Design of the polarization ‐ preserving optics system for motional Stark effect diagnostics Jinseok Ko a , Jinil Chung a , and Maarten De Bock b a National Fusion Research Institute, Daejeon, Korea b Eindhoven University of Technology, Eindhoven, The Netherlands Tue 25 Feb 2014, KSTAR Conference, Jeongseon, Korea 20140225tu, j ko – kstar conf, jeongseon, korea 1
Challenges for commercial fusion plants – Info on internal B comes in • Control of a burning plasma • Non ‐ inductive steady state operation • Plasma wall interactions • First wall materials and divertor heat flux • Radiation ‐ resistant structural materials • Tritium breeding and fuel cycle 20140225tu, j ko – kstar conf, jeongseon, korea 2
Challenges for commercial fusion plants – Info on internal B comes in • Control of a burning plasma • Non ‐ inductive steady state operation • Plasma wall interactions • First wall materials and divertor heat flux • Radiation ‐ resistant structural materials • Tritium breeding and fuel cycle 20140225tu, j ko – kstar conf, jeongseon, korea 3
Challenges for commercial fusion plants – Info on internal B comes in • Control of a burning plasma • Non ‐ inductive steady state operation • Plasma wall interactions • First wall materials and divertor heat flux • Radiation ‐ resistant structural materials • Tritium breeding and fuel cycle • Control of plasma properties (temperature, density, current etc) and their profiles is essential for plasma equilibrium and stability. 20140225tu, j ko – kstar conf, jeongseon, korea 4
Challenges for commercial fusion plants – Info on internal B comes in • Control of a burning plasma • Non ‐ inductive steady state operation • Plasma wall interactions • First wall materials and divertor heat flux • Radiation ‐ resistant structural materials • Tritium breeding and fuel cycle • Control of plasma properties (temperature, density, current etc) and their profiles is essential for plasma equilibrium and stability. • Magnetic field profile is one of the most important (and hardest ‐ to ‐ measure) quantities and a direct measure of the equilibrium and stability. 20140225tu, j ko – kstar conf, jeongseon, korea 5
Challenges for commercial fusion plants – Info on internal B comes in • Control of a burning plasma • Non ‐ inductive steady state operation • Plasma wall interactions • First wall materials and divertor heat flux • Radiation ‐ resistant structural materials • Tritium breeding and fuel cycle • Control of plasma properties (temperature, density, current etc) and their profiles is essential for plasma equilibrium and stability. • Magnetic field profile is one of the most important (and hardest ‐ to ‐ measure) quantities and a direct measure of the equilibrium and stability. • MSE (Motional Stark Effect) diagnostic is under development at KSTAR for this purpose. 20140225tu, j ko – kstar conf, jeongseon, korea 6
Challenges for commercial fusion plants – Info on internal B comes in • Control of a burning plasma • Non ‐ inductive steady state operation • Plasma wall interactions • First wall materials and divertor heat flux • Radiation ‐ resistant structural materials • Tritium breeding and fuel cycle • Control of plasma properties (temperature, density, current etc) and their profiles is essential for plasma equilibrium and stability. • Magnetic field profile is one of the most important (and hardest ‐ to ‐ measure) quantities and a direct measure of the equilibrium and stability. • MSE (Motional Stark Effect) diagnostic is under development at KSTAR for this purpose. • Today’s talks focus on the design of the front optics and filters: • Diagnostic principles / development timeline • Spatial / temporal resolutions • Front optics (etendue, polarization preservation) • Bandpass filter design and selection 20140225tu, j ko – kstar conf, jeongseon, korea 7
Motional Stark effect: Doppler ‐ shifted polarized light gives local field information E = v B n = 3 n = 2 20140225tu, j ko – kstar conf, jeongseon, korea 8
Motional Stark effect: Doppler ‐ shifted polarized light gives local field information E = v B n = 3 n = 2 20140225tu, j ko – kstar conf, jeongseon, korea 9
Motional Stark effect: Doppler ‐ shifted polarized light gives local field information E = v B n = 3 n = 2 20140225tu, j ko – kstar conf, jeongseon, korea 10 10
Motional Stark effect: Doppler ‐ shifted polarized light gives local field information E = v B n = 3 n = 2 20140225tu, j ko – kstar conf, jeongseon, korea 11 11
Motional Stark effect: Doppler ‐ shifted polarized light gives local field information E = v B n = 3 n = 2 Bandpass filtering 20140225tu, j ko – kstar conf, jeongseon, korea 12 12
Overall plan: Commission in 2015 • PEM (Photo ‐ Electric Modulator) ‐ based MSE (30 channels) under design phase collaborating with Eindhoven Univ. of Tech (TU/e), The NBI MSE Netherlands. • The development path also includes the Li ‐ beam Zeeman effect diagnostic for the pedestal region. LiBeam • Design of (polarization ‐ preserving) front optics 2013 (Collection optics layout) – Sharing the collection optics with the existing CES: Signals will be separated into 550 nm and 650 nm via a dichroic beam splitter. – Low thermal ‐ birefringence materials will be used to avoid random change in the polarization in thermally harsh environment. • Design of bandpass filter module (Prototype of filter module) – Precise characterization of the bandpass filters and development of the filter module prototype. – PC ‐ based software to control the filter pass band (tilting). • Procurement and assembly (after 2014 campaign) 2014 • Commission 2015 • Application to the real ‐ time feedback control of J(r) 2016 20140225tu, j ko – kstar conf, jeongseon, korea 13 13
Better spatial resolution than ITER ‘Center of mass’ of the emission and the central point of the line of sight are different because of the asymmetry in NBI MSE the manner of intersection. LiBeam Emission profile (Ion source 1 in NBI1) r/a (%) Number of Machine min max channels • The ITER MSE requirements: r/a 5 % for ITER 2.5 8 20 JET 2 6 25 reasonable q profiles for NTM feedback (q = 1.5, JT ‐ 60U 8 10 16 2) and reversed shear control. 15T 38 23 10 315T 1.5 15 16 DIII ‐ D • The FWHM of the emission profiles due to the 195TL 8 11 8 45T, 195TU < 1.5 8 9, 16 NBI of the 30 lines of sight with realistic beam NSTX 3 5 12 emission and intersection geometry is regarded C ‐ Mod 10 40 10 as the ‘radial resolution’ MAST 5 5 35 KSTAR 2 8* 30 *R = 1.75 m 20140225tu, j ko – kstar conf, jeongseon, korea 14 14
r = 1 – 3 cm (mostly 1 – 2 cm) MSE coverage (using ION1) 20140225tu, j ko – kstar conf, jeongseon, korea 15 15
t = 10 – 50 msec seem to accommodate the various characteristic time scales • Theoretical limit: PEM fundamental frequencies 20 kHz • Practical limit: the amount of photons from the beam emission tens of msec • The typical H ‐ mode energy confinement time in KSTAR 100 msec • The current relaxation time in KSTAR 1.4a 2 Te 1.5 (keV)/Zeff 1.4 (0.5) 2 1.8 2 1.5 / 2 1 sec • The real time equilibrium reconstruction planned for KSTAR discharges can tolerate as low as 50 Hz. 20140225tu, j ko – kstar conf, jeongseon, korea 16 16
Space challenge: optics should reside in the M ‐ port cassette “cassette” I p B t 1 • Very small and narrow space to accommodate the necessary optical elements. • Faraday effect vs thermal birefringence? – Faraday effect: static and systematic (SFL6 glass materials; unacceptably high thermal birefringence constants) – Thermal birefringence: random and varying in time (NSSK5 and NSF15 glass materials; high Verdet constants) • Keep toroidal components of the lens surface normal vectors as low as possible. 20140225tu, j ko – kstar conf, jeongseon, korea 17 17
Sharing challenge: collection optics will be shared with CES • Do not degrade the current etendue of the CES diagnostic. • Dichroic beam splitter is under consideration to separate the CES (~ 550 nm) and MSE (~ 650 nm) signals. • Need to be custom ‐ made with a large aperture. From thorlabs.com 20140225tu, j ko – kstar conf, jeongseon, korea 18 18
Two challenges are implemented into the final optics design PEM Mirror MSE signals go up after beam splitter (reflected) Beam splitter CES signals go straight beam splitter (transmitted) 20140225tu, j ko – kstar conf, jeongseon, korea 19 19
Etendue is well reserved (close to an ideal optics) NA = 0.37 (hard polymer) d i f = 39 Image (or source) f o d o Optics NA = 0.22 (silica) f = 25.4 Fiber o d f f 20140225tu, j ko – kstar conf, jeongseon, korea 20 20
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