TRIDENT materials shots (Pink Elephant), July-August 2002

Contents

• Objectives
• Schedule and statistics
• Sample materials
• Targets
• Drive conditions
• Diagnostics
• Shot summary
• Notable events
• Experimental observations
• Evaluation
• Acknowledgements

Objectives

1. To shock and recover samples of two compositions of NiTi, with VISAR recording to determine loading conditions, and covering a wider range of pressures than in the experiments performed in summer 2000. 90-100% successful.
2. To perform scoping experiments on pairs of NiAl crystals of different orientation, to see whether differences in wave speed can be resolved in laser experiments. ~60% successful.
3. To perform scoping experiments on dynamic ellipsometry. 100% successful.

Opportunistic:

1. "To shock and recover samples of Si crystal." Si was used as a readily-available shiny material for trial shots, e.g. to test alignment and timing. We took advantage of this to recover samples in such a way that there could be no question about which sample came from which shot, by ensuring that they remained in the target holder. ~80% successful.
2. "To shock and recover LLNL shear-banding targets." Bruce Remington and Jim McNaney of LLNL had asked if we might fire scoping experiments that fell off the end of the OMEGA schedule, as a step towards a closer relationship on materials at NIF. ~80% successful.
3. "To obtain VISAR profiles of laser-induced shocks in Sn." Sn was used as a trial material for ellipsometry tests (as was Si); however, we have no previous experience with directly driven Sn. Sn exhibits strength, polymorphism and melt, and is interesting in its own right for its multi-wave shock structures. 100% successful.

Schedule and statistics

Shot allocations
	Plan	Actual	Comments
Timing etc	~5	2	Actual excludes some Si recovery
NiTi LDRD	20-30	29	Used all but 2 available.
NiAl LDRD	6-10	7	Used all provided.
Ellipsometry	10-20	17	Includes Sn and Si calibration shots.
Si recovery	0	6	Excludes calibration shots for ellipsometry.
Shear-banding	0	2
Total	40-50	63

Firing duration and rate
	Plan	Actual	Comments
Experiment time	6.5	5.5	Unanticipated delays and down-time.
Shots/day	7.0	11.5	Higher shot rate at low energy.

Sample materials

Targets

The NiAl samples were prepared by polishing after gluing to a window. The only windows available in the necessary time scale were too large to be held in the clamp-type holder. A holder was constructed based on a lens holder, with three radially-sliding bars to grip the sample. It was found necessary to use a little "tacky wax" to secure the sample to the holder and eliminate any propensity for the surfaces to slip.

Drive conditions

The Fresnel zone plate was used to smooth the beam spatially. With the unsmoothed beam at best focus, inserting the phase plate gave a focal spot nominally 4 mm in diameter. However, the undiffracted light produced a hotspot in the centre of the disc, so the usual procedure was followed of defocusing the drive beam by 3/4". The resulting spot was ~5 mm in diameter, and was estimated to contain 85% of the nominal drive energy reported for each shot.

In the VISAR `shots the target was close to normal to the drive beam. In the ellipsometry shots, the target was rotated significantly, so the intensity of the drive beam was lower.

Diagnostics

Line VISAR

Fringe constant

Timing relative to drive beam

sweep speed	start	from 14910	from 14922	from 14948
(ns)
50	2962	2340	2190	2390
20	3002		2230	2430
10	3013		2241	2441
5	3023		2251	2451
camera from laser	-	499	649	449

Fiducial markers

Using the fiducial markers, the camera scale and nominal offset were as follows:

sweep speed	temporal scale	location of drive pulse		first fiducial pulse
(ns)	(ps/pixel)	(pixels from start of record)	(ns from start of record)	(pixels from start of record)	(ns from start of record)
50	41.4±0.5	502	20.76±0.25	400	16.54±0.20
20	18.1±0.1	496	8.96±0.10	266	4.81±0.05

(The location of the drive pulse is with the delay set to the value for the drive to occur nominally at the centre of the record.)

For a given value of the delay d, reference delay d_r and drive pulse time p on the reference record, times in the record are expressed with respect to the drive pulse by setting the the record to start at d - d_r - p. Alternatively, if the Nth fiducial pulse occurs at a time f in the record compared with a time f_r in the reference record, times in the record are expressed with respect to the drive pulse by setting the the record to start at f_r - (p + f) + (N - 1) t_p, where t_p is the interval between fiducial pulses.

Spatial scale

Ellipsometry

The VISAR probe laser was used as the source, with the omission of the cylindrical lens used to illuminate a line on the sample and the addition of a depolariser at the same point in the system. The angles of incidence and reflection were 32±0.5^o to the sample normal. The target was rotated to send specular reflections from the probe laser towards the detecting optics; thus as designed it was not possible to use VISAR and ellipsometry with specular reflection on the same experiment in this initial scoping study. Light reflected from the sample was collected with a 200 mm f/5 achromat positioned with the sample at its focal length. A polarising cube was used to split the two components, and these were recorded using a second Hamamatsu streak camera, with an S-1 photocathode.

The two channels were focused onto the camera slit through the same lens (200 mm f/5 achromat); the resulting beams diverged too far to be captured by the camera so a piece of tape was stuck over the slit to act as a diffuser.

Shot summary

• Target stack listed from drive side.
• Drive energy measured by calorimetry, as usual where the beam enters the target area and also for later low-energy shots with a more precise Laser Precision detector looking at an uncoated optic nearer the target chamber. Value listed is the raw LP reading; empirical relation is to scale the LP reading by 20481.
• Some of the uncertain values will be established later e.g. energy from analysis of the photodiode intensity history.

Shot	Target	Line VISAR		Ellipsometer		Pulse	Energy		Comments
		sweep	delay	sweep	delay	(ns)	standard	LP: raw
		(ns)	(ns)	(ns)	(ns)		(J)	(mJ)
30 Jul
14906	Al/Si (100), 30 µm	50	2982	-	-	2.5?	20±5	-	omitted camera filter: swamped by stray drive light
14907	Al/Si (100), 30 µm	50	2982	-	-	2.5?	20±5	-	very fast jumpoff
31 Jul
14909	NiTi 3D, 90 µm (#1)	50	2982	-	-	2.5	20±1	-
14910	NiTi 3D, 198 µm (#10)	50	2380	-	-	2.5	18±1	-	timing offset changes
14911	NiTi 3D, 400 µm (#13)	50	2430	-	-	2.5	21±1	-
14912	NiTi 3D, 99 µm (#2)	50	2355	-	-	1.0	21±1	-	no VISAR
14913	NiTi 3D, 101 µm (#4)	50	2355	-	-	1.0	23±1	-
14914	NiTi 3D, 210 µm (#12)	50	2377	-	-	1.0	27±1	-
14915	NiTi 3D, 408 µm (#14)	50	2420	-	-	1.0	33±1	-
14916	NiTi 3D, 95 µm (#3)	50	2360	-	-	1.0	6±1	-
14917	NiTi 3D, 182 µm (#11)	50	2380	-	-	1.0	7±3	-
14918	NiTi 3D, 397 µm (#15)	50	2420	-	-	1.0	8±3	-
14919	Si (100), 380 µm	50	2400	-	-	1.0	28±1	-	sample recovered
1 Aug
14921	Si (100), 380 µm	50	2400	-	-	1.0	28±1	-	poor probe level; sample recovered
14922	Si (100), 380 µm	50	2250	-	-	1.0	27?	-	new timing offset; poor probe level; sample recovered
14923	Si (100), 380 µm	50	2250	-	-	1.0	15±1	0.820	sample not recovered
14924	NiTi 3D, 91 µm (#5)	50	2210	-	-	1.0	15±1	0.761
14925	NiTi 3D, 103 µm (#9)	50	2200	-	-	1.0	~4	0.0512
14926	NiTi 3D, 89 µm (#7)	50	2200	-	-	0.4	~3	0.1308
14927	NiTi 3D, 87 µm (#8)	50	2200	-	-	0.4	-	0.0418
14928	NiTi 3D, 105 µm (#6)	50	2200	-	-	2.5	-	-	energy to be deduced from photodiode record, cf previous shot
14929	Si (100), 380 µm	50	2260	-	-	2.5	13	0.651	missed shock; recovered sample
14930	Si (100), 380 µm	50	2240	-	-	2.5	17	0.825	sample in small pieces
14931	Al/Si (100), 30 µm/LiF, 2 mm	20	2230	-	-	2.5?	-	0.621	recovered sample
5 Aug
14934	Al/Si (100), 30 µm/LiF, 2 mm	20	2230	-	-	1.0?	27	1.360	recovered sample
14935	NiTi 5D, 105 µm (#1)	50	2200	-	-	1.0	28	1.428	probe intensity low but usable
14936	NiTi 5D, 194 µm (#6)	50	2240	-	-	1.0	29	1.354
14937	NiTi 5D, 408 µm (#11)	50	2280	-	-	1.0	24	1.298
14938	NiTi 5D, 101 µm (#3)	50	2210	-	-	1.0	~17	0.565
14939	NiTi 5D, 200 µm (#7)	50	2240	-	-	1.0	13	0.614
14940	NiTi 5D, 409 µm (#12)	50	2280	-	-	1.0	8	0.549
14941	NiTi 5D, 87 µm (#2)	50	2210	-	-	1.0	8	0.437
14942	NiTi 5D, 196 µm (#8)	50	2240	-	-	1.0	6	0.393
6 Aug
14944	NiTi 5D, 399 µm (#13)	50	2270	-	-	1.0	10	0.449	no probe signal
14945	NiTi 5D, 99 µm (#4)	50	2210	-	-	1.0	-	0.0557
14946	NiTi 5D, 185 µm (#9)	50	2230	-	-	1.0	-	0.0537	probe signal weak but usable
14947	NiTi 5D, 402 µm (#14)	50	2270	-	-	1.0	-	0.0427	probe multi-moded; signal weak but may be usable
14948	NiTi 5D, 394 µm (#15)	50	2470	-	-	2.5?	-?	0.566?	new VISAR timings
14949	NiTi 5D, 200 µm (#10)	50	2430	-	-	0.4	5	0.216
14950	NiAl (110)+(111), ~120 µm / BK7 (P2)	50	2410	-	-	1.0	33	1.509
14951	NiAl (110)+(111), 149 µm / BK7 (P3)	50	2420	-	-	1.0	25	1.198	probe laser multi-moded but some fringe motion visible
14952	NiAl (110)+(111), 122 µm / BK7 (P1)	50	2415	-	-	1.0	32	1.293	little fringe motion (glue?) but evidence of side-to-side variation
14953	NiAl (110)+(111), 84 µm / BK7 (P4)	50	2415	-	-	1.0	29	1.365	forgot filter: fringes obscured by scattered green
14954	NiAl (100)+(111), 185 µm / BK7 (P9)	50	2410	-	-	1.0	27	1.308	fringes moved one side only
7 Aug
14956	LLNL Al/groove/wax (B)	-	-	-	-	0.8	~180	-	no calorimeter reading
14958	LLNL Al/groove/wax (A)	-	-	-	-	2.5	158	-	photodiode scale same as 14958 except extra OD 0.3: use to deduce 14956's energy
14959	Sn, 25 µm	20	2435	-	-	1.0	27	1.376	Focus preshot for scale: ~5 mm dia holder. Probe multimoded; sample destroyed.
14960	Sn, 25 µm	20	2435	-	-	2.5	25	1.205
14961	Sn, 25 µm	20	2435	-	-	2.5	~2?	-
14962	Sn, 25 µm	20	2438	-	-	2.5	13	0.497
14963	Sn, 25 µm	20	2438	-	-	2.5	-	0.112
14964	Sn, 25 µm	20	2438	-	-	2.5	19	0.848
14965	Sn, 25 µm/LiF	20	2438	-	-	2.5	13	0.425
8 Aug
14967	Al/Si (100), 30 µm/LiF	-	-	50	2972	2.5	33	1.577	omitted camera filter: swamped by stray drive light
14968	Al/Si (100), 30 µm/LiF	-	-	50	2972	2.5	54	2.55	saw effect
14969	Al/Si (100), 30 µm/LiF	-	-	50	2972	2.5	88	4.27	saw effect
14970	Sn, 25 µm/LiF	20	2438	50	2972	2.5	9	0.428	no effect (?); diffuse VISAR signal
14971	Sn, 25 µm/LiF	20	2438	50	2972	2.5	40	2.279	no effect (?)
14972	Sn, 25 µm/LiF	20	2438	50	2972	1.0	54	2.77	saw effect
14973	Sn, 25 µm	20	2438	50	2972	1.0	55	2.90	VISAR washed out; ellipsometer not useful.
14974	NiAl (100)+(111), 203 µm / BK7 (P10)	50	2424	-	-	2.5	8	0.625	spatial variation in fringe motion
14975	NiAl (110)+(111), 145 µm / BK7 (P7)	50	2414	-	-	2.5	18	0.767	spatial variation in fringe motion
14976	Sn, 25 µm/LiF	20	2438	-	-	2.5	10?	0.829	Ramp wave. Forgot to replace filter: fringes washed out by stray drive light.

Notable events

1. 1 Aug: visit by Linda Brzuchalski (and friend), summer student in C-div (working on laser flyers and explosive initiation).
2. 5 Aug: water leak in capacitor bay, laser down for a couple of hours.
3. 6 Aug: after steady decrease in signal from probe laser, Randy changed the flash-lamp which made a big improvement.
4. 6 Aug: some difficulty lining up bicrystals and obtaining VISAR through glue layer.
5. 7 Aug: 3 aborts on charging; turned out to be blown fuse; ~hour down time.
6. 7 Aug: visit by Martin Braithwaite, William Penney Fellow at RMCS Shrivenham, UK (specialist in properties of energetic materials). His first visit to a laser materials facility; expressed surprise that anyone should want to use anything else to measure EOS, strength and high-pressure reaction properties of the constituents of explosives.
7. 8 Aug: visit and seminar by Martin Greenaway, PDRA at Cavendish Lab, Cambridge, UK (working on laser flyers and explosive initiation) and postdoc candidate for C-ADI.

Experimental observations

1. It was found possible to alternate flash lamps on shots of ~45 J or less (green), allowing the firing rate to be doubled compared with high energy shots.
2. No loss of signal was observed from plasma or hot electrons, despite energies up to ~30J and significant gaps between target and holder.
3. The target holder based on a lens holder worked very well, if "tacky wax" was included.
4. Gap between bicrystal samples did not cause problems in VISAR record i.e. no interference from drive beam.
5. The Sn samples produced a diffuse reflection. Weak VISAR fringes were recorded back down the direction of incidence when ~25^o from the surface normal.
6. Ellipsometry of a point seemed to work fine.
7. Low-energy calorimetry system worked fine.

Evaluation

1. "To shock and recover samples of two compositions of NiTi, with VISAR recording to determine loading conditions, and covering a wider range of pressures than in the experiments performed in summer 2000."

   15 samples of alloy 3D and 14 samples of alloy 5D were shocked and recovered; intensity not exactly as specified for each but within accuracy found possible in practice. One VISAR record lost; pressure pulse captured on all other shots (despite long transit time for the thick samples). One extra sample of each composition was not fired, as the shots already obtained were deemed to cover the pressure range planned, and time was needed to compensate for laser outage.

2. "To perform scoping experiments on pairs of NiAl crystals of different orientation, to see whether differences in wave speed can be resolved in laser experiments."

   Problems were experienced with glue joints between the samples and the BK7 release window, and the alignment of the interface may have been poor on early shots. Spatial variations were seen clearly in later shots, with some evidence in early shots.

3. To perform scoping experiments on dynamic ellipsometry.

   Time-varying differences in the absolute and relative intensity from each channel was observed clearly in the experiments on Si and in the higher-pressure experiments on Sn. Pressure-dependent birefringence of LiF may contribute to the observed effects. Complementary experiments to calibrate the response of Sn to laser-induced loading produced VISAR records of reasonable quality, though they did not cover the higher intensity range where strong ellipsometry effects were observed.

Secondary objectives:

1. "To shock and recover samples of Si crystal." Samples were recovered unambiguously (i.e. with definite knowledge of the shot each sample came from). Bare samples were recovered from the target holder (clamp with 5 mm hole) at 380 µm thick. Samples 20 µm thick were recovered from the target holder clamp when a LiF release window 2 mm thick was included. The samples exhibited small length-scale surface damage at the drive side, and many were fractured, but the fragments were typically ~1 mm in size or larger. The Si experiments were limited in number, and may not have explored the full pressure range desired.
2. "To shock and recover LLNL shear-banding targets." The samples were fired when the ellipsometry system was being aligned, and so caused essentially no delay to the rest of the program. The samples were mounted quite efficiently with the target holder based on a lens holder. The energy in the drive beam was considerably less than desired, so the loading history may not have reached the intended pressures. However, the crater produced by the drive laser appeared to contain a pinhole on both shots, suggesting that at its centre the ablated layer reached through to the machined groove. In both cases, the Al-6061 assembly was pushed several hundred microns down the Cu sheath.
3. "To obtain VISAR profiles of laser-induced shocks in Sn." Compared with the other samples used, the Sn had a dull surface and the VISAR had to be aligned more carefully to obtain fringes. Although the intensity was correspondingly lower, it was possible to obtain reasonable signals with a 20 ns sweep period. Velocity histories were obtained on bare samples and also with a LiF release window. Samples 25 and 50 µm thick were ordered, but instead of the latter, discs 500 µm thick were delivered. All experiments were performed on the thinner discs to avoid decay of the shock wave. The VISAR records captured the shock wave and the following release wave. Multi-wave structures were evident in the rising part of the wave.

Acknowledgements

Distribution