TRIDENT materials shots (Flying Pig 2), August 2002

Contents

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

Objectives

Primary objectives:

1. To test the newly-delivered Kentech intensifier. 100% successful.
2. To perform further trials of polycrystal TXD. ~80% successful.
3. To investigate the use of image plates for recording TXD signals. ~80% successful.
4. To test TXD schemes with the X-ray source much closer than previously to the sample. ~20% successful.

Opportunistic:

1. To perform further VISAR trials and experiments in Si, Sn and Al. Transient X-ray diffraction experiments are relatively slow, and in many cases we needed to wait for the results of one experiment before performing the next. VISAR shots were slipped into natural gaps between TXD shots. ~80% successful.

Schedule and statistics

• Plan refers to TRIDENT schedule ~June 2002.
• We were asked ~30 Jul whether the start could be brought forward by a week, but this was not possible as targets would not have been ready.
• The schedule was only remotely workable because we were able to leave the VISAR equipment in place from the previous series which finished two weeks before.
• In order to help accommodate the short pulse series which followed ours, we were asked to allow short pulse development work in the north target area at the same time, and to avoid firing more than once every 45-60 mins. This parallel working was not too awkward for us, though it was necessary to wear IR protection goggles while setting up between shots, we had to be sparing in requests for Randy Johnson to help with VISAR alignment, and the two target areas occasionally inconvenienced one another by operating at incompatible lighting levels.
• As the following series was in the north target area, we were able to dismantle and tidy our equipment at leisure during the following week.

Shot allocations
	Plan	Actual	Comments
Timing etc	~10	8
Intensifier trial	0	0	combined with timing and data shots
Polycrystal TXD	5-10	8
Image plate trial	~5	3	3 additional shots combined with data shots
Close-in TXD source	~5	1
VISAR	10-20	11
Total	40-50	31

Firing duration and rate
	Plan	Actual	Comments
Experiment time	6.0	3.5	Unanticipated delays and down-time.
Shots/day	7.0	6.6

• Calculations of shot rate ignore timing shots.
• Plan did not take account of the Labor Day holiday (in the hope that we might have fired on one of the Fridays).

Sample materials

Targets

For all TXD shots other than with a close-in backlighter, the re-usable TXD holder developed for the Aug 2001 TXD series was used. This holder incorporated a detachable collimator for polycrystal diffraction experiments. The collimator was an aluminium disc 5/8" in diameter and 1 mm thick, with a 1 mm diameter hole; it was rotated in the holder to alter the spot size on the sample. The backlighter was the standard TRIDENT design of a metal foil in a gold cone; this was press-fitted into the holder and secured with "Tacky Wax" to reduce the risk of losing the cone during handling.

For the TXD shots with a close-in backlighter, the design attempted was to attach the backlighter directly to the sample with a spacer made of CH-based plastic. A spacer disc of plastic was glued by its edges to the sample, and the backlighter foil was glued to the spacer. A shield to protect the X-ray diagnostics from direct shine was constructed from copper sheet (obtained from the Black Hole, Los Alamos) bent to form ~half of a truncated cone. The target assembly was attached to then VISAR clamp using pieces of "Tacky Wax". This concept was inspired by a design used by Dan Kalantar at OMEGA.

Drive conditions

Diagnostics

Transient X-ray diffraction

X-rays from the plasma were incident on the sample material, and diffracted according to the Bragg condition. For single crystal samples, a large part of the sample was illuminated and the Bragg return occured from a circular arc in the surface. For polycrystal samples, the X-ray source was collimated through a hole in a thick Al disc so that a small spot was illuminated on the surface of the sample; only crystallites oriented in the appropriate direction contributed to Bragg reflections.

Diffracted X-rays were recorded on Kentech X-ray streak cameras situated to the north and south sides of the target; the slit of each camera was horizontal. The sweep period of the Kentechs was set to 2 ns throughout, and the backlighter operated 3.69 ns after the drive beam. The image from the Kentech intensifiers was recorded on optical film. Static film packs (DEF film) were mounted in front of the slit in each camera. In some shots, a static image plate was placed vertically in front of one or other of the cameras.

The X-ray detectors were shielded from direct shine from the source by an attenuating layer. This was the standard Au truncated cone around the backlighter foil, except in the close-in trials where a section of a truncated cone of Cu (thicker than the Au) was used.

The X-ray source was monitored with a spectrometer mounted with the "Tubby" film detector.

Line VISAR

Fringe constant

Timing relative to drive beam

sweep speed	delay
(ns)	(ns)
50	2390
20	2430
10	2441
5	2451
camera from laser	449

Fiducial markers

Shot summary

• Target stack listed from drive side.
• Drive energy measured by calorimetry, as usual where the beam enters the target area.
• Some of the uncertain values will be established later e.g. energy from analysis of the photodiode intensity history.
• X-ray streak cameras were run at 2 ns sweep.
• In all dual-beam cases, the backlighter was timed to operate ~2.5 ns after the drive beam.
• LiF: windows 2 mm think in all cases.
• Aluminised Si: Al layer ~1000 Å
• Except for 15007, all TXD shots used the target holder made by Tom Ortiz in 2001.
• For the 1.0 ns pulses, TRIDENT supplied elements 8 to 13.
• For the Si shots with pairs of samples of different thickness, the thicker sample was not coated with Al, and the thinner was oriented with the aluminised side facing the VISAR.

Shot	Target	Backlighter	Line VISAR		Laser drive			Comments
			sweep	delay	duration	loading energy	backlighting energy
					(ns)	(J)	(J)
27 Aug
14988	-	-	-	-	-	-	-	A/B timing
28 Aug
14989	-	-	-	-	-	-	-	A/B timing
14991	-	Au disc	-	-	-	-	-	X-ray streak timing
14992	-	Au disc	-	-	-	-	-	X-ray streak timing
14993	-	Au disc	-	-	-	-	-	X-ray streak timing
14994	-	Au disc	-	-	-	-	-	X-ray streak timing
14995	-	Au disc	-	-	-	-	-	X-ray streak timing
14996	Al, 29 µm / LiF	-	50	2395	2.5	9 (IR: 30)	-	fringes weak and close together but some motion; no drive history
14997	Al, 29 µm / LiF	-	20	2438	2.5	10 (IR: 34)	-	low VISAR signal; nice ramp drive
14998	Si (100), 30 µm / Al	-	20	2438	2.5	? (IR: 9)	-	very low VISAR signal; low intensity ramp and reverberation
29 Aug
15000	Be foil, ~25 µm	Ti	-	-	1.0	-	145	collimator flat on; thick band on S camera, nothing on N
15001	Si (100), 30 µm / Al	-	20	2438	2.5	11 (IR: 34)	-	good ramp drive and VISAR record
15002	Be foil, 55 µm	Ti	-	-	1.0	50	209	collimator twisted; apparently clear double-line structure plus extra fatter line on S camera
3 Sep
15004	Be foil, ~25 µm	Ti	-	-	1.0	-	179	image plate at N side; surface damaged by fringes of A beam
15005	Si (100), 30 µm / Al	-	20	2438	2.5	36 (IR: 88)	-	lumpy drive pulse; shock structures in VISAR data
15007	Si (100), 30 µm / Al	Mn	-	-	1.0	-	206	close-in design; image plate with 25 µm CH2 shield; plate surface still damaged
15008	Si (100), 30 µm / Al	-	20	2438	2.5	21 (IR: 53)	-	good ramp drive and VISAR record
15009	Be foil, 55 µm	Ti	-	-	1.0	49	208	S streak: apparently unshocked and shocked lines
15010	Be foil, 55 µm	-	20	2438	2.5	21 (IR: 53)	-	VISAR signal very low
4 Sep
15012	Be foil, 55 µm	Ti	-	-	1.0	103	198	S streak: apparently unshocked and shocked lines; deviation varied with time
15013	Al foil, 29 µm	-	20	2438	2.5	24	-	VISAR signal very low
15014	Be foil, 55 µm	Ti	-	-	1.0	115	168
15015	Si (100), 30 µm / Al	-	20	2438	2.5	27	-	low VISAR signal, but ramp wave evident
15016	Si (100), 30 µm / Al	Mn	-	-	1.0	-	205	Bragg signal apparent
5 Sep
15018	Si (100), 30 and 59 µm	-	20	2438	2.5	33	-	Clear isentropic waves.
15019	Si (100), 380 µm	Mn	-	-	1.0	-	182	Image plate shielded with 12.5 µm Al and CH2; no PCD record
15020	Si (100), 30 and 59 µm	-	20	2438	2.5	62	-	VISAR signal low, but clear isentropic waves.
15021	Al foil, 29 µm	Ti	-	-	1.0	-	198	Image plate to N, shielded with 12.5 µm Al and CH2.
15022	Si (100), 30 and 59 µm	-	20	2438	2.5	13	-	Clear isentropic waves.
15023	Al foil, 29 µm	Ti	-	-	1.0	-	197	Image plate to N, shielded with 26 µm Al.
15024	Be foil, 55 µm	Ti	-	-	1.0	222	209	Image plate to S, shielded with 26 µm Al.; local damage from foil debris.

Notable events

1. 4 Sep: abort when charging for a final shot; traced to malfunction of laser head chiller; fixed mid-morning 5 Sep.

Experimental observations

1. New Kentech intensifier worked very well.
2. One of the new Kentech photocathodes produced pronounced regions of considerably different signal. It was suggested that the photocathode may have had variations in the thickness of the deposited layers. An older photocathode was installed instead.
3. Diffracted X-rays were apparently detected from Be foils, using a collimated source.
4. A ramped wave 2.5 ns long was suitable for generating isentropic compressions to ~10 GPa in samples several tens of microns thick.
5. A gap between Si crystals did not cause the VISAR record to be obscured by plasma or hot electrons, though the small amount of drive light transported through the VISAR optics saturated the streak camera locally and distorted the fringe pattern for a short period.
6. Image plates were able to detect X-rays from the backlighter and possibly also from diffraction. They were prone to surface damage unless shielded with at least 12.5 µm of Al. Plenty of signal passed through with 26 µm of Al shielding.

Summary of TXD results

Shot	N streak	N time-integrated	S streak	S time-integrated	Source spectrum	Comments
15000	-	n/a	broad, faint line(s)	n/a	Tubby problem
15002	-	?lines at edges	strong line(s)	broad, curved line	Tubby problem
15004	-	-	?lines/mess against dark background	broad, curved line	n/a
15007	-	-	complicated structures	overexposed	Tubby problem
15009	-	-	lines: 1 strong + 1 weak	weak, broad, curved line	OK Ti
15012	-	?line at edge	broad bending lines: 1 strong + 1 medium	medium, broad, curved line	OK Ti
15014	-	?lines at edges	broad bending lines: 1 strong + 1 medium	~smooth background	OK Ti
15016	-	-	very broad lines + structured noise	weak line + structured blob	no signal at all
15021	n/a	n/a	broad line + noise	n/a	OK Ti
15023	-	-	very broad lines + structured noise	-	no signal at all
15024	-	-	broad bending lines: 1 strong + 1 weak	high background + light spot	OK Ti

Evaluation

1. "To test the newly-delivered Kentech intensifier."

   The new intensifier worked without problems on the south Kentech, and seemed to be a considerable improvement over the old intensifiers.

2. "To perform further trials of polycrystal TXD."

   Using a collimated beam from a Ti backlighter, diffraction signals were apparently detected on the static film, X-ray streak record and possibly the image plates, from Be foils. This is a confirmation of the results obtained previously. Also apparently, the polycrystal TXD signal was intense enough to be observed when the collimated beam was narrow enough to allow the doublet backlighter wavelengths to be distinguished. The experiments were repeated with a shock wave in the sample; a pair of lines was observed, with variations of deviation in time and with different drive intensities. The records require further analysis to allow a clearer interpretation, because there were some unexpected systematic features about them.

   Experiments were also performed with an Al foil sample and a Ti backlighter; analysis of the image plate records is to determine whether any signal was obtained.

3. "To investigate the use of image plates for recording TXD signals."

   Sample image plates were obtained, used in TXD shots, and scanned to reveal varying degrees of exposure and possibly diffraction lines. Schemes were investigated to shield the plates from damage, and also to attenuate the X-ray intensity. Where closest to the experiment (~5 to 10 mm), the image plates were always saturated. We did not optimise X-ray filtering to best bring out the TXD lines. (Apparently, image plates have an effective dynamic range of 10⁵ to 10⁶, and it is often possible to extract quantitative data from saturated images by repeated scanning - up to 20 times. We scanned some of the plates three times; analysis will be reported separately.)

   We had hoped to perform trials with the image plates cut into pieces and folded or bent so that a large solid angle could be monitored around a TXD experiment. We spent a significant amount of time developing methods to shield the surface of the image plate from stray but intense laser light from the "A" or "B" beams, deposited material from the plasma, and fragments from the sample or backlighter assembly. We were offered the use of two scanners: one in DX-div which was capable of scanning oddly-shaped pieces but required a Q-cleared person for access, and one in P-22 which we were able to borrow for the last couple of days of the experiments but which was restricted to letter-sized image plates with no creases. For these reasons, the only image plate data obtained were from complete letter-sized plates; there seems to be no reason why target-enveloping detectors based on image plates could not be developed in a fairly short time. The work required would be the optimisation of X-ray filters and the design of a suitable holder, including a scheme to align the target.

4. "To test TXD schemes with the X-ray source much closer than previously to the sample."

   A range of designs was considered, two prototype targets were assembled, and one was fired. It is not yet clear whether diffraction was obtained, but useful experience was obtained for future experiments. No further experiments were performed because there was insufficient time for improved targets to be assembled. Furthermore, the amount of shot time available was less than originally requested; something had to be cut back and it was this.

Secondary objectives:

1. "To perform further VISAR trials and experiments in Si, Sn and Al."

   Further line VISAR records were obtained in Si crystals and Al foils, with sweep periods down to 20 ns. The clarity of the data was relatively poor for the Al foils, and no usable fringes were obtained from the relatively diffuse Sn foils - so none of these were fired. It is likely that fringes would have been obtained more easily in a dedicated series of experiments with time to adjust the VISAR alignment more carefully, but this was not appropriate for experiments slipped in between TXD shots where convenient.

   The highlight of the VISAR experiments was the opportunity to evaluate different loading paths. Gentle ramp waves were produced very successfully by the TRIDENT staff. At 2.5 ns long, these proved eminently suitable to generate quasi-isentropic compression histories in Si and Al samples up to over 60 µm thick at pressures up to at least ~10 GPa, with a small fraction of the laser energy available. Example experiments were performed with two thicknesses of Si fired side-by-side, which should provide usable isentropic compression data.

   There seems to be no reason why TRIDENT could not be used to simulate a Taylor wave or the release behind a von Neumann spike, as I have suggested previously. This was not attempted in the present series of experiments because of a lack of time: shock and release experiments of this nature need to be planned carefully to avoid undesired spall.

Acknowledgements

Distribution