FI FSC Progress meetingJune 1st, 2005

Todd DitmireUniversity of Texas at Austin

Fusion Science Center Research at UT: Hot Fusion Science Center Research at UT: Hot electron and x-ray generation from cone electron and x-ray generation from cone

shaped targets shaped targets

T.E. Cowan, T. Ohkubo, J. Rassuchine, N. Renard-Le Galloudec, Y. Sentoku

Nevada Terawatt Facility, The University of Nevada at Reno

Gilliss Dyer, Byoung-Ick Cho, Stefan Kneip, Daniel Symes and Todd Ditmire

Texas Center for High Intensity Laser Science, The University of Texas at Austin

S.A. Pikuz, Jr. , A. FaenovInstitute for High Energy Density, Russian Academy of ScienceE. Foerster, O. WehrhanIOQ, X-ray Optics Group, Jena University

We propose a change to the FSC logo:

The spot from which x-rays originate is found to

be considerably larger than the laser spot

Our contribution to the FI FSC is in the study of Our contribution to the FI FSC is in the study of hot electron generation in cone targetshot electron generation in cone targets

Proposed approach:

Study x-ray source size and brightness from cone targets in silicon with x-ray converter layers

Opening angle < 90°

Small Kα spot

Tip size< 5m

Laser Spot

~ 20m

Many hot electronsHigh energy

conversion efficiency

Bright Bright KKαα

EmissionEmission

Based on modeling at UNR, we have begun a study of Based on modeling at UNR, we have begun a study of hot electron generation in cone targetshot electron generation in cone targets

Contour of Bz/B0

Y. Sentoku, et al., Laser Light and Hot Electron Micro Focusing using a Conical Target; Phys. Plas, 11, 3083 (2004)

3D structure of the magnetic field

x

y

z

Bpola

rizat

ion

polariz

atio

n

°=×=

×=−

26105.5

/104321

0

2180

θcmncmWI

Magnetic field (Bz)

p-po

lp-

pol

Incident & reflective electric fields

Surface current

Magnetic field

Push electrons out to the interior of the cone

x

y

Simualtions indicate that electrons are accelerated Simualtions indicate that electrons are accelerated along the cone surface toward the tip of the conealong the cone surface toward the tip of the cone

Magnetic field (Bz)

Sheath field (En)

Electron angular distribution in p-space Bz & Ey

Electrons are driven back by the sheath field

Forces are balanced

Electrons are trapped on the surface

x

y

z

Bpola

rizat

ion

polariz

atio

n

°=×=

×=−

26105.5

/104321

0

2180

θcmncmWI

E

Electron flow

x

y

Y. Sentoku, et al., Laser Light and Hot Electron Micro Focusing using a Conical Target; Phys. Plas, 11, 3083 (2004)

Contour of EM Energy Electron energy density

Hot electrons and laser light are guided inside the Hot electrons and laser light are guided inside the conical target and focused at the tipconical target and focused at the tip

Electron temperature @ tip

A portion of the energy in the tip region is higher than that in the same region of a flat target

The conical target appears to increase the number of electrons accelerated forward by more than one order of magnitude

A portion of the energy in the tip region is higher than that in the same region of a flat target

The conical target appears to increase the number of electrons accelerated forward by more than one order of magnitude

Y. Sentoku, et al., Laser Light and Hot Electron Micro Focusing using a Conical Target; Phys. Plas, 11, 3083 (2004)

We use the fact that the KOH etching rates in Si are We use the fact that the KOH etching rates in Si are strongly affected by the crystallographic orientationstrongly affected by the crystallographic orientation

CrystallographicOrientation

Etching Rate (m/min)

(100) 0.797 (0.548)

(110) 1.455 (1.000)

(111) 0.005 (0.004)

Anisotropic KOH Etching Rates vs. Orientation

@ 70ºC, 30% KOH

Surface Orientation <100>

<111>

54.74º

Diamond Structure of Silicon

(111) the Close-Packed Surface

Double FCC Slow Chemical Reaction

Surface Orientation <110>

<111>

Immerse in KOH solution

Si2N3 Deposition

Furnace @ 950ºC

Si2N3 Deposition

Furnace @ 950ºC

Optical Lithography

PR coat, UV, Develop

Optical Lithography

PR coat, UV, Develop

Reactive Ion Etching

Vacuum @ 10-5 torr

Reactive Ion Etching

Vacuum @ 10-5 torr

KOH Etching

~ 7 hours @ 60ºC

KOH Etching

~ 7 hours @ 60ºC

30% KOH solution is used to etch the Si 100 plane 30% KOH solution is used to etch the Si 100 plane at a 1at a 1m/min etching ratem/min etching rate

Metal Layer Coating

Low Z material (Ti, Cu)

Metal Layer Coating

Low Z material (Ti, Cu)

The tip of pyramid goes to the reverse side as close as possible

KOH etching done

Deposit low Z material to generate characteristic Kα photons

We then deposit a metal layer with thickness We then deposit a metal layer with thickness optimized for maximum optimized for maximum KK yieldyield

Si2N3 Deposition

Furnace @ 950ºC

Si2N3 Deposition

Furnace @ 950ºC

Optical Lithography

PR coat, UV, Develop

Optical Lithography

PR coat, UV, Develop

Reactive Ion Etching

Vacuum @ 10-5 torr

Reactive Ion Etching

Vacuum @ 10-5 torr

KOH Etching

~ 7 hours @ 60ºC

KOH Etching

~ 7 hours @ 60ºC

Metal Layer Coating

Low Z material (Ti, Cu)

Metal Layer Coating

Low Z material (Ti, Cu)

Titanium or Copper Vapor

Thickness will match about

the length of KK photon

mean free path ~ 30 m

2mm2mm

Arrays of sharp tipped pyramid shaped guides have been fabricated successfully

500500μμmm

500500μμmm

50μm

We have achieved pyramid cones with tips We have achieved pyramid cones with tips smaller than 1 smaller than 1 mm

We have achieved pyramid cones with tips We have achieved pyramid cones with tips smaller than 1 smaller than 1 mm

100100μμmm

< 5 m

We have conducted a series of experiments on the We have conducted a series of experiments on the 20 TW THOR Ti:sapphire laser at UT20 TW THOR Ti:sapphire laser at UT

Characteristics

Pulse Width ~ 40 fs

Energy ~ 750 mJ

Intensity >1019 W/cm2

Repetition Rate 10Hz

Characteristics

Pulse Width ~ 40 fs

Energy ~ 750 mJ

Intensity >1019 W/cm2

Repetition Rate 10Hz

Regenerative amplifier (25 passes)

1.4J @ 10Hz YAG

0.2J @

10Hz YAG

5W

Mil

len

ia

4-pass amplifier

5-pass amplifier

40fs to

Target

1 nJ 20fs

600 ps

2 mJ

20 mJ

1.2 J2

0 f

s T

i:S

ap

ph

ire

O

sc

illa

tor

Single Grating Stretcher

1.4J @ 10Hz YAG

Vacuum Compressor

40 mJ

150 mJ

A 1D imaging spectrometer was employed to measure the spatial extent of titanium K

magnetsmagnets

Spherically Bent Mica Crystal

• Bragg reflection, 7th order

• Oriented to reflect K1 and K2

• Spatially images horizontal direction

• Demagnification of 1.15

targettarget

Kodak RAR 2492 Film

• Directly exposed by Ti K x-rays

• High (5m) resolution1

• Requires ~30 shots integrated

X-rays in the vicinity of K1 and K2 are focused by an off-axis spherically bent crystal which is fine-tuned to give spatial resolution in 1 dimension

Spa

tial

Spectral

K1

K2

Kodak RAR 2492 Film

• Integrate ~50 shots for good signal

X-rays in the vicinity of Ti K and K pass through a filter window and are diffracted and line-focused by a cylindrically bent crystal. Blue lines indicate path of detected x-rays.

Cylindrically Bent PET Crystal

• Bragg reflection, 2nd order

• Oriented to detect 2.21Å - 2.79Å

• Line focus increases intensity of spectrum

K2 & K1 K

A Von Hamos spectrometer measured yield of Ti Kand K

R= 250 mm7th order

A bent mica spectrometer was used to image the Ti K in one dimension

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

Clear spatial side bands were observed on flat Ti targets

y

K 1 K 2

-800 -600 -400 -200 0 200 400 600 800

Space dimentions, um

Mean K Max K Blue edge

Side peaks from fountain effect?

K1

~ 520 um

Spatial sidebands are seen throughout the K line

30x10-3

25

20

15

10

5

0

-5

-2mm -1 0 1 2Spatial extent in horizontal (polarization) direction [mm]

Side peakSide peak

Plateau

Spatial Extent of K1 11 0From µm Ti Foil Shot at º

18Run 19Run

30x10-3

25

20

15

10

5

0

-5

-2mm -1 0 1 2Spatial extent in horizontal (polarization) direction [mm]

Side peakSide peak

Plateau

Spatial Extent of K1 11 0From µm Ti Foil Shot at º

18Run 19Run

A well-imaged K line from flat targets showed well-defined side peaks

K1 spatial of flats: With best imaging conditions and 35 integrated shots, clear side peaks become visible for 11m Ti foils shot at 0º.

Is there an electron “fountain” effect?

• Electrons leaving front and back surfaces are pulled back by space charge but could also be influenced by an azimuthal magnetic field

• L≈ 125m H~20-30kG

• Electrons responsible for Ti K-shell ionization are largely in the 10-100keV range

?Ti K

b

righ

tne

ss [

ph

oto

ns/

µm

2 /sh

ot @

film

]

Hot electrons

800760720680640600560520480440400 800760720680640600560520480440400

Run 19 (false color)

120

110

100

90

80

70

60

50

40

30

20

10

03.02.82.62.42.22.01.81.61.41.21.00.80.60.40.20.0 mm

330

320

310

300

290

280

270

260

250

240

230

220

210

740720700680660640620600580560540520500

30x10-3

25

20

15

10

5

0

-5

Ti K1

[brightness K

1

/photons µm

2

/ ]shot on film

-2mm -1 0 1 2 ( ) [ ]Spatial extent in horizontal polarization direction mm

,11Flat Ti µm71 º square cones25 µm Ti on back

.0 Direct brightness comparison of cones vs º flats

Cone targets exhibit lower yield but substantially smaller source size when compared to flat Ti targets

Flat Ti target

Si cone with Ti fluor layer target

60x10-3

50

40

30

20

10

0

-2mm -1 0 1 2Spatial extent in horizontal (polarization) direction [mm]

Spatial Extend of K1 From 11µm flat Ti foil and 71º P-wedges and cones with 25µm Ti on Back Surface

11µm Ti foil shot at 0º

p-oriented Si wedge

with 25µm Ti

Si pyramid with 25µm Ti

60x10-3

50

40

30

20

10

0

-2mm -1 0 1 2Spatial extent in horizontal (polarization) direction [mm]

Spatial Extend of K1 From 11µm flat Ti foil and 71º P-wedges and cones with 25µm Ti on Back Surface

11µm Ti foil shot at 0º

p-oriented Si wedge

with 25µm Ti

Si pyramid with 25µm Ti

The spatial extent of Kfrom cone targets showed no side peaks or plateau

Ti K

b

righ

tne

ss [

ph

oto

ns/

µm

2 /sh

ot @

film

]

K1 spatial from flats, p-wedges, and cones (offset): Cones and wedges do not show a plateau or side peaks.

Laser in Laser in

Fountain screening: In cones, electrons which “fountain” >50m from center on the front side will be separated from Ti by enough Si to stop 100keV electrons. This could account for the lack of side peaks & plateau for cone and wedge targets

Far-flung electrons stopped by Si

Si Ti

Ti

We investigated the role of polarization by elongating the pyramid and comparing P and S polarization

Investigation of polarization effectsInvestigation of polarization effects

Various lengths of grooves will be necessaryVarious lengths of grooves will be necessary

Investigation of polarization effectsInvestigation of polarization effects

Various lengths of grooves will be necessaryVarious lengths of grooves will be necessary

Sharp end in polarization plane Wide end in polarization plane

Anisotropic etching of Si is used to produce shaped targets

point line 100µm

A wedge is a “1-D” cone

p s Flat, 0º

“p” and “s” refer to wedge orientation relative to laser polarization

Si TiTi

S-oriented wedges showed higher K yield than p-oriented wedges, but less than flat foils

2.73 2.74 2.75 2.76 2.77Wavelength (Å)

-0.05

0

0.05

0.10

0.15

Pho

tons

/µm

2 /sh

ot o

n fi

lm Flat, 0º

S -wedge

P -wedge

X-ray yield from 25µm foil for flat, s-wedge, and p-wedge Targets

50 shots integrated for each target type

Possible explanations

• Imperfect coupling between wedge and foil

• Mid-temperature (~10keV) electrons stopped by Si bulk material;

• Minimal surface guiding of electrons towards tip

• P polarized produced more hot electrons (>100 keV, which interact less with Ti) than s-polarized wedges

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

B

Photomultipliertube

NaI scintillatorcrystal

Al, Cu and filters

Pb shielding

tube

Pb

THOR laser

f/2.8

Filter Cutoff energy

32 mm Pb 600 keV

48 mm Pb 800 keV

95 mm Cu 1200 keV

NaI detectors

We measured hot electron production with NaI hard x-ray detectors

Hard x-ray detectors show greater hot e- production for P-pol shots than S-pol or flat targets

~50 shots averaged for each target type

Hard X-ray detector1 2 3

0

100

200

300

400S

ign

al @

sco

pe

[nV

s] Flat, 0º

S -wedge

P -wedge

Hard X-ray yield from 25µm foil for flat, s-wedge, and p-wedge Targets

>0.8MeV4.8cm Pb

>1.2MeV9.5cm Cu

>0.6MeV3.2cm Pb

Explanation:•Hard X-ray yields suggest, that p-polarized wedges are able to create more hot electrons than s -polarized wedges.•Hotter electrons in case of p-wedges will lead to lower Kalpha production since they pass the thin Titanium slab unaffected.

Future/current projects

• Further experiments and modeling of side peaks in spatial distribution of Ti

K for flat targets

• Seek laser parameters for which significant electron channeling can occur

(e.g. higher intensity)

• Spectroscopy of Si (better sensitivity from crystals)

• Study higher Z Ka source materials (e.g. gold) for which hot electrons

>100keV are more relevant (crystal spectrometers do not operate at Au Ka

energies)

• Investigate narrowing of cone angle (e.g. “half pyramids”, laser machining,

etc.)

• Analyze and model K1 & 2 as per results from recent experiment at COMET

Near term plan will study sidebands (and lack of) and hot electron temperature from cones