Space Research & Planetary Sciences (WP)

ILENA

The Imager for Low Energetic Neutral Atoms experiment (ILENA) at the Physikalisches Institut of the University of Bern is used to evaluate charge state conversion surfaces for low energy neutral particle instruments.

In the experiment an ion beam is reflected at grazing incidence from a sample surface. The reflected particles are analyzed using a retarding potential analyzer and a position sensitive multi channel plate assembly providing coarse energy and detailed angular information. Electrostatic and magnetic deflection systems at the sample allow to distinguish between neutral and charged primary and reflected atoms, secondary electrons, and sputtered particles. Typical primary particles include oxygen and hydrogen atoms and molecules and noble gases at energies from 1500eV down to 10eV per atom.

Experimental Setup

he ILENA experiment consists of an ion source, a beam guiding system, a sample stage with housing, and a detection unit. All these units are contained in a single vacuum chamber pumped by a turbomolecular and a ion getter pump.

setup

Ions are formed in an electron impact ion source (Nier type), with the intensity of the primary ion beam of the order of 1pA. A 90° sector magnet allows to select the particle mass. The beam then passes through the beam guiding and modulation module to the entrance aperture of the sample housing. Diaphragms limit the beam size to a diameter of 1mm and the beam divergence.

The impact angle of the ion beam on the conversion surface can be varied between 0° and 90° with respect to the surface normal. The beam current is monitored by measuring the current from the sample to ground. A magnetic field may be applied parallel to the sample surface to retain secondary electrons. By varying this magnetic filed the secondary electron yield of a sample surface can be determined.

The reflected beam is recorded with a two-dimensional position-sensitive micro channel plate (MCP) detector with a viewing angle of +/- 12.5° in the azimuthal direction and polar direction. A retarding potential analyzer (RPA) consisting of three grids is mounted in front of the MCP detector. The detector unit, including the RPA, is shielded electrostatically and can be rotated independently of the conversion surface around the same axis. The outer grids of the RPA are grounded to shield the inner grid, which can be biased to suppress positive ions. An additional grid in front of the MCP detector at negative potential with respect to the MCP detector serves to reject secondary electrons originating from the preceding grids and the conversion surface. The MCP detector may be floated to a negative high voltage with respect to the conversion surface to eliminate negative particles. The whole assembly allows to determine the ionization yield and the reflection efficiency of a conversion surface.

After baking out the vacuum chamber a residual gas pressure of 5e-8 mbar is achieved. During operation the pressure may rise into the low 1e-7 mbar range as a result of the test gas leaking into the ion source chamber.

Ion Beam Source

Positive ions are generated in Nier type ion source. Beam currents of the order of 1pA are obtained from the source. Different ion species can be produced by leaking a suitable gas into the source, i.e. H2, CO2, O2, or noble gases. The ions extracted out of the source are mass analyzed using a 90° sector magnet with a mass resolution of approximately 45. The following species have up to date already been produced using the source: H+, H2+, CH+, CH2+, CH3+, CH4+, O+, OH+, H2O+, CO+, CO2+,CO22+ and various positive single charged fragments of C3H8. The elemental range is to be extended to He and Ne.

raw data
The mass range of the magnet extends from 1amu to approximately 50amu and the energy range from 10eV to 1500eV. The upper end of the mass and energy scale is limited by the power dissipation by the magnet coils as they are placed directly in the vacuum.

Pictures

Surfaces

Properties

Surfaces are selected regarding several key properties depending on the application:

  • Secondary electron yield:
    The number of emitted electrons per incident particle.
  • Ionization efficiency:
    The fraction of particles that get negatively ionized upon reflection of the surface
  • Reflection efficiency:
    A measure of the scattering properties of the surface. It indicates the percentage of particles that get reflected into the standardized field of view of the MCP.
  • Angular scattering:
    Angular resolved charge state dependent scattering properties of the surface.

These properties are measured dependent of primary particle type and energy, and of the angle of incidence of the primary beam upon the surface under test. Some properties are also time dependent and others change when the surface is exposed to UV radiation.

A Classification

Roughly the investigated surfaces fall into three different groups:

  • Metallic surfaces:
    These include mono and poly crystalline tunsten and tantalum surfaces. Tungsten surfaces were used in the LENA instrument on IMAGE.
  • Insulating surfaces:
    Oxydic single crystals like MgO and SrTiO3 fall into this group.
  • Multi layer surfaces:
    Most of the surfaces fall into this group. The idea is to combine some optimized top layer with an easy to handle substrate with a high flatness. Typical examples are CVD diamond or samples with dielectric layers to absorb UV more efficiently combined with a suitable top layer, i.e. tungsten or MgO.