RIKEN Atomic Physics Lab Seminar

　

" Nonlinear Properties of Laser-Generated Giant Surface Acoustic Wave Pulses in Solid Materials"

Hans A. Schuessler (Dept. of Physics, Texas A&M University)

11:00 a.m., Monday, Dec. 04, 2000

Meeting Room 624-626 at 6th Floor, RIKEN Main Building

ABSTRACT

The nonlinear evolution and formation of shock fronts in the profile of very high-amplitude surface acoustic wave (SAW) pulses with acoustic Mach numbers ~ 0.01 were studied in solid materials. The excitation of the giant SAW pulses was performed with a nano- second laser pulses through a strongly absorbing layer. The detection of the SAW pulses at two probe spots along the propagation path was performed in a single laser shot with a dual- probe-beam deflection setup. Nonlinear changes of a SAW pulse shape were calculated using a nonlinear evolution equation.

In polycrystalline aluminum and copper, formation of one negative (inward the solid) narrow peak in the registered normal surface velocity and a shock front in the in-plane velocity was observed. This nonlinear behavior corresponds to a positive main nonlinear acoustic parameter.

In fused silica, formation of two positive sharp peaks related to two shock fronts in the in-plane velocity was registered. In this case the main nonlinear parameter is negative. Changes of the SAW pulse shape were explained by considering effects of local and nonlocal nonlinearities and their simulation with a nonlinear evolution equation.

The mechanical forces associated with the surface acceleration in high-amplitude SAWs detach the particles from the surface. It is shown that for nanosecond SAW pulses the limit of the surface acceleration of about 1010 m/s2 is set by fracture of the material and corresponds to the removal of particles larger than about 0.05 mm. In addition the nonlinear transformation of the excited SAW pulses results in an increase of the surface acceleration and contributes to the cleaning process extending it to even smaller particle dimensions. The technique is applicable in vacuum and improves the energetic effectiveness of the cleaning due to removal of particles not only in the irradiated region, but also in the wider area covered by the SAW pulse propagation. It can be also used for the determination of the Hamaker constant of the adhesion force.

Nuclear and Atomic Researches with an Ion Trap Apparatus, Using Dense Electron Beams and Strings ---Results and project proposals

Evgueni D. Donets

(Joint Institute for Nuclear Researches, Dubna, Russia)

2:00 p.m., Monday, Dec. 18, 2000

Meeting Room 624-626 at 6th Floor RIKEN Main Building.

Contents

  1. Introduction (Fusion nuclear reactions at low energies; Electron impact ionization and mutual charge exchange of highly charged ions).
  2. Production and study of highly charged ions in dense electron beams (EBIS/EBIT)
  3. Reflex mode of EBIS operation and the electron string phenomenon observation and study
  4. The BeTa (Beam-Target) apparatus for studies of fusion reactions at low energies and of mutual charge exchanges of highly charged ions.
  5. Various modes of the BeTa operation
  6. Expected proton counting rates for d+d and d+3He reactions
  7. Conclusion

戻る