MECHANICAL DESIGN OF PLASMA ANALYSERS

1
MECHANICAL DESIGN OF PLASMA ANALYSERS

• J. Coker
• Mullard Space Science Laboratory

2
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).
• MCPs 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.

3
Principle

• Angular acceptance
• Typically 3o

Top Hat
e-
Outer Deflection Plate
Inner Deflection Plate
Field Defining Grid
MCP (amplifies charge)
Anode
4
Principle

• Analyser focuses electrons
• MCP detector

5
Cut Away CAD Model of CASSINI/CAPS Analyser
CAP/AMP BOARD
S/C INTERFACE
MCP ASSY.
GRID
OUTER HEMISPHERE (DEFLECTION PLATE)
LIGHT BAFFLES
HV. ELECTRONICS
TOP HAT
INNER HEMISPHERE (DEFLECTION PLATE)
DAUGHTER BOARDS
ENTRY APERTURE
MOTHER BOARD
ANODE
INSULATOR
6
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).

7
GIOTTO
The First integral analyser, electronics and
mounting package
Spun hemispheres and light sheet structure
Complete GIOTTO instrument
8
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.

9
Engineering Models, CASSINI, PEPI, CLUSTER
CLUSTER
CASSINI
PEPI
10
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 CTEs
• MCP clamping a particular worry
• High voltages
• Breakdown gaps must be maintained
• Connections and screening difficult

11
Multi Layer Ceramic Anodes
EARLY CLUSTER
CLUSTER Wedge Strip Pattern (laser cut)
12
TIMAS-Detector assembly for Toroidal Analyser
75mm outer radius Micro Channel Plates
13
Design Drivers Continued

• Vibration
• First Eigen frequency should be gt100Hz
• 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

14
Electronics and Supporting Structure
Components for CLUSTER DPU
CLUSTER analyser assemblies
15
Electronics and Supporting Structure
Analyser head body used to stiffen mounting
flanges (Dual purpose gives high structural mass
efficiency)
16
CASSINI Electronics Main Structure
A compact electronics package with good
interchangeability for most PCBs
Checking flexible circuit card for fit
Final installation package
17
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).

18
Blacking jigs, before and after blacking
Parts for PEPI lab analyser
19
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.

20
Sub assemblies for PEPI lab analyser
21
Principle

• Angular acceptance
• Typically 3o
• Require 30o

0 V
e-
HV
22
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.

23
Thank you for your time and attention