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Franco
RevMexAA (Serie Conferencias), 123--130 (2000)
EMISSION LINE SPECTRA FROM LOWíDENSITY LABORATORY
PLASMAS
Beiersdorfer, Brown, Drake,
2 M.íF. Kahn,
3
J. Lepson,
4 Liedahl,
C. Mauche,
1 Savin, Utter,
1 and Wargelin
2
RESUMEN
Estamos realizando mediciones laboratorio lÒÐneas emisiÒon
iones importantes
en astrofÒÐsica,
el rango espectral rayosíX entre
400
, cubrirÒan misiones espaciales Chandra, XMM, AstroíE EUVE.
Los datos, obtenidos ambiente controlado densidades similares
a
coronas estelares, son usados para verificar exactitud completez cÒodigos
modelaje espectral. Nuestro trabajo incluye
la compilaciÒon lÒÐneas emisiÒon
fierro, capa entre
6
Ú
A
y capa
a 200 Muchas lÒÐneas
identificadas por primera vez
y
se determinan flujos correspondientes, faltantes
los modelos. Nuestras medidas tambiÒen permiten determinar
la exactitud
cÒalculos excitaciÒon lÒÐneas, incluyendo excitaciÒon colisional, recombinaciÒon
dielectrÒonica excitaciÒon resonante. Estos resultados permiten calibrar casos
pecÒÐficos diagnÒosticos cocientes lÒÐneas. dan ejemplos datos obtenidos
recientemente. ABSTRACT
Using spectroscopic equipment optimized laboratory astrophysics,
performing systematic measurements emission from astrophysically releí
vant ions wavelength band between
1 and 400
Ú
A important Xíray missions
such Chandra, XMM, AstroíE, EUVE. Obtained
in
a controlled laboratory
setting electron densities similar those found stellar coronae, data
used spectral modeling codes for accuracy completeness. e#ort
includes compilation Líshell emission lines from 6--18 iron
Míshell emission lines from 50--200 Many lines have been identified first
time, and fluxes from lines missing
in spectral modeling codes assessed.
Our measurements assess accuracy line excitation calculations, including
direct electroníimpact excitation, dielectronic recombination, resonance excií
tation. These measurements yield calibration specific diagnostic ratios.
Examples our current measurements given.
Key Words: ATOMIC DATA ATOMIC PROCESSES LINE: FORí
MATION LINE: IDENTIFICATION
INTRODUCTION
The analysis
of spectral emission data from medium resolution Xíray EUV satellite
sions requires spectral models that both accurate and complete. Accuracy means that correct atomic
used, accurate line positions, excitation recombination cross sections, and radiative rates.
Completeness means that relevant lines atomic processes included models. test spectral
1 High Temperature Physics and Astrophysics Division, Lawrence Livermore National Laboratory, Livermore, USA.
2 HarvardíSmithsonian Center Astrophysics, Cambridge, MA, USA.
3 Department Physics, Columbia University, York, NY, USA.
4 Space Sciences Laboratory, Univ.
of California, Berkeley, CA, USA.Franco
BEIERSDORFER AL.
models accuracy completeness, comparisons with experimental obtained controlled laboratory
environment needed.
Among di#erent laboratory sources capable producing reliable atomic data, tokamaks electron beam
traps (EBIT) provide spectral data density and temperature regime most applicable Xíray emitting
astrophysical situations. Tokamak sources extensively used assess accuracy atomic data
related Kíshell emission line spectra from hydrogenlike and heliumlike ions, including calculations
dielectronic recombination, collisional excitation, and radiative cascades (Beiersdorfer 1989a; Bombarda
et 1988; Decaux 1991; Keenan
et 1991; Bitter
al. 1993; Smith 1993; Beiersdorfer
1995). An analysis
of iron Kíshell spectrum PLT tokamak extended such measurements
charge states low XVIII, providing
a very detailed Kíshell (Beiersdorfer
1993). tokamak observations exist astrophysically relevant Líshell xíray spectra because these emanate
from colder regions the plasma with
a correspondingly diminished diagnostic importance magnetic fusion
plasmas. exception
is measurement Líshell emission from
n
4
2
n
5
2
transitions through XXIV tokamak (Wargelin
et 1998) that covered
7
-- region. This measurement was performed with highíresolution crystal spectrometer built specifically
measuring Líshell spectra (Beiersdorfer 1989b). measurement successfully validated spectral
modeling calculations yielded lists with wavelength accuracies 40,000. Tokamaks
have provided astrophysically important spectra extreme ultraviolet region. For example, Stratton,
Moos, Finkenthal (1984) reported intensities several transitions XVIII through XXIII
been used emission line predictions, while Sugar
& Rowan (1995) have reported very accurate
wavelength measurements.
EBIT provides
a higher level control tokamaks producing spectra relevant astrophysics.
reason that EBIT provides almost complete control choice charge states excitation
processes, illustrate below. result, many our laboratory astrophysics investigations have involved
use EBIT.
the following, describe Livermore EBIT facility present recent results our work. These
include measurements
of contributions from resonance excitation dielectronic recombination satellite lines
3d emission lines XXIV, calibration singlet
to triplet
3d lines
neonlike ions, and line surveys Xíray and extreme ultraviolet region.
2. THE EBIT SPECTROSCOPIC SOURCE
The Livermore Electron Beam Ion Trap was specifically developed built for studying the spectroscopic
properties highly charged ions (Levine 1989). has been operation over decade during which
spectroscopic measurements have been optimized
to provide stateíofítheíart measurements the
1
-- 7000
Ú
A
region. Special emphasis been placed the Xíray and EUV regions provide laboratory data present
future astrophysics missions such ASCA, Chandra, EUVE, DXS, XMM, and AstroíE.
The EBIT modified electron beam source built study the interaction
of highly charged with
electron beam looking directly the trap. Magnetic fields confine focus electrons, which
accelerated energy between about eV 200,000 Neutral atoms ions with charge
injected nearly monoenergetic beam where they collisionally ionized excited.
electrons pass through trap region 2ícm length, the beam compressed diameter
of approximately
a 3íTesla magnetic field, generated
a pair superconducting Helmholtz coils. There,
longitudinally confined applying appropriate voltages three drift tubes through which
beam passes. Radial confinement provided electrostatic attraction electron beam,
freezing within magnetic field. three tube voltages
of
a potential
common drift tube voltage)
is supplied
a lowínoise highívoltage amplifier, and electron beam
energy
is determined sum these potentials. The electron beam density given beam energy
selected varying beam current. typically range
2
½
10
.
slots the drift tubes and aligned with vacuum ports permit direct lineíofísight access
trap,
as shown
in Figure
1.
is introducing atomic molecular gases trap
means
a ballistic injection system. The remaining five ports are used spectroscopic measurements.
typical arrangement Xíray astrophysics shown Figure windowless highípurity detector

Franco
EMISSION LINE SPECTRA FROM LABORATORY PLASMAS
Camera
Grazing Incidence
Spectrometer (10--400
Vacuum FlatíCrystal
Spectrometer (4--25
Hñmosítype
Curved Crystal
Spectrometer (1--5
j
r
Spectrometer
Š)
Vacuum FlatíCrystal
SolidíState Detector
(0.3--50 keV)
Radial cut through EBIT showing the diagnostic access
monitors overall Xíray emission from ions. signal used for tuning facility, count
normalization, monitoring presence impurity highíresolution bentícrystal spectrometer
provides detailed information Kíshell emission spectra below (Beiersdorfer 1990).
crystal spectrometers operating
in vacuo provide detailed Líshell and Kíshell spectra the
4
--
25
Ú
A region
(Brown 1999). flatífield spectrometer with either
a 1200 #/mm 2400 #/mm grating used study
-- 400 extreme ultraviolet region (Beiersdorfer
et 1999b).
A seventh port top EBIT
permits axial access used injection singly charged metal ions into
a
metal vapor vacuum source.
very important feature using EBIT the ability produce ions desired charge state and study
their emission selecting specific formation processes. so, rely the choose
energy electron beam within about eV. This provides
a powerful technique determine
which line comes from which ionization state. calculation
of iron ionization balance
a function
electron beam energy based electroníimpact ionization radiative recombination shown Figure
actuality, distribution somewhat smeared because charge transfer reactions between trapped
background neutrals. Figure illustrates the charge state selectivity given proper choice
electron beam energy. The spectrum shows the emission lines observed when
beam energy 1200 energy
is below 1250íeV ionization potential XVII only lines from
XVII seen. spectrum obtained 1300 higher than ionization potential
of XVII. Consequently, XVIII produced. Comparing spectra allows unambiguously
identify transition
at 17.623 transition XVIII. This particular line observed solar spectra
for some time was thought result from inneríshell ionization coronal Xírays and subsequent Líshell
fluorescence
of neutral ionized
in photosphere,
as described Drake
et (1999).
Figure show
a spectrum the Rydberg transitions VII. wavelengths
of these transitions
well known, have used them
as reference measurements
of lines. fact,
spectral measurements EUV and Xíray region calibrated situ recording lines from
hydrogenlike heliumlike reference lines whose wavelengths accurately known.
Line identifications wavelength measurements best made setting energy electron beam

Franco
BEIERSDORFER AL.












































  !#" $%'&)( *,+.-/1032
XVII
5
5
4'4
0
0
2. Calculated iron ionization balance
in EBIT function
of electron beam energy.
a constant value integrating the resulting spectral emission
a given period time.
In contrast,
excitation processes best studied sweeping energy the electron beam continuous fashion. Using
eventímode acquisition system each photon with energy the beam. spectral data
plotted function
of electron beam energy. This allows determine the excitation function
of
a
given spectral line. Resonant enhancement line emission, onsets
of radiative cascades from higher levels,
dielectronic satellite features blending with
a given thus measured the relative magnitude
process assessed di#erent electron energies Section
SPECTRAL CATALOGUES OF THE INTERMEDIATE CHARGE STATES IRON
The 30--140 region never been systematically investigated lowídensity laboratory plasmas. Moreí
over, region has received incomplete attention solar measurements. Consequently, many lines
in
region never been identified and missing from present spectral models,
as have shown recently
(Beiersdorfer 1999a). hampered correct interpretation shortíwavelengthíband spectra
from EUVE, causing
a significant underestimation
of flux wavelength band (Schmitt, Drake, Stern
1996; Drake, Laming,
& Widing 1997). example, measured
a spectrum and VIII,
shown Figure None observed
in less than
of flux from lines
standard spectral modeling codes. The same true emission lines from higher charge states
(Lepson these proceedings), e.g., more than
%
of flux from XIII wavelength band
below 140
is missing existing radiative loss models.
a result, the problems encountered
in modeling
flux charge states iron solarílike stellar coronae also pertain interpretation spectra
from coronae somewhat hotter than that Sun.
have recently completed inventory significant Líshell emission lines from XVII through
XXIV region (Brown
al. 1998; Phillips
et 1999; Brown
et preparation).
found, example, emission emanating from levels with high principal quantum number
XVII equalled about the
of strongest XVII line.
is significant amount
is concentrated
a small wavelength band
Ú
A XVII ionization limit.
Overall, our measurements have yielded about twice many lines listed, example, compilation
Kelly (1987).
4. CALIBRATION DIAGNOSTIC LINE RATIOS
The intensity ratio intercombination and resonance lines XVII 15.014 15.261
Ú
A respectively, has been proposed
a diagnotic optical depth and hence density coronal active

Franco
EMISSION LINE SPECTRA FROM LABORATORY PLASMAS
80
60
40
20
0
40
30
20
10
0 17.6
17.4
17.2
17.0
16.8 Wavelength
)
d
O
80
60
40
20
0
3. Flatícrystal spectrometer spectra near XVII 2p-3s transitions measured
a beam energy
of 1200 XVIII appears 17.62 beam energy
of (c) Rydberg transitions
wavelength calibration.
Wavelength
7000 6000
5000 4000 3000 2000 1000
0
1.0 0.8 0.6
0.4 0.2
0.0 140
100
4. Spectrum extreme ultraviolet: (a) measurement EBIT; (b) predictions
from MEKA model (Kaastra Mewe 1993). The latter includes only lines fromFranco
BEIERSDORFER AL.
EBIT
1985
Smith
et
al.
Year publication calculation
5. Comparison predicted values (filled circles) optically thin ratio the intensity
resonance intercombination lines XVII with measured value shown horizontal
band. The uncertainty measured value
is indicated width band. The author(s)
publication the prediction are given the abscissa.
regions (Schmelz, Saba, Strong 1992; Waljeski 1994). Because oscillator strength resonance
much larger than
of intercombination observed emission can, under certain conditions,
significantly reduced plasmas through resonance scattering photons out line sight (Rugge
&
McKenzie 1985). this diagnostic infer the optical depth however, impaired the simple
modern calculations electroníimpact excitation cross sections typically accurate between
5
fact, compilation
of various predictions the optically thin ratio based di#erent calculations
shows scatter the ratio from
to illustrated Figure knowing which calculation
optical thin ratio, measured ratio 2.9, example, may thus lead inferred optical depth between
0 and made measurement this ratio with EBIT found
a
of
‘ 0.12 (Brown 1998).
ratio agrees best with prediction made Bely Bely (1967). This illustrates that usual assumption
most recently published calculations are best'' fallacious. On other hand, the result does
imply that atomic model used Bely and Bely superior
to presentíday calculations.
DIELECTRONIC RECOMBINATION CONTRIBUTIONS TO LíSHELL EMISSION LINES
Our measurements focus formation electroníimpact excitation also indirect
formation processes, such those resonance excitation highín dielectronic satellite contributions
Líshell emission lines
et 1999). example contributions from these processes
formation
a line lithiumlike XXIV
is shown Figure The measurement clearly identifies
di#erent contributions
to line: direct electroníimpact excitation resonance excitation above
threshold excitation, unresolved dielectronicírecombination satellite lines below threshold.

Franco
EMISSION LINE SPECTRA FROM LABORATORY PLASMAS
n=5
n=6
4l6l'
n=7
Electron energy (keV)
1.2 1.4 1.6
0.0
0.5
1.0
1.5
6. Measured emission the
# transition
in lithiumlike
Fe 23+ function electron
energy showing dielectronic satellite contributions below threshold resonance excitation contributions above
threshold electroníimpact excitation. Dotted connects measured values; solid and dashed lines indicate
predictions from two di#erent models.
The contributions unresolved satellite lines are important accounting
of
a
given line. Calculations unresolved dielectronic satellites been carried out most Líshell lines.
measurements show that such satellites enhance some Líshell lines
in XXIV (Gu
1999) more, especially colder plasmas. Such resonances a#ect intercombination
resonance
in XVII, which discussed above, and hence may change the inferred optical depths.
order provide
a complete accounting contributions the
a given line, our current e#ort
concentrates measuring dielectronic resonance contributions strong Líshell emission lines.
results these measurements should
be available near future.
CONCLUSION
presented several recent results
of ongoing laboratory e#ort provide reliable atomic data
spectral modeling astrophysical plasmas. e#ort extends from compiling spectral catalogues
assessing formation processes and calibrating diagnostic ratios. employing properly designed
executed measurements, spectroscopic facility become analog computer where Nature computes
correct result inclusion spectral codes. With able address issues completeness
accuracy the spectral models. Most importantly, measurements provide uncertainty limits that
invaluable establishing believable bounds physical parameters inferred from spectroscopic observations
of astrophysical data.
This work was supported NASA grants NAG5í6731 and NAG5í5123 work order Wí19127
performed under auspices
of Department Energy under contract Wí7405íENGí48.
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Beiersdorfer, Brown, Liedahl, Mauche, Utter: High Temperature Physics Astroí
physics Division, Lawrence Livermore National Laboratory, 7000 East Ave., Livermore, 94550, USA.
J.
J. Drake, Wargelin: HarvardíSmithsonian Center Astrophysics,
60 Garden Street, Cambridge,
02138, USA.
M.íF. Gu, Kahn, Savin: Department
of Physics, Columbia University, 120th New
York, 10027, USA.
J. Lepson: Space Sciences Laboratory, Univ. California, Berkeley, CA 94720, USA.