The influence of an intense laser beam interaction with a preformed plasma on the

characteristics of emitted ion streams

L. Láska, K. Jungwirth, J. Krása, E. Krouský, M. Pfeifer, K. Rohlena, J. Skála, J. Ullschmied, A. Velyhan

Institute of Physics, A.S.C.R., Na Slovance 2, 182 21 Prague 8, Czech Republic

P. Kubeš Czech Technical University, Technická 2, 16627 Prague 6, Czech Republic

J. Badziak, A, Kasperczuk, P. Parys, T, Pisarczyk, M. Rosinski, L. Ryć, J. Wołowski

Institute of Plasma Physics and Laser Microfusion, Hery St. 23, 00-908 Warsaw, Poland

S. Gammino, L. Torrisi, INFN - Laboratori Nazionali del Sud, Via S. Sofia 44, 95123 Catania, Italy

Outline

1. Introduction (LIS)

2. Experimental arrangement

3. Laser pulse properties

4. Laser interaction with a preformed plasma

5. Characteristics of emitted ion streams 6. Summary

Laser Ion Sources (LIS)Depending on the irradiated target material and on parameters ofthe laser used, laser plasma emits ions of any atomic mass (anymaterial can be evaporated and ionized), with multiply chargestates (even above 50+) and with energies from hundreds of eV toabout hundreds of MeV (without external acceleration).

LIS is the most efficient ion source among any others (MEVVA,ECR), ion current density j ~ 10-100 mA/cm2

at a distance of 1 m(j > 10-100 kA/cm2 in the vicinity of the target).

Different demands on the kind and amount of ions and on theirenergy (a common demand - high ion current densities) influencethe choice of the used laser (its wavelength, pulse length, laser pulseenergy, repetition frequency).

LIS can be utilizedo - a highly charged heavy ion source for preinjector of large TW hadron accelerators (e.g. LHC -Geneva, TWAC – Moscow)o - low charge state ions with the energy of only hundreds of eV are used in the hybrid laser-ECR ion source (ECR ion source Coupled to a Laser Ion Source for charge State Enhancement).o - for medical purposes (C6+ - direct injection of ions into RFQ accelerator)o - for various technological purposes, like solid surfaces modification (ion implantation with, or without, electrostatic acceleration), very large scale integration circuit fabrication, etc.o – for laser driven inertial fusion

Experimental arrangement

o - laser (different wavelength, laser pulse energy and pulse length)

o - vacuum target chamber equipped with a target holder (movable in x, y, z and ϕ directions)

o - target (foils - slabs- solid materials)

o - irradiation: normal (with regard to the target surface), obliqe

o - focusation: lenses (aspherical) , mirrors (parabolic, ellipsoidal)

o - extraction (back side, rear side) and acceleration system (transformation into ion beam)

High-power photodissociation iodine laser PALSat the PALS Research Center in Prague(λ = 1.315 µm, EL ≤ 1 kJ, τ ~ 300 ps, IL ≤ 8 x 1016 W/cm2, possible conversion to 2ω and 3ω)

Factors, which influence the laser ion production

o - laser wavelength laser power density (laser energy, laser pulse length and shape, diameter of the focal spot) o - angle with respect to the target normal, in which target is irradiated or ions are extracted (recorded) o – interaction with pre-formed plasma o - optimum focus setting (the threshold laser intensity IL for the appearance of the non-linear processes in the pre-formed plasma plume was found ~1x1014 W/cm2) and multiple shots to the same target place

intensity

diameter

a.u.

-2000 -1500 -1000 -500 0 500 1000 1500 20000

10

20

30

40

50

60

z max

FP [µm]

3w30a 3w30b 1w30 3w00 1w00

-2000 -1000 0 1000 20001E13

1E14

1E15

1E16

225J 78 J 25 J 10 J

I L [W

/cm

2 ]

FP [µm]

-2000 -1500 -1000 -500 0 500 1000 1500 20000

10

20

30

40

50 Ta

1ω, 215J 3ω, 220J 3ω, 80J 3ω, 80J 3ω, 80J

z max

FP [µm]

0,01 0,1 1 100

10

20

30

40

50

60

z max

IL [x1016 W/cm2]

zm31P zm32N zm32P zm33N zm34N

Ta05, 1ω, T=00,Ei/z = 30 keV

-1500 -1000 -500 0 500 1000 1500

0

1

Ta, 3ω, 00

150 J

H S

X-r

ay [

a.u.

]

FP [µm]

-1500 -1000 -500 0 500 1000 15001E-3

0,01

0,1

1

10

100

Ta, 3ω, 0o, 150J

2 3 4 5 6 7 8 9 10 11 12 13 14

E i [M

eV]

FP [µm]

-2000 -1500 -1000 -500 0 500 10000

10

20

30

40

50

60

j1 j2 j3 j4 j5 j6 j7 j8 j9 j10 j11 j12 j13

j max

[m

A/c

m2 ]

FP [µm]

-1500 -1000 -500 0 500 1000 15000

5

10

15

20

25

30

Ta, 3ω, 0o, 150J

8 9 10 11 12 13 14

j max

[m

A/c

m2 ]

FP [µm]

-1500 -1000 -500 0 500 1000 15000

5

10

15

20

Ta, 3ω, 0o, 150J

2 3 4 5 6 7

j max

[m

A/c

m2 ]

FP [µm]

-1500 -1000 -500 0 500 1000 15000

10

20

30

Ta, 3ω, 0o, 150JEi/z = 30keV

11+ 12+ 13+ 14+ 15+

Am

plitu

de [

mV

]

FP [µm]

-1000 -500 0 500 10000

500

1000

1500

2000

2500

3000

3500

inte

nsity

[a.

u.]

FP [µm]

X1-10keV X2-20keV

X-rays

-1500 -1000 -500 0 500 1000 15000

10

20

30

Ta, 3ω, 0o, 150JEi/z = 30keV

26+ 27+ 28+ 29+ 30+

Ampl

itude

[a.

u.]

FP [µm]

-1500 -1000 -500 0 500 1000 15000

10

20

30

40

50

Ta, 3ω, 0o, 150JEi/z = 30keV

41+ 42+ 43+ 44+ 45+

Am

plitu

de [

a.u.

]

FP [µm]

-2000 -1000 0 1000 2000

400

600

800

1000

1200

Focus Position (µm)

Ta, 1ω, φ = 0°, EL= 371 JSp

ot D

iam

eter

(µm

)

o The higher the laser power density IL, the higher plasma temperature can beachieved (from tens of eV to tens of keV) and thus us charge states and energies ofthe ions produced. Demands for the highest intensities necessitate high laserenergy, or shortening of the laser pulses.

o Experiments on generation of highly charged ions are, in fact, currentlyaccompanied by the laser interaction with a pre-formed plasma. It can beproduced using separate laser pre-pulse preceding the main pulse, however, themain pulse itself also significantly participate in the pre-plasma formation: due tothe length of pulse (self-created plasma from the front part of the long pulses >100ps), or due to a long lasting background of the short pulses < 1 ps (low contrastratio).

o The interaction of the intense laser radiation above some threshold witha pre-plasma of defined properties (plasma density and temperature,expanding plasma length) may significantly increase the charge state andenergy of the produced ions due to the appearance of various non-lineareffects, including self-focusing (ponderomotive, relativistic, magnetic),which can reduce the irradiation channel diameter down to about one λ(laser intensity may increase up to several orders of magnitude).

Summary

o Laser plasma emits ions with a broad range of charge states and energies, as well as soft and hard X-rays. Emitted ions (X-rays) can be used for a large spectrum of applications

o In addition to the thermal, fast and slow ion groups, generally accepted, the existence of the fourth (superfast) group was proved, which is connected with the presence of non-linear processes in a pre-formed plasma.

o A longitudinal structure of the self-focused laser beam was deduced from the performed experments

Corpuscular diagnostics

o - ions are recorded in the far expansion zone (~ 2 m) o - based on the time-of-flight method: Faraday cups (FC) or ion collectors (IC), or combined with the imposed electric and/or magnetic fields: cylindrical electrostatic ion analyzers (EIA), Thomson parabola spectrometer (TPS) Ion Collectors o - charge-integrated time-resolved signal, from which the mean (peak), as well as the maximum, ion velocities (energies) and ion currents can be evaluated. Electrostatic Ion Analyzer o - identification of the ion species produced, (mass-to-charge ratios, energies and abundance) Thomson Parabola Spectrometer o - capable to distinguish different kind of ions (with the same m/q ratio), as well as to record their velocity (energy) distribution in a single laser shot

0 1 2 3 4 50,0

0,5

1,0

1,5

2,0

Ta

ampl

itude

[V

]

time [µs]-10 0 10 20 30 40 50

0,0

0,5

1,0

1,5

2,0

Ta

ampl

itude

[V]

time [µs]

IC signal IEA spectrum (Au plasma - 1ω, EL = 27 J)

0 1 2 3 4 5 6

0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

1.08 MeV

Au

530 keV

1.75 MeV

IC a

mpl

itude

[V]

time [µs]2,0 2,2 2,4 2,6 2,8 3,0 3,2

-0,15

-0,10

-0,05

0,00

Au50+

Au41+Au

IEA

am

plitu

de [V

]

time [µs]

-1500 -1000 -500 0 500 1000 15000,01

0,1

1

10

Ta, 3ω, 0o, 150J

2 3 4 5 6 7 8 9 10 11 12 13 14

v p [x

108 cm

/s]

FP [µm]

-1500 -1000 -500 0 500 1000 15000

10

20

30

40

50

60

Ta, 3ω, 00

150J,30keV 150J,50keV 140J,20keV

z max

FP [µm]

1E8 1E10 1E12 1E14 1E16 1E18 1E20 1E220

10

20

30

40

50

1ω, 215J 3ω, 220J 3ω, 80J 3ω, 80J 1ω, SF 3ω, SF Nd:YAG A

Ta

z max

IL [W/cm2]

-1500 -1000 -500 0 500 1000 15000

10

20

30

Ta, 3ω, 0o, 150JEi/z = 30keV

31+ 32+ 33+ 34+ 35+

Am

plitu

de [

a.u.

]

FP [µm]

-1500 -1000 -500 0 500 1000 15000

10

20

30

40

50

Ta, 3ω, 0o, 150JEi/z = 30keV

36+ 37+ 38+ 39+ 40+

Am

plitu

de [

a.u.

]

FP [µm]

-1500 -1000 -500 0 500 1000 15000

10

20

30

40

50

60

Ta, 3ω, 00, 150 J Ei/z=30keV

40+ 20+ 10+

I i [a

.u.]

FP [µm]

-1500 -1000 -500 0 500 1000 15000

10

20

30

Ta, 3ω, 0o, 150J

2 10 12 14

j peak

[m

A/c

m2 ]

FP [µm]

-2000 -1500 -1000 -500 0 500 1000 15000

100

200

300

400

500

600

700

800

900 crater diameterTa, 1ω

215J,N,30deg 215J,P,30deg 500J,N,0deg 500J,P,0deg FS meas LA

D [µ

m]

FP [µm]

S.J. Gitomer at al., Phys. Fluids 29 (1986) 2679

X-rays [G.J. Tallents et al., Aust.J.Phys. 39 (1986) 253]

Laser energy absorption - inverse Bremsstrahlung (thermal electrons) - resonance absorption, parametric instabillities (fast elecrons)

Depending on the laser wavelength λ (on the absorption coefficient α,reflectance R):

~103 W/cm2 - evaporation~107 W/cm2 - the vapour becomes partially ionized~1010 W/cm2 - plasma~1015 W/cm2 - hot and dense plasma

A part of the absorbed laser energy is transferred in directed particlekinetic energy, a part in X-ray radiation. Laser radiation is reflected in thehot dense plasma at the place where the critical electron density nc makesthe plasma frequency ωp equal to the laser frequency: nc = (ωp

2εom) / e2 ~ 1/ λ2

The laser plasma emits various neutral particles (atoms, molecules,clusters) as well as electrons and ions with a varying charge-to-mass ratioand also differing in the energy distribution. TH ~ Iλ2