSTS
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
ION BEAMS IN MAGNETIC RECONNECTION EXHAUSTS
ION BEAMS IN MAGNETIC RECONNECTION EXHAUSTS
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.
ION BEAMS IN MAGNETIC RECONNECTION EXHAUSTS
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
ION TEMPERATURE ANISOTROPY DRIVEN INSTABILITIES
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
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