H. Haseroth

Physical mechanisms leading to high currents of highly charged ions in laser-driven ion sources

Heavy ion sources for the big accelerators, for example, the LHC, require considerably more ions per pulse during a short time than the best developed classical ion source, the electron cyclotron resonance (ECR) provides; thus an alternative ion source is needed. This can be expected from laser-produced plasmas, where dramatically new types of ion generation have been observed. Experiments with rather modest lasers have confirmed operation with one million pulses of 1 Hz, and 10 11 C 4+ ions per pulse reached 2 GeV/u in the Dubna synchrotron. We review here the complexities of laser-plasma interactions to underline the unique and extraordinary possibilities that the laser ion source offers. The complexities are elaborated with respect to keV and MeV ion generation, nonlinear (ponderomotive) forces, self-focusing, resonances and hot electrons, parametric instabilities, double-layer effects, and the few ps stochastic pulsation (stuttering). Recent experiments with the laser ion source have been analyzed to distinguish between the ps and ns interaction, and it was discovered that one mechanism of highly charged ion generation is the electron impact ionization (EII) mechanism, similar to the ECR, but with so much higher plasma densities that the required very large number of ions per pulse are produced.

IODINE LASER PRODUCTION OF HIGHLY CHARGED Ta IONS

The results of systematic studies of multiply charged Ta ion production with the fundamental frequency of an iodine laser (λ=1.315μm), and its 2nd (0.657μm) and 3rd (0.438μm) harmonics are summarized and discussed. Short laser pulse (350 ps) and a focus spot diameter of 100μm allowed for the laser power densities in the range of 5×1013–1.5×1015 W/cm2. Corpuscular diagnostics were based on time-of-flight methods; two types of ion collectors and a cylindrical electrostatic ion energy analyzer were used. The Ta ions with charge state up to 55+ were registered in the distance of 210 cm; the maximum amplitude of the signal of a high energy ion group was found to belong to the ions with the charge state around 43+, depending on the laser power density. The ion energy distribution was measured for all three wavelengths, however, in a different energy range; the maximum registered ion energy was 8.8 MeV. The occurrence of highly charged ions in the far expansion zone is discussed in view of the mechanism of charge distribution “freezing” during two-temperature isothermal plasma expansion.

Ion production by lasers using high-power densities in a near infrared region : Laser-Ion Sources

Results are presented of experiments on ion production from Ta targets using a short pulse (350-600 ps in focus) illumination with focal power densities exceeding 10 14 Wcm -2 at the wavelength of an iodine photodissociation laser (1.315 μm) and its harmonics. Strong evidence of the existence of tantalum ions with the charge state +45 near the target surface was obtained by X-ray spectroscopy methods. The particle diagnostics point to the existence of frozen high charge states ( 4 MeV) for the highest observed charge states. A tentative theoretical explanation of the observed anomalous charge state freezing phenomenon in the expanding plasma produced by a subnanosecond laser pulse is given.

The CERN laser–ion source

This paper describes the first results of a feasibility study undertaken at CERN to determine whether a laser-produced plasma can be used as a source of intense highly charged heavy ion beams. A variety of important measurements have been made, and the results are encouraging. Furthermore, a beam of highly charged light ions produced by the laser ion source has been accelerated successfully in a radio frequency quadrupole (RFQ) structure.

Laser ion source for heavy ion synchrotrons (invited)

A status overview of the development of laser ion sources suited to heavy ion synchrotrons is presented. The results of experimental and theoretical studies, recently obtained at a number of laboratories, for laser-produced highly charged heavy ions are summarized for plasmas heated by long wavelength lasers. Design of a powerful repetition rate CO2 laser, target interaction chamber, and extraction system suited for reliable long term operation mode with real accelerators is discussed. Requirements for the final performance of the laser ion sources for ion beam injectors at the ITEP-Moscow and CERN accelerator facilities are given.

Highly charged Ta ions produced by the photodissociation iodine laser with subnanosecond pulses

The results of the generation of tantalum ions with a charge state of 46+ are reported. Iodine laser pulses of about 15 J energy and 350 ps in duration were used for a Ta plasma production. Multiply charged ions were recorded by the diagnostic techniques based on the time‐of‐flight method in the distance of approximately 200 cm. The plasma temperature estimated from the ion measurements was found to be about 1750 eV.

Properties of iodine laser-produced stream of multiply charged heavy ions of different elements

Properties of laser produced stream of ions of high- and medium-Z elements were analyzed in the far expansion zone using time-of-flight method and Thomson parabola spectrograph in experiments performed with the iodine laser system PERUN at IP ASCR, Prague (λ=1.315 μm, EL∼45 J,τL∼300–700 μs). Ta, Au, and Pb, as well as Co, Sn, and Ag were mainly used as targets onto which a laser power density up to 1×1015 W cm−2 was delivered. Attention is paid to the evolution and comparison of different produced ion groups, and to the number, charge state, energy, and angular distribution of emitted ions.

Laser ion source for heavy ion synchrotrons

A status overview of the development of laser ion sources suited to heavy ion synchrotrons is presented. The results of experimental and theoretical studies, recently obtained at a number of laboratories, for laser-produced highly charged heavy ions are summarized for plasmas heated by long wavelength lasers. Design of a powerful repetition rate CO2 laser, target interaction chamber, and extraction system suited for reliable long term operation mode with real accelerators is discussed. Requirements for the final performance of the laser ion sources for ion beam injectors at the ITEP-Moscow and CERN accelerator facilities are given.

Multiply charged ion generation from NIR and visible laser‐produced plasma

Results of experiments on ion production from Ta target using ∼350 ps pulses of the iodine photodissociation laser PERUN are presented and analysed. Ta ions with charge states higher than 45+ and with energies 8 MeV were registered in a far expansion zone (∼2 m). Theoretical interpretation of the results is possible by considering that the plasma temporal evolution follows the mechanism of two‐temperature isothermal expansion.

Status of the CO2 laser ion source at CERN

A laser ion source using a CO2 laser focused onto a solid target is under study at CERN for the production of high currents of highly charged heavy ions, for possible use in the preinjector for the large hadron collider. A new expansion and extraction layout was installed in this test facility, improving the alignment and making the target to extraction distance more flexible. A two solenoid beam transport system was studied for providing the matching of the beam to a radio-frequency quadrupole. An electrostatic beam transport using gridded electrostatic lenses was designed and constructed as an alternative to a magnetic system. Results show an increased overall current transmission for the electrostatic case. Investigation of the laser parameters required for the production of 1.4×1010 Pb25+ ions in a 5 μs pulse, has been performed using the TIR-1 laser facility at power densities up to 1014 W cm−2 for a focal spot size of 65 μm. The results of the latest scaling are presented.A laser ion source using a CO2 laser focused onto a solid target is under study at CERN for the production of high currents of highly charged heavy ions, for possible use in the preinjector for the large hadron collider. A new expansion and extraction layout was installed in this test facility, improving the alignment and making the target to extraction distance more flexible. A two solenoid beam transport system was studied for providing the matching of the beam to a radio-frequency quadrupole. An electrostatic beam transport using gridded electrostatic lenses was designed and constructed as an alternative to a magnetic system. Results show an increased overall current transmission for the electrostatic case. Investigation of the laser parameters required for the production of 1.4×1010 Pb25+ ions in a 5 μs pulse, has been performed using the TIR-1 laser facility at power densities up to 1014 W cm−2 for a focal spot size of 65 μm. The results of the latest scaling are presented.