A UHV clean aluminium foil of 25µm thickness was installed in the monitor, but also
a 200 µm thick aluminium plate was prepared. A solution of sub-µm was not used in
the prototype due to its fragility, price and electrostatic forces that can deform a thin film. It was decided to make the first tests with a standard UHV clean foil available in the lab instead of a thick plate to investigate the difficulties with its installation and behaviour in the real setup.
Figure 6.17: A cross-section view of the secondary emission monitor setup.
has 80 lines per inch which is approximately 3 wires per millimetre. Each wire is 25µm
thick and the hole size is 293µm in diameter. The parameters of the mesh correspond
to a transmission of 85%.
The MCP and phosphor assembly, model BOS-40, was procured from Beam Imaging Solutions [276]. It consists of a chevron type microchannel plate and a P-43 phosphor screen housed in a ceramic base. The beam energy range specified for the model is 1 eV
to over 50 keV. Should it be used for beam currents greater than 3.2 nA/mm2, a 90%
attenuation needs to be used. The assembly is UHV compatibile and the maximum
bakeout temperature is 300◦C. Vacuum of at least 10−6 mbar or better is required to
operate the MCP. An exploded view of the assembly is shown in Fig. 6.19.
The chevron type microchannel plate is a stack of two MCPs, each of 50 mm
diameter and effective imaging area of 44 mm diameter. Their channels are 10µm in
diameter with 12 µm pitch and 8◦ bias angle. The thickness of a single plate is 0.46
6.4 Prototype Design and Construction Foil/mesh assembly MCP/phosphor assembly CCD camera flange Beam Ø20 mm a) b) MCP/phosphor assembly CCD camera flange Beam Ø20 mm
Figure 6.18: Two possible configurations of the monitor: (a) a foil-based SEM, and (b) an MCP placed directly in the beam path.
is approximately 2·104 for a single plate at 1000 V, and more than 107 for the chevron
configuration at 2000 V.
The phosphor screen is an aluminised P-43 (Gd2O2S:Tb3+) on a glass (Pyrex) plate.
Its typical peak wavelength is 545 nm, and the fluorescent colour is green, whereas the
decay time is of the order of 1 ms. The thickness of the phosphor layer is 10 – 15 µm
and of the aluminium layer is 250 – 500 ˚A. The glass plate is 1.5 mm thick and its
outer diameter is 50 mm, with a diameter of the imaging area of 44.9 mm.
6.4.3 Electrical Design
A 3-channel HV power supply 19” THQ from ISEG is used to supply voltages to the MCP, phosphor and foil. Two DPS series modules with a switchable polarity and 5 kV SHV output connectors can supply up to 3 kV/4 mA and 5 kV/2mA to the MCP and the phosphor, respectively. Peak-to-peak ripple and noise for DPS modules are typically less than 2 mV and 7 mV maximum. A single CPS module with a fixed polarity and 16 kV LEMO output connector can supply up to –10 kV/1 mA to the foil.
Ceramic spacer Single MCP Contact rings Phosphor screen Ceramic base Single MCP Ceramic spacer
Figure 6.19: Exploded view of the BOS-40 chevron type MCP and phosphor assembly.
Peak-to-peak ripple and noise for the CPS modules is typically less than 200 mV and 500 mV maximum.
In order to apply proper voltages with a single HV power supply and to vary them for both MCPs at the same time, a custom-made voltage divider is used. It incorporates two micro-ammeters in series with the power supply to monitor current flow across the MCPs, two digital voltmeters to read the voltage divided between the MCPs and two current limiting resistors to help prevent damage to the MCPs in the event of a HV breakdown within the internal detector assembly. The schematic circuit of the voltage divider is shown in Fig. 6.20.
For the interface between air and vacuum, four 5 kV SHV feedthroughs and one 10 kV SHV feedthrough were welded to the actuator flange. The first three feedthroughs supply voltage to the front, middle and rear of the MCP assembly, the fourth 5 kV SHV is used for the phosphor screen, whereas the 10 kV SHV is for the foil.
The MCP and phosphor assembly is built as a sandwich with metal rings placed between the components, see Fig. 6.19. Kapton-insulated wires, rated up to 5 kV, were spot welded to the rings separating the MCP plates.
For the foil, a more stiff Kapton-insulated wire, rated up to 10 kV, is used.
6.5
Experiments with Electrons
6.5.1 Experimental Setup
Prior to tests in a beam line, the monitor was assembled and tested in terms of its vacuum and electrical performance as well as response to a low-energy electron beam. The setup shown in Fig. 6.17 was used with an electron gun, ELS5000 from PSP Vacuum
6.5 Experiments with Electrons
+ +
+
+
Figure 6.20: Voltage divider for the chevron type MCP: a schematic diagram (top) and the practical implementation (bottom). See text for details.
Technologies, attached to deliver the beam in the monitor direction. Additionally, the same vacuum gauge and pumps were used as described in Section 5.4. The detector was tested in both configurations shown in Fig. 6.18.
Two flanges were used to install viewports for monitor observation. The custom- made vacuum window reductor was installed as shown in Fig. 6.17, but a large size DN100 viewport was also mounted downstream of the detector. The latter made it easier to observe images in the stand-alone MCP configuration.
The main purpose of the tests was to establish whether no major mechanical, electri- cal or vacuum problems could be found prior to shipping the equipment for experiments with protons. For this reason, only qualitative data related to electron beam imaging are reported.