Celebrating the achievements of
Alan Gabriel
Laboratory spectroscopy
Exploring the process of dielectronic
recombination
S. Volonte
IAS 20 June 2013
Laboratory spectroscopy
1969 -1973
• Under Alan’s supervision development of an electromagnetic T-tube
• Study of high density effect in He plasmas
• Plasma Polarization Shift (PPS) of lines from ions due to partial
screening from neighbouring perturbing electrons
• Studies by Griem et al
• Controversial theoretical and experimental results
IAS 20 June 2013
Laboratory spectroscopy
•
First shift measurements of He II resonance series up to n=4
•
Blue shifts measured and predicted ~n4 scaling confirmed
•
Shift measurements in overall quantitative agreement only with quantum
estimate (Griem 1970) based on hydrogenic continuum wavefunctions for
perturbing electrons
•
Study showed that if perturbing electrons are assumed as bound
in high orbitals the effect corresponds to emission of high dielectronic
satellites and is negligible in high density plasmas
•
Conclusion: only free perturbers produce measurable PPS (Volonte PhD
1972 , Gabriel and Volonte 1973a,b)
IAS 20 June 2013
Exploring dielectronic recombination
•
In same period Alan’s main laboratory activity-production of highly ionized
spectra with theta-pinch machine for simulation of spectra of solar sources
•
In parallel, research work on satellite lines-faint features on red side of
resonance lines of H-like and He-like ions (Edlen and Tyren 1939)
•
With co-workers (a.o C. Jordan) he showed that production process was
dielectronic recombination of free electron bound to parent ion in excited state
to form an auto-ionzing doubly excited state of recombined ion from which
satellites are emitted
•
In developing the theory of dielectronic satellites he derived the intensity ratio
of the satellite to the resonance line as given by
Is/Ir ~ Ar/Te exp((E0-Es)/kTe) with Ar~Z4
The ratio varies only with temperature and the n scaling shows that satellites
are much stronger in heavy ions (factor ~100 from O to Fe)
IAS 20 June 2013
Exploring dielectronic recombination
•
The theory was further developed for inner-shell satellites produced by
collisional excitation of inner-shell transition in the recombined ion.
•
For inner-shell satellites the intensity ratio to resonance line is given by
Is’/Ir~ (NHe/NLi)(C’/C)F(A)
Here the intensity ratio varies with the relative abundancies (Te dependant)
which depend on the ionisation equilibrium of the source (i.e. wether
ionising, recombining or steady state)
•
These satellite intensity ratios give independent access to the electron
temperature and the ionization state of the source in coronal plasmas.
IAS 20 June 2013
Exploring dielectronic recombination
• This early work was devoted to the red shifted prominent satellites
produced from doubly-excited states with n=2 for the most solar
abundant He-like ions (O to Fe)
• Confirmation of these calculations was obtained from the first ever
observations of solar flare iron spectra (Fe XXV near 1.85 A)
observed by the Intercosmos-4 satellite (1972).
• Best fit between the observed and computed spectra provided the
electron temperature and the ionization state of the flaring plasma
thus demonstrating the diagnostic capabilities of the satellite line
theory
IAS 20 June 2013
Exploring dielectronic recombination
1974 -1986
• The next step undertaken by Alan was to improve the diagnostic
capability of the method by extending the work to include satellites
emitted from auto-ionizing states with n>2 for H-like and He-like ions
• This entailed calculation of a very large number of atomic
parameters
• This work was realised through an extended collaboration with the
atomic physics groups at the Nice observatory (F. Bely-Dubau, P.
Faucher, L. Steeman-Clark) and Meudon (J. Dubau, M. loulergue,
M. Cornille) and myself at Mons
IAS 20 June 2013
Exploring dielectronic recombination
• A series of workshops were held regularly mainly at Nice (see
photos) but also at Meudon and Culham and occasionally at Mons
to discuss progress of the work
• The early results of the collaboration concerned the high n satellite
lines of Fe XXV (Bely-Dubau, Gabriel and Volonte 1979)
• The results showed the importance of including these lines to
improve agreement between observed and calculated spectra thus
enhancing the accuracy of the diagnostics.
IAS 20 June 2013
Exploring dielectronic recombination
• A series of workshops were held regularly mainly at Nice (see
photos) but also at Meudon and Culham and occasionally at Mons
to discuss progress of the work
• The early results of the collaboration concerned the high n satellite
lines of Fe XXV (Bely-Dubau, Gabriel and Volonte 1979)
• The results showed the importance of including these lines to
improve agreement between observed and calculated spectra thus
enhancing the accuracy of the diagnostics.
IAS 20 June 2013
Nice workshop
IAS 20 June 2013
Nice workshop
IAS 20 June 2013
Exploring dielectronic recombination
• The collaboration worked actively on the atomic parameters needed
in preparation of NASA’s Solar Maximum Mission (SMM 1980)
• Satellite line data up to n=5 were obtained for the most solar
abundant H-like and He-like
• Examples of solar flare spectra from the BCS spectrometer on board
SMM are shown below. The calculated spectra have adjusted to
provide the best fit with the observations. Note the Te values and the
ionization diagnostics obtained from the satellite line intensities.
IAS 20 June 2013
Exploring dielectronic recombination
• The work on satellite spectra was eventually tested on
spectra obtained in fusion machines (a.o. Princeton and
Frascati Tokamak)
• Calculated and observed spectra were in good agreement
• The temperatures derived from the satellite intensities showed
excellent agreement (few %) with independent measurements from
Thomson scattering provided high satellites were accounted for
• This was a further confirmation of the powerful diagnostic
capabilities of the satellite lines
IAS 20 June 2013
Exploring dielectronic recombination
IAS 20 June 2013
Exploring dielectronic recombination
IAS 20 June 2013
Conclusion
Through his leadership, scientific insight and as
an instigator of international collaboration Alan‘s
work in laboratory spectroscopy, theory and
interpretation of spectra has provided not only
important data but also new diagnostics
methods (e.g. satellite lines) which had not been
available earlier. These research topics are
among the many other important contributions
that Alan has provided throughout his scientific
career.
IAS 20 June 2013