Development of a fast EUV movie camera for Caltech spheromak jet experiments

Transcription

1 P1.029 Development of a fast EUV movie camera for Caltech spheromak jet experiments K. B. Chai and P. M. Bellan ` California Institute of Technology kbchai@caltech.edu

2 Caltech Spheromak gun 2 Target: study spheromak formation & astrophysical jets Plasma characteristics Discharge type: pulsed power Applied voltage: ~ 5 kv Current: ~ 100 ka Lifetime: a few tens of s Gas: H 2, Ar, N 2 Diagnostics Magnetic probe array: B Rogowski coil: I p High voltage probe: V p Interferometer: n e Fast framing camera: visible emission 1.58 m Chamber drawing Jet experiment 1.48 m

3 Evolution of spheromak jet 3 8 flux ropes formed Merged into jet Jet stretching Kink instability Rayleigh-Taylor instability Plasma torn apart

4 Motivation 4 Strong EUV burst: observed when there is magnetic reconnection 1 st reconnection: when loops merge 2 nd reconnection: when Rayleigh-Taylor instability occurs Study EUV: can learn about magnetic reconnection Goal: measure EUV as a movie with few hundreds ns interframe time EUV signal Plasma ignited Bursts of EUV during magnetic reconnection Rayleigh-Taylor s A. Moser and P. Bellan, Nature 482, 379 (2012)

5 EUV movie camera issues 5 Our camera cannot detect EUV directly We need something to convert EUV into visible: Use scintillator Rayleigh-Tayor instability lasts very short time: a few s Need fast decay scintillator Visible light should be blocked Metal thin film is good for blocking visible light Need EUV focusing optics: concave mirror 1 st method: High incident angle to surface normal 2 nd method: EUV multilayer mirror

6 EUV camera with parabolic mirror 6 EUV YAG:Ce Scintillator (Al coated at front side) Mirror Aperture (D=5.08 mm) Visible Parabolic mirror (Au, f=5.08 cm) (incident angle ~ 45 ) CCD camera with lens

7 Efficiency of YAG:Ce scintillator 7 For converting EUV into visible Efficiency for powder YAG:Ce scintillator 30 ev= 41.3 nm 60 ev= 20.7 nm 2.54 cm 5 m thickness 10 m thickness 75 m thickness Material: 100 m single crystal YAG:Ce with 100 nm Al coating Customized one for better imaging from Crytur Ltd. (Czech Republic) Decay time: 70 ns Efficiency for EUV (30-60 ev): more than 1% A. Baciero et al., J. Synchrotron Rad. 7, 215 (2000)

8 Reflectance Au off-axis parabolic mirror reflectance 8 Incident angle=45 For focusing EUV Photon energy (ev) 30 ev= 41.3 nm 60 ev= 20.7 nm Mirror material: Au (chemically stable and high reflectivity) Reflectance for EUV (30-60 ev) at 45 : 5-20%

9 Transmission Transmission of Al coating 9 For blocking visible Photon energy (ev) 30 ev= 41.3 nm 60 ev= 20.7 nm Coating thickness: 200 nm (enough to block visible light) Transmittance for EUV (30-60 ev): 60-70%

10 Efficiency Total efficiency of optics x 10-3 x10-3 Efficiency: converting rate of EUV into visible through all the optics Photon energy (ev) 30 ev= 41.3 nm 60 ev= 20.7 nm Maximum efficiency: 32 ev (= 38.7 nm) Average efficiency for EUV (30-60 ev): % Efficiency is low but enough for us!

11 EUV radiation power (Ar jet) 11 For showing that such a low efficiency should be okay Supposed all the photons are 30 ev and plasma jet is covered by line of sight of photodiode AXUV diode signal: Ohm 0.6 ma Responsivity of AXUV diode for 30 ev: 0.2 A/W [1] 5 mw Consider Al filter transmission for 30 ev: 60% 8 mw Area of AXUV: 1 mm 2 Therefore, total plasma jet EUV emission : 8 mw 4 R 2 /1 mm 2 = 50 kw [1] IRD homepage.

12 How many photons we will get 12 For showing that such a low efficiency should be okay Total EUV emissivity of 30 ev photon: 50 kw Distance between plasma and detector: 58.4 cm Radiation power at 2.54 mm diameter aperture: 50 kw (1.27 mm) 2 / 4 (58.4 cm) 2 = 50 mw Convert to 30 ev photon flux: s -1 = 10 7 ns -1 Optics efficiency: 10-3 # photons incident on the scintillator: = # of CCD pixels covering the object: =10 4 Set exposure time at 100 ns: photons will come Quantum efficiency of CCD: 35% 35 photons/100 ns/pixel

13 Spatial resolution 13 Phantom image (Laptop) Through EUV optics Diameter of dots: 0.25 (6 mm) Distance between dots: 0.25 (6 mm) Spatial resolution of optics ~0.25 (6 mm)

14 Image distortion check 14 Not real plasma 25.4x25.4 mm grid 12.5x12.5 mm grid ϕ12.5 dots Book Due to parabolic mirror, image is distorted Can be corrected by reconstruction method

15 Field of view 15 Plasma jet 25 cm 5 cm FOV: covers 5-25 cm with < 0.6 cm resolution Almost entire jet structure is Electrodes covered by this system Plasma jet

16 Visible light for optics check 16 Log-scaled images No EUV yet 7 s 9 s 11 s 13 s Jet structure moving away from electrodes Plasma Gas: Ar V p : 5 kv I p : 100 ka t p :20 us Camera Princeton ICCD (single frame camera) Exposure time: 100 ns Camera iris: f8.0 Position: inside the chamber

17 Visible light for optics check 17 Log-scaled images No EUV yet 28 s 28 s 29 s 30 s Rayleigh-Taylor Rayleigh-Taylor Kink instability Kink instability Plasma Gas: Ar V p : 5 kv I p : 100 ka t p :40 us Camera Princeton ICCD (single frame camera) Exposure time: 100 ns Camera iris: f8.0 Position: inside the chamber

18 EUV from actual plasma 18 Log-scaled images Visible leakage 6 s 7 s 8 s 9 s Jet structure moving away from electrodes Plasma Gas: Ar V p : 5 kv I p : 100 ka t p :40 us Camera Princeton ICCD (single frame) Exposure time: 100 ns Iris: f1.4 Position: inside the chamber

19 EUV from actual plasma 19 Log-scaled images 30 s 30 s 30 s 30 s Kink instability can be observed Plasma Gas: Ar V p : 5 kv I p : 100 ka t p :40 us Camera Princeton ICCD (single frame) Exposure time: 100 ns Iris: f1.4 Position: inside the chamber

20 EUV from actual plasma 20 Log-scaled images cathode FOV 6.50 µs 6.75 µs 7.00 µs µs 7.50 µs 7.75 µs 8.00 µs 8.25 µs Plasma Gas: Ar V p : 5 kv I p : 100 ka t p :40 us Camera Imacon ICCD (multi-frame) Exposure time: 100 ns Iris: f1.4 Position: inside the chamber

21 EUV from actual plasma 21 Log-scaled images cathode FOV` 27.8 µs 28.5 µs 29.2 µs µs 30.6 µs 31.3 µs 32.0 µs 32.7 µs Plasma Gas: Ar V p : 5 kv I p : 100 ka t p :40 us Camera Imacon ICCD (multi-frame) Exposure time: 200 ns Iris: f1.4 Position: inside the chamber

22 Comparison with diode data 22 Red line is time of picture Exposure time: 200 ns cathode FOV 27.8 µs 28.5 µs 29.2 µs 29.9 µs Corresponds with EUV diode

23 Comparison with visible image 23 Same plasma cathode EUV FOV Visible 27.8 µs 28.5 µs 29.2 µs 29.9 µs Both EUV and visible were measured at the same time EUV exposure time: 200 ns, Visible exposure time: 20 ns Strong EUV signal was captured when Rayleigh-Taylor happened Difference: Kink structure closer to cathode is bright in EUV

24 Conclusion 24 Successfully measured EUV movie 2 distinct EUV bursts corresponding to EUV diode peaks Merging phase (7-8 µs): rectangular structure bright when 8 flux loops merge Kinking phase (28-32 µs): kink close to the electrode bright when RT happens Problems: Low sensitivity: best performance with 200 ns exposure time Small field of view: got something happens just outside of FOV Difficulty in triggering right before Rayleigh-Taylor happens Image distortion Plans: FOV change to see upper part of kink New triggering circuit is under development Develop reconstruction method for our geometry

25 EUV camera with multilayer mirror 25 Mirror EUV Mirror Visible YAG:Ce Scintillator (Al coated at front side) Multilayer Mirror CCD camera with lens

26 Reflectivity Reflectance of multilayer mirror BB mirror (1) For focusing EUV Centered at 34 ev (36.5 nm) Photon energy (ev) Customized one for ev EUV from NTT-AT (Japan) Reflectance of 34 ev EUV at normal incident: 13%

27 EUV multilayer mirror 27 Principal: constructive interference between periodically placing two different materials (Bragg reflection) Periodicity: condition for constructive interference (Bragg s condition) dsinθ=λ/2 (d: periodicity, θ: angle to mirror surface, λ: wavelength) Difficulty: thickness should be a few tens of nanometers for EUV so extremely sophisticated coating technique is required Use: EUV measurement from solar corona and EUV lithography

28 Efficiency Total efficiency of optics x 10-4 Efficiency: converting rate of EUV into visible through all the optics Photon energy (ev) Maximum efficiency: 34 ev (= 36.5 nm) Average efficiency for EUV (25-50 ev): 0.05 % Conversion efficiency is half of previous one however, there is no aperture

29 Image distortion check 29 Using visible light It not a real plasma Due to the spherical mirror, images are distorted Can be corrected by reconstruction method

30 Field of view 30 Plasma jet 18 cm FOV: covers from electrode to 18 cm Electrodes Most interesting part will be covered by this system Plasma jet

31 Visible light for optics check 31 For checking optics (No EUV yet) using Al spherical mirror 1 us 5 us 9 us 13 us Arches merge Spider legs are merging together Plasma Gas: Ar V p : 5 kv I p : 100 ka t p :40 us Camera Princeton ICCD (single frame) Exposure time: 100 ns Camera iris: f22 Position: outside the chamber

32 Visible light for optics check 32 For checking optics (No EUV yet) using Al spherical mirror 21 us 28 us 29 us 30 us Kink kink instability Plasma Gas: Ar V p : 5 kv I p : 100 ka t p :40 us Camera Princeton ICCD (single frame) Exposure time: 100 ns Camera iris: f22 Position: outside the chamber

33 Conclusions 33 Visible test: spatial resolution looks okay Advantage: 100 times more sensitivity than parabolic mirror optics due to absence of aperture Multilayer mirror has been ordered from NTT-AT, Japan (will be done by end of February) Plans Install multilayer mirror optics and debugging Measure EUV images of actual plasma Study magnetic reconnection