RoentDek DLD120, Manual

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MCP Delay Line (Version 6.2.90.5) Page 21 of 80
2.4.6 Operation of the Delay-line Detector – General Description
Always evacuate the vacuum chamber slowly (50mbar/sec) in the presence of an MCP detector. The maximum
recommended operating pressure for the detector is 2
×
10 -6 mbar) and the MCP should be in vacuum for at least one hour
before applying bias.
However, after first installation a startup procedure is required. Please refer to the chapter “getting started”.
Generally, it is recommended to apply the bias voltages slowly (100V/sec), also to the anode, in order to avoid
possible damage to electronic modules connected. When applying voltage to the detector never exceed relative
voltages of 100V between the reference and signal wire and of 500V between holder and signal wires or holder and
MCP back.
The MCP can be operated with a relative bias voltage up to about 1300V for each MCP in the stack for MCP with L/D 60:1
or 80:1 (only 1000V per MCP with L/D of 40:1, for 80:1 even higher bias up to 1500V per MCP may be required). Lower
relative bias on the MCP often yields sufficient performance and can increase the lifetime of the MCP stack. Higher voltages
are not recommended and will only improve the performance if the amplifiers still have sufficient dynamics.
It is advisable to control the amplifier signals with an oscilloscope when applying voltage (see “getting started”). For
supplying the MCP operation voltages it is strongly recommended to use power supplies with current limitation and fast
shutdown for protection (as available from
RoentDek
). The optimal potential of the MCP front side with respect to
ground depends on the particles to be detected. Ions should be pre-accelerated onto the detector with a potential of -2000V
or higher. For most ion species it is suitable to operate the MCP back side on ground potential, thus the front side is in the
range of –2kV to –3kV. Electrons should be accelerated to at least 300eV to ensure high detection efficiency. Thus the MCP
front should be 300V or higher with respect to the electron source for low energetic electrons. For UV photon
detection the MCP front side potential is arbitrary.
The wire array consists of two double delay-line helical propagation lines (Lecher-line). For each dimension a wire pair is
formed by a collection (signal) wire and a reference wire. A potential difference of about +20V to +50V of the signal wire
with respect to the reference wire ensures that the electron cloud emerging from the MCP is mainly collected on the signal
wires, shared almost equally between both wire layers. The anode holder has to be supplied with an intermediate potential
with respect to the anode wires and the MCP back potential to ensure proper charge cloud propagation and spatial
broadening in the drift zone between MCP and anode wires. The optimal voltage depends on the distance between the MCP
holder plate and the anode wires.
Typically the wires should have about 300V more positive potential than MCP back side and the holder about +150V with
respect to the MCP back.
Typical voltage settings are
Ion or Photon Detection Electron Detection
MCP front +300V −
2400V
MCP back 0V +2700V
Anode holder 0V to 250V +2700V to +2950V
Reference wires +250V +2950V
Collecting (Signal) wires +300V +3000V
Table 2.3: Detector voltage settings
Avoid penetration of strong external electrical and magnetic fields into the electron cloud drift region (between MCP and
wire anode). Electrical fringing fields can produce image distortions, magnetic fields (> 50Gauss) disturb the proper charge
cloud broadening and will lead to malfunction of the anode.
When applying voltage to the MCPs most high-voltage power supplies have a too low input resistance (also the
RoentDek
power supply). Then it is possible that when you increase the MCP front voltage to –2kV the back voltage also increases
although the high voltage power supply for MCP back is set to zero. This effect (the MCP back potential is “drawn away”)
usually disappears when the power supplies are switched to different polarity. It is anyway suitable when MCP front and back
need to be set to the same polarity to use only one high voltage power supply for the MCP stack and a resistor to ground for
the lower voltage end. The dimension of the resistor depends on the MCP stack resistance R
MCP
and should be of value
back MCP
front MCP MCP
U
U R M
R
× + Ω
=
) 2 (
Equation 2.6