118 PI-MAX/PI-MAX2 System Manual Version 5.F
Minimum FWHM for a PI-MAX
MG
system is in the 8 to 15 ns range, compared to 1.5 ns
to 2 ns for a standard PI-MAX. Pulse repetition rate is limited to 1 kHz.
Setup and Operation
The PI-MAX
MG
is set up in the same way as a standard PI-MAX and is compatible with
the DG535 and the PTG (preferred) timing generators. The cables used for a standard
PI-MAX are used for the PI-MAX
MG
. Note that when using the DG535, the set value of
gate width is significantly larger than the optical FWHM. Typically, a width setting of
37 ns produces a FWHM in the range of 7 to 9 ns.
1. Start with Shutter Mode operation if possible, to verify initial operation, to
focus, etc.
2. Then, switch to Gate Mode and a relatively long gate to acquire the
phenomenon of interest.
3. Finally, narrow down the gate to the desired operation.
When possible in the experiment, it is a good idea to use bracket pulsing to limit the
photocathode ON time. The required “lead time” or pretrigger time for the photocathode
bracket pulse is 300 ns (compared to 500 ns for a standard PI-MAX MCP bracket pulse).
Note: Pulse repetition rate is limited to 1 kHz.
Gain Variation
MCP gain approximately doubles for each 50 V increase in voltage. Therefore, small
ripples in the MCP voltage as a result of the gating waveform will cause gain changes
that vary with time after the rising edge of the gate pulse. A gain overshoot of 20 to 30%
during the first 20 ns of a gate pulse is typical, with smaller variations later in time if a
wider gate pulse is used. For a given gain setting and pulse width, these variations are
reasonably repeatable, and may be calibrated.
Fluorescence Experiment
A typical laser-induced fluorescence experiment might incorporate a pulsed laser that
excites a sample with the laser beam and that additionally provides a trigger to the DG535.
When the laser pulse hits the sample, some atoms are raised to a higher energy state and
then spontaneously relax to the ground state, emitting photons as they do to generate the
fluorescence signal. This signal can be applied to a spectrograph that spreads the
fluorescence spectrum across the photocathode of the PI-MAX. The spectrum would then
be intensified and applied to the PI-MAX’s CCD array.
Cabling for MCP Gated Operation
Figure 73 illustrates the cabling for this experiment using a Stanford Research DG535 Timing
Generator. The laser trigger output is applied to the Trig. In connector of the DG535 to
initiate the timing sequence. The A and B edge outputs of the DG535 gate the MCP on and
off. The pulse output gates the photocathode on and off if bracket pulsing
*
has been
selected. These three signals, A, B, and , are applied to the PI-MAX Timing Gen
connector. Another timing generator output, D, is applied to the Ext. Sync input of the
controller to initiate readout. Finally, to prevent artifacts from the laser from degrading
*
Bracket pulsing is not available for camera’s having a Gen III Intensifier. Also, if the camera has
a Gen III Intensifier, for proper operation, it is the DG535 output that must be
connected to the camera Timing Gen connector and not the DG535 output.
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