Transcript Experimental Setup - Queen`s University Belfast

The Apparatus….
Neutral target studies….
Ionic target studies….
Queens University Belfast
University College London
Schematic of the Ion Beam Apparatus
Primary
Beam
Collector
Charged
Fragment
Detector
Extraction and Focussing Lenses
Einzel
lens
45o Parallel Plate
deflectors
Neutral Fragment
Detector
-ions are accelerated to 1-2 keV
Deflection
Plates
Laser
Beam
Ion Source
-ions produced via
Interaction
Region
discharge
Selection Magnet
Plasma Discharge Ion
Source
- creates a plasma of ions and electrons
In the fringe region between the anode and
cathode, electrons are stripped off the
introduced gas by the strong electric field.
The electrons then spiral due to the
surrounding magnetic field thus colliding with
more atoms ionizing them.
The ions produced are self-extracted through
the aperture due to the electric field distribution
and are then focused and transported.
Mass Selection Magnet
- used to select the particular ion species
to study.
Ions passing through the magnet field
region are deflected due to the Lorentz
force.
For a particular magnetic field setting, ions
of different mass/charge ratios are
deflected through different angles.
By varying the magnet field we can select
a particular mass/charge ratio ion to exit
through the final aperture.
Interaction Region
- interaction of ions with focused laser beam.
The laser system used is the RAL Astra
790nm Ti:Sapphire laser giving 50fs, 25mJ
pulses at 10Hz.
When focused with a relatively soft focusing
lens, we obtain intensities between 1016 –
1017 Wcm-2.
The ionized ions are carried along with the
initial momentum of the primary ion beam.
A set of plates can also be used to bias the
interaction region if needed.
Parallel Plate Analyser
- used to separate product ions from primary
ion beam.
An electric field is applied across the 45o
parallel-plate region.
Ions of higher charge/mass ratio are deflected
more by the electric field thus one can
separate out the ionized products from the
primary ion beam.
The main ion beam is dumped in a well-baffled
Faraday cup whilst the product ions are
detected by a CEM.
An on-axis CEM is used to detect any
neutral fragments from molecular studies.
Neutral Target Apparatus
Focussed
Laser Beam
The gas is effused into the interaction region via a
hypodermic needle. The focused laser interacts with the
gas target and the product ions are extracted by a weak
electric field across the interaction region. After drifting up
the drift-tube region, the product ions are detected by a
pair of microchannel plates with their time-of-flight
recorded.
The system can be operated in Spatially-resolved
geometry mode or Wiley-McLaren mode.
Scanning Techniques….
Intensity Scan
y
focussed laser direction
x
z
With the Intensity scan, interact
the ion beam with the centre of
the laser focus.
Then vary the peak focus
intensity by varying the input
power of the laser.
ion beam
direction
Thus measure Ion yield
(ionization) as a function of laser
intensity.
Scanning Techniques….
Intensity Selective Scan
y
focussed laser direction
With the Intensity Selective
Scan, drive the laser focus
through the ion beam.
x
z
ion beam
direction
By interacting with different
intensity slices of the focus, we
again measure the Ion yield as a
function of intensity.
This technique has the added
advantage in that the low
intensity regions are enhanced
by increased volume.