RoentDek DLD40 - The Ft4(Tp) for Ft16(Tp) and Det40;75

An integral part of most signal decoupling circuits is a serial resistor in the connection line between the MCP stack and the bias
input, e.g. from the high voltage supply. The resistor forms a chain with the MCP stack resistance (and more resistors in the
line, e.g. a serial resistor on the other MCP stack side) and alters the effective potential present on the MCP surface: it is shifted
by a value ΔU towards the potential on the other MCP side, compared to the set potential coming from the high voltage supply.
These serial resistors are usually small compared to the MCP resistance but the effect cannot always be neglected.
)( DfDbMCP
Df
MCP
frontMCP
RRR
R
UdU
++
×∆=
Equation 2.1
)( DfDbMCP
Db
MCPback
MCP
RRR
R
UdU
++
×∆=
dU
MCP
front/back are the potential shifts between set and effective potential on the MCP stack ends, R
MCP
is the MCP stack
resistance and R
Df
/R
Db
the serial resistors’ values in the respective decoupling circuits and ΔU
MCP
is the set voltage difference
across the MCP (ΔU
MCP
= U
MCP back
- U
MCP front
). Usually, the term (R
Df
+ R
Db
) can be neglected in the denominator of the
equations.
Example: R
MCP
= 60 MΩ, R
Df
= 10kΩ, R
Db
= 1MΩ, U
MCP front
= -600V, U
MCP back
= +1800V
→ U
MCP front effective
= -599.6V, U
MCP back effective
= +1760V, ΔU
MCP effective
= 2360V
In the FT12TP decoupler R
Df
and R
Db
are 10kΩ and 1MΩ, respectively, older FT12TP plugs may have 1MΩ as default value
for both sides, also like the standard
RoentDek FT4TP with HFSD and HFST single line decouplers (see next section). It
is possible to measure the actual resistance values with an Ohm meter between the high voltage input socket and the pin that
connects to the respective feedthrough. The sum (R
MCP
+ R
Df
+ R
Db
) can also be measured through the decoupler’s high
voltage inputs for U
back
and U
front
if all connections to the MCP are made. Note that the measured value R
MCP
can differ
compared to operation conditions if the stack is at atmospheric pressure. R
MCP
is also temperature dependent.
Attention: Although the outputs of the
FT4TP
and
FT12TP
are delivered with DC-coupling to ground potential, a
discharge on a detector can damage the electronics which is used to analyze or amplify the signals.
Before vacuum-baking of the experimental setup all air-side connections on the
FT12
and
FT4
feedthroughs must
be removed. The
FT12TP
plugs and
HFSD/T
connectors are not rated for typical bake-out temperatures
2.4 The FT4(TP) for FT16(TP) and DET40/75
For the RoentDek HEX detector and the timing detectors of type DET40/75 the FT4, a four-fold MHV feedthrough set
(optionally SHV) on CF35 flange is used to supply cable contacts to MCP front, MCP back, Holder (or the timing anode,
respectively) and an optional mesh. For signal pickup or termination, individual
HF-signal-de-coupler plugs
(HFSD/HFST)
(
Figure 2.7) for each detector contact can be provided to complete the product set FT4TP. This combines with the
FT12TPhex to the FT16TP. The
HF-signal-de-coupler plug
for the Hexanode “Holder” is of type HFST and has a signal
terminating poti. The same type is used on one of the MCP contacts, while the other MCP contact (and the timing anode
contact, respectively) is supplied via a HFSD signal decoupler with a lemo output socket for the signal to further process it to
an amplifier and timing circuit. An
Adjustable
LEMO Terminator
(AST) can be optionally supplied to turn a HFSD into a
double-use unit (as HFSD or HFST). The latest version of the HFSD has a poti in the signal line (see red arrow in
Figure 2.7)
which can also be used to improve the signal quality in combination with the other potis (like with the FT12TP connector plug
and DLD)
*
. Optionally, the HFSD and HFST can be supplied with a 10kΩ serial resistor in the line to the high voltage power
supply (default is 1MΩ).
*
On older systems the HFSD needs to be opened for accessing the in-line poti. Although most care has been taken to insulate
all high voltage holding parts of the circuit for your safety please be careful to
only touch the poti screw
and
only with an
insulating screw driver
when operating on an open HFSD.
MCP Delay Line Detector Manual (11.0.1304.1) Page 29 of 83