Transcript MAST Thomson scattering systems

MAST Thomson Scattering Upgrade
Laser Control System: Energy Monitors
João Figueiredo - October 2008
European Doctorate in Fusion Science and Engineering
MAST
• First MAST campaign January-June 2000
• Plasma cross-section and current
comparable to ASDEX-U and DIII-D
• Adaptable fuelling systems - inboard &
outboard gas puffing plus multi-pellet injector
• Very good access for diagnostics and
camera views
Thomson Scattering
Thomson scattering - scattering from free electrons
Rayleigh scattering - incoherent scattering from bound electrons in atoms and ions
Stray light component - laser radiation scattering from surfaces and windows
MAST Thomson scattering systems
Edge Nd:YAG TS system
R = 1.25-1.45m,1cm resolution
Single pulse 10J ruby laser
4 x 50Hz, 1.2J Nd:YAG lasers
Edge TS System Optics
Edge System View
Laser Beam Path
Core System View
Core ruby, 300 pts
Core Nd:YAG 19 pts
Core System Optics
Core TS data
• The profiles shown were
obtained over the course of a
long ELM free H-mode
• Edge density builds up over
the course of the H-mode and
becomes significantly higher
than the core density
H-mode - Inter ELM period
• Profiles taken during the inter ELM period. Taken
with laser separation of ΔT = 5μs.
• Camera picture is also taken during the inter-ELM.
• No significant variation seen in the Thomson
scattering ne and Te profiles. This is typical of the
inter-ELM period.
H-mode ELMing
• Laser separation: ΔT = 200μs
• During the ELM there are large protrusions of the plasma edge
from the pre-ELM LCFS (Last Close Flux Surface).
ELM filaments
• Laser separation: ΔT = 5μs
• As well as protrusions, filamentary structures are seen.
• Here 3 sets of filaments ordered by distance from pre-ELM LCFS
•It may be seen that the filament temperature falls off rapidly as the
filaments move from the plasma edge.
Edge TS data
The resolution in the edge region is sufficient to resolve the
pedestal gradients.
Pellet Deposition
• Pellet deposition profiles
have been measured using
the Ruby laser system.
• Here, three similar pellet
injections from three shots
are shown. The timing with
respect to pellet injection is
obtained from interferometer
data. (TS system triggered
by the pellet).
• Profiles approximately
constant along flux surfaces.
Reasons for Upgrade
• ITB measured in
counter injection shot
• 3/2 island structure
measured in the Ruby
profile
• Currently these profiles can only be measured once per shot.
• We want to measure similar profiles using the Upgraded Nd:YAG system.
Reasons for Upgrade
Core:
• density peaking
• sawteeth
Inboard & Outboard:
• Plasma Rotation from
density asymmetry
• Constraining EFIT
NTMs
ITBs
Edge Physics:
• gradients
• ELM filaments
Project
In a joint project with York University, implement
improvements to the MAST Nd:YAG Thomson
scattering system to give better temporal resolution
and improved capability to track fast transient events
ESPRC / European Milestone
Project
Lasers
Spectrometer
Room
Overview of MAST and relevant areas
Project
YAG Laser Room
New Laser Layout
Laser 1
Laser 2
Beam Combining Unit
Project
Control unit
FPGA box
(Red)
24 way patch
Panel
MD 03/04
(Red)
24 way fibre bundle
(Cyan)
Analogue fibre
(Orange)
MD 38/39
ADC box
MD 05/06
(Yellow)
ADC box
2+2 digital fibres
(Cyan)
Ruby Laser (Red)
Optical Receiver
Network connection
(Green)
Pellet route
(Purple)
Control room
MAST Area
Digital fibre
(Cyan)
Compass Area
Pellet control / detect
(Red)
Control System Schematic
Nd:YAG TS upgrade
To be fully implemented by 2009.
Technical specification
• 120 spatial points, ~10mm resolution, 240Hz
• Laser energy increased to 1.6J
• Number of lasers doubled to 8 (but each laser 30Hz vs 50Hz at present)
enhancing burst mode capability for NTM, ELM studies etc.
Applications
• Transport analysis (spatial resolution comparable to CXRS) including eITB evolution
• Transport in and around magnetic islands
• Pellet ablation and associated particle transport
• Transient events (e.g. ELMs and other filamentary structures)
Energy Monitors
System Goals and Requirements
• Check in real time proper laser operation
• Measure representative fraction of laser power
• Implement a robust and flexible system
• Future minor operational changes should have
none or minor impact in power monitors data output
System Characteristics
Photodiodes
• Slow
• Cheaper
• Straightforward
implementation
• 1 detector per beam solves
slowness
• 1 monitor per beam allows
more detailed control over the
TS system
APD’s
• Fast
• Expensive
• Difficult and time consuming
implementation
• 1 detector for all beams,
otherwise too expensive
• Data Consistency checks
would be easier because data
acquisition is performed with
APD’s
•Photodiodes to be implemented
•Future implementation of APD system is a possibility
Design
SM1ST - SM1 (1.035"-40) to
ST Fiber Connector Adapter Plate
Fiber - 62,5/125 µm
BK7 Uncoated Plano-Convex Lens, D=1.0’’, f=2’’
Unmounted Ø1" Absorptive ND Filter, OD: 1.0 2X
HeNe Beam
Parasitic
Transmission
<1 %
YAG-HeNe Mirror
Polarizer Film IR 3’’ X 3’’ TS
Ground Glass Diffuser, D=1.0", 220 GRIT
or
1" Round 20º Circle Pattern Diffuser (Engineered)
YAG Beam
1.6 J
Short Wave Pass
Dichroic Beamsplitter
Specifications
Short Wave Pass Dichroic Beamsplitter
■ Reflection > 99.9 % @ 1064 nm
■ Reflected Polarization: S-pol
■ 1’’ diameter
■ 0.25’’ thick
■ Material: BK7
■ Incidence Angle: 45º
■ Transmitted Wavelength (nm): 633
■ Transmitted Polarization: Unpolarised
Example for different wavelengths
Best solution for low power losses for the YAG beam – reflection better than 99.9%
Specifications
Ø1" Absorptive ND Filter, OD: 1.0 2X
Worst case scenario without filters ~13mJ after diffuser. Photodiode ~5A !
NE10 ~10% transmission (1064nm). Two filters combined ~ 50 mA.
It is assumed flat top profile in the diffuser and 75% power inside 20° solid angle
Specifications
Polarizer Film IR 3’’ X 3’’
Specifications
BK7 Uncoated Plano-Convex Lens, D=1.0’’, f=2’’
High power loads - coating unnecessary
Ground Glass Diffuser, D=1.0", 220 GRIT
DG10-220 – Low Price Best Option (20° ~ 75% of Power)
Specifications
Ground Glass Diffuser, D=1.0", 220 GRIT
High power loads. Transmission values could be lower with no problem.
Specifications
1" Round 20º Circle Pattern Diffuser (Engineered)
Scattered Properties
■ Scatter Shape: Circular
■ Divergence:1
ED1-C20 ED1-C50
20º (Flat Region) 50º (Flat Region)
27º (50%-Max) 54º (50%-Max)
36º (10%-Max) 60º (10%-Max)
■ Incident Beam Size: 0.5mm or Greater !!!
■ Transmission Efficiency: 90%
■ Design Wavelength: Achromatic (400-700nm)
Physical Properties
■ Material: Injection Molded ZEONOR
■ Index of Refraction: 1.53
■ Size: Ø1" (Ø25.4mm)
■ Thickness: 1.5mm
■ Clear Aperture: 95% of Diameter
■ Transmission Spectrum: 380-1100nm
■ Maximum Temperature: 120°C
Angles defined for 633nm and
collimated illumination. Actual
angles may differ from nominal
values for other wavelengths or
degree of collimation.
Specifications
Flat top profile for a wider angle. Price 10x.
Will be used if tests rule out ground glass diffusers.
Fiber - 62,5/125 µm
SM1ST - SM1 (1.035"-40) to
ST Fiber Connector Adapter Plate
Design
Acceptance Angle
31,924 +/- 1,718º
Value calculated from typical numerical aperture for a 62,5/125 µm fiber
Final Design
Fibre
Lens
Polarizer ND Filters
25.4 mm
Diffuser
25 mm
50 mm
75 mm
Photodiode OPF482 (1064 nm) – 0.1 A/W ~ 50mA
Laser Beam: Ø10mm
MAST Thomson Scattering Upgrade
Laser Control System: Energy Monitors
João Figueiredo - October 2008
European Doctorate in Fusion Science and Engineering