Overdense gas jets for ion acceleration studies

Transcription

1 Overdense gas jets for ion acceleration studies Nicholas Dover John Adams Institute at Imperial College London 2 nd Ion Instrumentation Workshop, École Polytechnique 7-8 th June

2 fx(1:6,1) = 0, 0, 1.8, 1.8, 1.8, 1.8, Acknowledgements Imperial College London J. Cole, G. Hicks, C.A.J. Palmer, A. Rehman, M.J.V. Streeter, Z. Najmudin LMU Munchen/Max-Planck-Institut fur Quantenoptik J.Schreiber Brookhaven National Laboratory Accelerator Test Facility I.V. Pogorelsky, M. Babzien, M. Ispiriyan, M.N. Polyanskiy, V. Yakimenko

3 Outline Overview of experimental gas jet studies at BNL Review of sonic/supersonic gas jet nozzles Needle nozzle design and characterisation Thin gas jets High density gas jets

4 Outline Overview of experimental gas jet studies at BNL Review of sonic/supersonic gas jet nozzles Needle nozzle design and characterisation Thin gas jets High density gas jets

5 Overview of experimental gas jet studies at BNL λ=527 nm Pulse length=6 ps Drive Beam!=10.6 µm "= ~6 ps # 0 =70 µm a 0 =~ 0.5 Circular Polarisation Shadowgraphy Interferometry Experiments performed at the Accelerator Test Facility (ATF), part of the Brookhaven National Laboratory, in collaboration with Stony Brook and the University of Maryland

6 Overview of experimental gas jet studies at BNL Monenergetic ion beams from radiation pressure effects!"#!"$ $"% $"& %"$" #"!" See: Palmer et al., PRL, 106 (2011)

7 Overview of experimental gas jet studies at BNL Monenergetic ion beams from radiation pressure effects 0!"#!"$ 0.5 Hydrogen Plasma $"% $"& y (mm) 1 Laser %"$" #"!" See: Palmer et al., PRL, 106 (2011) Gas Jet z (mm)

8 Overview of experimental gas jet studies at BNL Transverse shadowgraphy and interferometry just after interaction shows 50 µm hole bored into an overdense plasma ρ (nc)

9 Overview of experimental gas jet studies at BNL Also see shock propagating into overdense plasma t=200 ps t=500 ps t=1500 ps Shadowgraphy: Density profile: What if target size was less than 50 µm? Could get light-sail acceleration

10 Overview of experimental gas jet studies at BNL To follow on from these experiments, two goals for gas jet targetry: Objective 1: Constructing thin gas jets for light sail/shock breakout studies at BNL Objective 2: Constructing high density gas jets to scale experiments at BNL to optical lasers Critical density at λ=1 µm (1 x cm -3 ) is 100x higher than for CO2 lasers

11 Outline Overview of experimental gas jet studies at BNL Review of sonic/supersonic gas jet nozzles Needle nozzle design and characterisation Thin gas jets High density gas jets

12 Review of supersonic gas jet nozzles Widely used in underdense plasma studies Flat-top density profiles can be created using supersonic gas jet nozzles See Semushin & Malka (Rev. Sci. Instrum. 2001) Pressurised gas Gas density proportional to backing pressure Density profile optimised by varying Dexit, Dcrit and Lopt

13 Review of supersonic gas jet nozzles Typical radial gas jet profiles for Dexit = 2 mm nozzle: Phase shift (derived from interferogram) Gas density profile Density (m 3 ) 9 x ρ(r )for2mmat100bar 2500 µm 1000 µm 250 µm Gas profile top hat near nozzle exit Typical density gradients ~400 microns, varies with distance from nozzle exit Maximum density limited by nozzle width at throat Can sacrifice flat top density profile and increase density by matching Dexit to Dcrit x(µm)

14 Outline Overview of experimental gas jet studies at BNL Review of sonic/supersonic gas jet nozzles Needle nozzle design and characterisation Thin gas jets High density gas jets

15 Needle nozzle design and characterisation Our objectives: Sacrifice flat top profile to increase maximum density Allow the laser to be focussed as close to the top of the nozzle as possible Try to limit the size of the gas jet as much as possible Use the needles off medical syringes Needles come in many sizes, are cheap and well engineered

16 Needle nozzle design and characterisation

17 Needle nozzle design and characterisation Added height useful for fast focusing lasers - can shoot closer to the nozzle Gauge Din (mm) Dout (mm) Needle diameters as low as 80 micrometres inner diameter up to > 1 mm

18 Needle nozzle design and characterisation Interferometry set-up for characterisation (Mach-Zehnder): Data retrieval uses standard phase retrieval and numerical trapezoid method Abel inversion technique done automatically with in-house Matlab software

19 Outline Overview of experimental gas jet studies at BNL Review of sonic/supersonic gas jet nozzles Needle nozzle design and characterisation Thin gas jets High density gas jets

20 Phase map Density map Work mostly performed by J. Cole at Imperial College Thin gas jets One goal was creating thin gas jet targets for experiments at BNL Require maximum electron density ~5x10 19 cm -3, corresponding to 2.5x10 19 cm -3 molecule density for He or H2 gas - easily achievable Require overdense region of <100 microns Typical images: Raw interferometry FFT

21 Work mostly performed by J. Cole at Imperial College Thin gas jets Comparison of 1 mm and 2 mm conical nozzles, and needle nozzle, Din=410 microns: Needle nozzle provides: Higher density close to the nozzle exit Smaller transverse extent close to nozzle exit Characteristic parabolic density profile

22 Work mostly performed by J. Cole at Imperial College Thin gas jets Comparison of 1 mm and 2 mm conical nozzles, and needle nozzle, Din=410 microns: Needle nozzles also provide shorter scale lengths - useful for front surface radiation pressure acceleration

23 Work mostly performed by J. Cole at Imperial College Thin gas jets Radial size vs axial distance from needle for Din=160, 260 and 410 microns Within a few hundred microns of the nozzle exit, Target size FWHM Din

24 Outline Overview of experimental gas jet studies at BNL Review of sonic/supersonic gas jet nozzles Needle nozzle design and characterisation Thin gas jets High density gas jets

25 High density gas jets Interested in scaling BNL experiments to optical lasers Requires >100x higher electron density ~5x10 21 cm -3 - difficult to achieve! Needle nozzles combine high density near nozzle exit with thin walls, allowing laser to be focused very close to the nozzle exit - good candidate Figure 18: Axial density profiles for the three needles Instead of worrying about target size, focus on maximum electron density Smaller targets cause a reduction in maximum density

26 High density gas jets Density vs axial distance from nozzle for Din= 1.2 mm with 100 bar backing pressure: 6 x mm long Gauge 16 nozzle Particle Density (m 3) Near nozzle, already have molecule density 5x10 26 m -3, giving 1x10 27 m -3 in electron density for hydrogen/helium This is the non-relativistic critical density at λ=1µm Distance from nozzle (microns)

27 High density gas jets Can vary Din and L Varying L for Din= 1.2 mm at different pressures: x bar 75 bar 50 bar 25 bar G x 1026 Varying Din for L= 20 mm at different pressures: 100 bar 75 bar 50 bar 25 bar 20mm length 2 Density/m Density/m Nozzle length/mm Stays roughly constant, but may show effect of imperfect nozzle surface Nozzle inner diameter/mm Shows slight increase in maximum density with increasing nozzle diameter

28 High density gas jets Can potentially achieve even higher densities with higher Z gases or higher backing pressure, but both have problems! Clustering will change properties of plasma and inhibit radiation pressure Hagena parameter: Γ (d/ tan α)0.85 = k T P 0 k very high for Ar, CH4, Xe, but very low for He... Increases linearly with backing pressure Also, rating gas system for higher pressures can be very expensive! Many issues addressed in Sylla et al. Rev. Sci. Instrum. (2012) using a pneumatic pressure booster

29 Conclusions We have developed thin < 100 micron gas jets for use on overdense plasma studies using λ=10µm CO2 laser Possible to extend densities up to the critical density for optical lasers Future work will look at slit nozzles to eliminate transverse density gradients, and modelling with CFD code