Mullard Space Science Laboratory
Mechanical Design Office
MECHANICAL DESIGN OF PLASMA ANALYSERS
J. Coker
Mullard Space Science Laboratory
1
Mullard Space Science Laboratory
Mechanical Design Office
Heritage of MSSL Plasma Analysers
• The Hemispherical Micro Channel Plate (MCP) based
Analysers were developed from earlier versions with
Channeltron detectors. (Effectively just one poor of an
MCP).
• MCP’s provided angular resolution and a much wider
field of view for the first time.
• The first MCP analyser was flown on the AMPTE mission
in 1984.
• Multilayer ceramic substrate anodes with “wedge and
strip” pattern laser cut maximised angular resolution.
2
Mullard Space Science Laboratory
Mechanical Design Office
Principle
• Angular acceptance
– Typically + 3o
Top Hat
eOuter Deflection Plate
+ HV sweep
Inner Deflection
Plate
Field Defining
Grid
Anode
MCP
(amplifies charge)
3
Mullard Space Science Laboratory
Mechanical Design Office
Principle
• Analyser focuses
electrons
• MCP detector
4
Mullard Space Science Laboratory
Mechanical Design Office
Cut Away CAD Model of CASSINI/CAPS Analyser
CAP/AMP BOARD
MCP ASSY.
S/C INTERFACE
GRID
OUTER HEMISPHERE
(DEFLECTION PLATE)
LIGHT BAFFLES
HV. ELECTRONICS
TOP HAT
INNER HEMISPHERE
(DEFLECTION PLATE)
ENTRY APERTURE
DAUGHTER BOARDS
ANODE
INSULATOR
MOTHER BOARD
5
Mullard Space Science Laboratory
Mechanical Design Office
The Two Main Design Drivers
• Mass, one of the main considerations in deciding suitable
design and manufacturing concepts
-Sheet metal parts are the lightest but less accurate and
more difficult to produce with good repeatability.
-Turned/milled parts more accurate but higher mass.
-Electro Discharge Machining (EDM) enables lighter
machined parts with better accuracy.
• Accuracy, high accuracy requirements demand stiffer parts
and dictate production techniques. (Always a compromise
with mass).
6
Mullard Space Science Laboratory
Mechanical Design Office
GIOTTO
The First integral analyser, electronics and mounting package
Spun hemispheres and light sheet
structure
Complete GIOTTO instrument
7
Mullard Space Science Laboratory
Mechanical Design Office
Accuracy (Tolerance)
• Cluster required 0.030mm tolerance on the 3.0mm analyser
gap, (1/4 of a human hair) with absolute max. tolerance of
0.3mm
• This tolerance was governed by 9 individual tolerances on 5
different parts.
• Two of these parts are Ceramic and required CTE clearances.
• All parts required assembly clearances.
• The best tolerance likely to be achieved was assessed by
calculating the root of the sum of the squared individual
tolerances and gave +/- 0.790mm
• However as most of the tolerances related to turned parts
inherent concentricity was in our favour.
• Final instrument performance suggested that we were within
the 1% to 10% tolerance requested.
8
Mullard Space Science Laboratory
Mechanical Design Office
Engineering Models, CASSINI, PEPI, CLUSTER
CLUSTER
CASSINI
PEPI
9
Mullard Space Science Laboratory
Mechanical Design Office
Design Drivers Continued
• Volume/FOV.
– Spacecraft accommodation and mounting options.
• Radiation screening.
– Should be incorporated into the design, ie integral, not
added as an after thought.
• Brittle materials
– Hard to guarantee survival
– Usually have significantly different CTE’s
– MCP clamping a particular worry
• High voltages
– Breakdown gaps must be maintained
– Connections and screening difficult
10
Mullard Space Science Laboratory
Mechanical Design Office
Multi Layer Ceramic Anodes
EARLY
CLUSTER
CLUSTER Wedge & Strip
Pattern (laser cut)
11
Mullard Space Science Laboratory
Mechanical Design Office
TIMAS-Detector assembly for Toroidal Analyser
75mm outer radius Micro Channel Plates
12
Mullard Space Science Laboratory
Mechanical Design Office
Design Drivers Continued
• Vibration
– First Eigen frequency should be >100Hz
– Usually design for 100g dynamic load
– All fixings must be locked. (difficult for small screws)
• Thermal
– Must ensure adequate CTE allowance
– Must consider instrument thermal control. (Usually
involves careful choice of finishes and thermal blanket
provision to maintain acceptable electronics temperatures.
• Contamination
• Electronics
– Normally required to be in close proximity to analyser and
generally housed in integral structure
13
Mullard Space Science Laboratory
Mechanical Design Office
Electronics and Supporting Structure
Components for CLUSTER DPU
CLUSTER analyser assemblies
14
Mullard Space Science Laboratory
Mechanical Design Office
Electronics and Supporting Structure
Analyser head body used to stiffen mounting flanges
(Dual purpose gives high structural mass efficiency)
15
Mullard Space Science Laboratory
Mechanical Design Office
CASSINI Electronics & Main Structure
A compact electronics package with good interchangeability for most PCB’s
Checking flexible circuit card for fit
Final installation package
16
Mullard Space Science Laboratory
Mechanical Design Office
Blacking considerations
• Ebanol-C surface conversion for light rejection.
• The process involves immersing the part to be blacked
in a succession of hot and cold chemical baths.
• Problems in sealing non blacked surfaces on delicate
parts.
• Parts can only be “reclaimed” once if process fails.
• Blacking jigs have limited life due to chemical erosion.
• Difficult to handle and transport blacked parts.
• Difficult assembly, (if blacked surface is touched finish is
damaged).
17
Mullard Space Science Laboratory
Mechanical Design Office
Blacking jigs, before and after blacking
Parts for PEPI lab analyser
18
Mullard Space Science Laboratory
Mechanical Design Office
Future Developments
• The trend is to work towards smaller, lower mass
instruments, but this accentuates the problems already
facing us as :– Accuracy requirements become more acute. (SWRI) have
produced a smaller instrument but at the cost of a larger
percentage inaccuracy.
– Operating voltages stay the same, so prevention of
breakdown is more difficult.
– Fixings, lack of volume to accommodate them.
PEPI instrument is the first with modular mcp assembly
achieved by supporting inner hemisphere on spokes.
Unfortunately the volume of the mcp assembly forces
instrument size back to CLUSTER volume.
19
Mullard Space Science Laboratory
Mechanical Design Office
Sub assemblies for PEPI lab analyser
20
Mullard Space Science Laboratory
Mechanical Design Office
Principle
• Angular acceptance
– Typically + 3o
– Require + 30o
0V
e-
+ HV
+ HV
MCP
MCP
21
Mullard Space Science Laboratory
Mechanical Design Office
Specifying Instrument requirements
• Think carefully about prime concerns/aims. Eg mass,
accuracy, volume etc. as these will affect science
capability.
• Try to give realistic minimum requirements.
• Minimise restrictions.
• Remember that the final design will be a compromise
between all requirements.
22
Mullard Space Science Laboratory
Mechanical Design Office
Thank you for your time and attention
23