March 15th 2009, Argonne, EFI Chicago,
Jean-Francois Genat and Edward May
Estimation of the Transmission linestoMicro-Channel Plate coupling using a grounded electrode.
1- Measurement set-up
Two fast transmission line cardshave been tested in order to understand the effect of their coupling them to Micro-Channel Plate (MCP) devices.One card was glued with silver epoxy to a regular 25 m MCP device (Figure 1), the other was bare, at the exception of 50 terminations at the end of all channels not in use. An electrodemade of a grounded plate of copper of the size of an MCP (2”x 2”) has been capacitive coupled to the transmission lines (T-lines) with different thicknesses of dielectric (paper), as shown Figure 2.
Figure 1. 25um Micro Channel Plate mounted on a Transmission line card.
Figure 2- T-line card and grounded copper electrode.
These measurements allow estimating the minimumsecond gap for future MCP devices intended to have transmission lines as anodes for 2D readout. This minimum gap is measured as the distance below which reflections of more than 10% of the pulse amplitude are produced, due to the impedance mismatch to 50, which is the nominal impedance of the T-lines card.
2- Measurements.
The T-line card equipped with a 25 MCP have shown a reflection coefficient of 3/17.5 = 17%.
The corresponding impedance is Z as: , so .
This impedance change is due to all couplings between the T-line and the MCP device.
When using a grounded planar electrode coupled to the T-lines through some layers of paper sheets, in order to mimic the effect of the MCP bottom plate electrode, the coupling inducing reflections appearedmuch sensitive to the electrode’slateral position when using T-lines close to the edge (#2). When centered, looking at T-line #17, there was no more sharp dependence of the reflections with the copper plate lateral position. This was understood as the fact that the capacitance seen by the T-line does not change as much with the lateral position of the electrode when at the center,as at the sides.
Pulser
Ch1
50
Figure 2-2 Reflection mode
Figures2-1and 2-2 show the transmission mode, where a fast pulse from the TDS 6154C oscilloscope was sent to the card through a 5’ fast cable, returning from the card to Ch1 with another similar 5’ cable, and in the reflection mode where the same pulse is sent straight to the oscilloscope channel-1 and from there to the card through the fast 5’ cable, the other end of the T-line being terminated on 50.
Figures 3-1 and 3-2 show the response of the bare card, in the transmission mode, for thicknesses of paper of 0, 4.6, 10.2, 14.6, and 20.4 mils.
Figures 4-1 and 4-2 show the response of the bare card, in the reflection mode, under the same conditions.
Figure 5 shows the T-line characteristic impedance as a function of the paper thickness calculated from , and is taken to be 50.
-3 Conclusions
No reflection was observed from the bare card alone. Above 20.4mils (.52mm) of insulating thickness, the electrode producedreflections below 4.5%, i.e. 8.6% changes in the characteristic impedance of the T-line as it appears from Figure 4.
These measurements allow concluding that a minimum width of 175m is required for the second gapto control the T-line impedance within 10%(520m/3 since the paper relative dielectric constant is 3), unless the T-lines are implemented with animpedance lower than 50, which would lead to a larger thickness of dielectric (glass/ceramic).It is assumed here that there is no coupling with the electrodeother than the MCP-electrode capacitance, as it would be the case if the top electrodes of the micro-strip lines were themselves the MCP anodes, in the vacuum, as it is planned to be for future devices implementations.
Figure 3-1 Transmission. No electrode, Figure 3-2 No electrode, electrode +
electrode + 117microns of paper 117-468 microns of paper (zoom)
Figure 4-1 Reflection. No electrode, Figure 4-2 No electrode, electrode+
electrode + 117 microns of paper 117–468 microns of paper (zoom)
Figure 5. Characteristic impedance vs coupling (microns of paper dielectric).
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