Ameland, day 2
Hard-core atomic physics: highly charged ions José R. José R. Crespo López-Urrut Crespo López-Urrutia Max-Planck-Instit Max-Planck-Institut für Kernphysik für Kernphysik Heidelberg Heidelberg
The warm-hot intergalactic medium The warm-hot intergalactic medium (59 ± 9) % of baryons are missing To hot for visible light, too diffuse for direct X-ray detection Cosmological simulations (Cen & Ostriker) predict a warm-hot interstellar medium (WHIM) heated by gravitation to 10 6 K containing most of the missing baryons
Fe L-shell autoionizing resonances carry 60% of total photoabsortion strength at T 500 eV -15 10 2 ) PI cross section (cm -16 Direct PI cross section 10 Total PI cross section -17 10 -18 10 -19 10 -20 10 -21 10 Total strength 2 eV) Resonant strength -16 3x10 Direct photoionization strength Integrated PI strength (cm Planck continuum -16 2x10 Resonances dominate! -16 1x10 Cross section weighted with Planck continuum 0 0 500 1000 1500 Photon energy (eV) Fe ions with a few remaining electrons can resonantly absorb photons and also be excited by monoenergetic electrons
Fe XVII at base of the solar convection zone Fe XVII ion has largest contribution to opacity L shell T = 2 × 10 6 K absorption N e = 10 23 cm − 3 M shell absorption Plasma temperature T=190 eV Parameter u=hv/kT Figure 5 from: Solar Mixture Opacity Calculations Using Detailed Configuration and Level Accounting Treatments Christophe Blancard et al. 2012 ApJ 745 10
Contribution of Fe to total opacity H, H, Fe Fe He He Figure from: Solar Mixture Opacity Calculations Using Detailed Configuration and Level Accounting Treatments Christophe Blancard et al. 2012 ApJ 745 10
How do we make them in the laboratory? • Fusion machines, magnetically confined plasmas • High power lasers, X-ray lasers • Ion accelerators • Electron beam ion traps
I onization potential rises from 1 0 to 1 3 0 0 0 0 eV Ionization potential (keV) uranium 100 tungsten barium krypton 10 argon neon 1 0.1 0.01 0 10 20 30 40 50 60 70 80 90 100 Ion charge state q+
Highly charged ions at accelerators ion source accelerator stripper foil storage ring Storage ring = synchrotron without acceleration • Take ions at half the speed of light (e. g. at GSI Darmstadt) • send them through a thin foil: outer electrons are stripped • Very hot highly charged ions are produced and stored • Disordered thermal motion reduces resolution • Deceleration and cooling in progress (HiTrap project, GSI)
Making HCI by electron impact ionization Continuum 12 keV n=3 n=2 31 keV With increasing charge state: • Higher binding energy • Smaller cross section Electron with (sufficient) energy E k n=1 130 keV
Electron beam ion source • In the electron beam ion source (EBIS), a fast, dense, electron beam interacts with atoms and produces ions. • Ions are confined radially by the potential well in the electron beam and axially by ring electrodes. • Ions can be accumulated in or expelled out of it. • As the interaction time between electrons and ions defines the highest charge state achievable, high current density (of the order of 1000A/cm 2 ) electron beams are required. • Since normal cathodes are limited to less than 10A/cm 2 , beam compression by means of a strong magnetic field is needed.
Space charge potential: a line charge Poisson‘s equation in cylindrical coordinates Resulting potential with boundary conditions
Space charge potential of the electron beam 140 Space charge potential (V) Ebeam=2162 eV Ibeam=40 mA 120 100 center drift space charge potential tube radius 80 60 40 20 electron density (normalized to 50) 0 1000 2000 3000 Distance from axis ( m)
Space charge potential of the electron beam 140 Space charge potential (V) Ebeam=2162 eV Ibeam=40 mA 120 space charge potential center drift 100 tube radius 80 electron beam radius 60 40 electron density (normalized to 50) 20 0 0.1 1 10 100 1000 Distance from axis ( m)
EBIT (electron beam ion trap) invented at LLNL Currently: 10 EBITs worldwide, of which 3 of the largest are in Heidelberg (Levine & Marrs 1986)
The electron beam ion trap (EBIT) • As electrons collide with the ions in the beam, they strip off electrons until the energy required to remove the next electron is higher than the beam energy • The original LLNL EBIT (1986) is capable of an electron beam energy of about 30 keV, enough to make neon-like uranium (U 82+ ) • From this EBIT-I, a high-energy EBIT, named SuperEBIT, was built. It has an electron gun that can achieve an electron beam energy of 200 keV, enough to make bare uranium (U 92+ )
HCI production with electron beam ion trap radial potential radial potential I beam beam =450 =450 mA mA electr electron beam on beam space charge space charge 15000 A 15000 A/cm cm 2 axial potential axial potential e 10 n e 10 13 13 e - /cm /cm 3 electrodes electr odes Electron beam drives ionization, excites and traps the ions inside a cylindrical volume
Tim e evolution of the charge state 0.3 Charge state fraction Hg 70+ Hg 10+ 0.2 Hg 20+ Hg 52+ Hg 30+ Hg 40+ Hg 78+ 0.1 0.0 0.01 0.1 1 10 Ionization time (s) Calculated for Hg ions at 50 keV electron beam energy by numerically solving a set of coupled differential equations for the ionization and recombination processes:
Der Feind: Rekombination Ladungsaustausch: N Neutrale Atome geben Elektronen ab Ne 9+ n n n Lösung: Vakuum bei 10 -13 mbar n n (Weltraumbedingungen) n n n n n (1000 Atome/cm 3 ) Photon emittiert Einfang freier Elektronen Lösung: höhere Elektronenenergie n n n n strahlungsmäßige Rekombination n radiative recombination (RR)
Electron beam ion traps • An electron beam produces, traps and excites HCI • Diagnostics from the optical to the hard x-ray range • Additional ionic species particle diagnostics • Studies from N 3+ to Hg 78+
Electron beam ion traps superconducting electron gun magnet collector trap region
Evaporative cooling
Evaporative cooling • collisions with beam electrons heat up ion ensemble • light, less tightly trapped ions (e.g. Ne 10+ ) evaporate removing thermal energy: a single Ne 10+ takes away 2 keV (1 second additional life for a heavy ion) • heavy, highly charged ions (e.g. Ba 53+ ) remain trapped indefinitely Ion temperat Ion temperatures from 1000 ures from 1000 eV eV to 10 eV to 10 eV Doppler width / 1/20.000 (Ba Doppler width 1/20.000 (Ba 53+ 53+ ) High resolution High resolution spectroscopy spectroscopy
Evaporative cooling: energy distribution function relative to trapping potential 2000 Evaporating fraction Potential energy (arb. units) D B 1500 Trapping potential 1000 Light ions 500 Heavy ions 0 0.0 0.2 0.4 0.6 0.8 1.0 Relative energy distribution function
EBI Ts are good to reproduce the conditions prevailing in astrophysical plasm as transient plasmas, strong density and temperature gradients EBITs: stationary, homogeneous conditions Density and temperature space sampled by different spectroscopic light sources P. Beiersdorfer, Annu. Rev. Astron. Astrophys. 4 1 ( 2 0 0 3 ) 3 4 3 -3 9 0
X-ray diagnostics: Bragg’s law
Absolute m easurem ents CCD 2 Γ = 180 ° - 2 Θ 180 ° - 2 Θ crystal EBIT |a/b| 180 ° -2 Θ ξ CCD 1 Bond method (W.L. Bond, Acta Cryst. 13, 814 (1960))
Absolute measurements Side-on vs. end-on spectra: line/point source
The Lyman- spectrum of hydrogenic Ar 17+ Testing QED Screening and Two-Loop Contributions with He-Like Ions, H. Bruhns, J. Braun, K. Kubi č ek, J. R. Crespo López-Urrutia, and J. Ullrich, Phys. Rev. Lett. 99, 113001 (2007) K. Kubi č ek, P. H. Mokler, V. Mäckel, J. Ullrich, and J. R. Crespo López-Urrutia, Transition energy measurements in hydrogenlike and heliumlike ions strongly supporting bound-state QED calculations, Phys. Rev. A 90 90, 032508 (2014)
Lyman- α and w in S, Ar HCI: Scaled spectra Testing QED Screening and Two-Loop Contributions with He-Like Ions, H. Bruhns, J. Braun, K. Kubi č ek, J. R. Crespo López-Urrutia, and J. Ullrich, Phys. Rev. Lett. 99, 113001 (2007) K. Kubi č ek, P. H. Mokler, V. Mäckel, J. Ullrich, and J. R. Crespo López-Urrutia, Transition energy measurements in hydrogenlike and heliumlike ions strongly supporting bound-state QED calculations, Phys. Rev. A 90 90, 032508 (2014)
GSI Gumberidze et al., PRL 94, 94, 223001 (2005)
LLNL Beiersdorfer et al., PRL 95, 95, 233003 (2005)
Beiersdorfer … JRCLU et al., Measurement of QED and Hyperfine Splitting in the 2s 1/2 - 2p 3/2 X-Ray Transition in Li-like 209 Bi 80+ , Phys. Rev. Lett. 80 80, 3022 (1998)
The Lyman- α spectrum of hydrogenic Ar
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