Plasma Diagnostics Lecture.Key

LA3NET School | Salamanca, Spain | October 1st, 2014

Advanced diagnostics

Plasma density proﬁle measurements and synchronisation of lasers to accelerators

J. Osterhoff and L. Schaper Deutsches Elektronen-Synchrotron DESY Outline

> Importance of the plasma density proﬁle > Measurement techniques > Interferometry > Absorption spectroscopy > Rayleigh scattering > Raman scattering > Laser induced ﬂuorescence > Synchronisation of lasers to accelerators > Summary

Jens Osterhoﬀ | plasma.desy.de | LA3NET School, Salamanca | Oct 1, 2014 | Page 002 Access to novel in-plasma beam-generation techniques requires control over plasma proﬁle in LWFA/PWFA

> Density down-ramp injection J. Grebenyuk et al., NIM A 740, 246 (2014)

IB & 1kA

> Laser-induced ionization injection (Trojan Horse injection) B. Hidding et al., Physical Review Letters 108, 035001 (2012)

IB & 5kA

> Beam-induced ionization injection A. Martinez de la Ossa et al., NIM A 740, 231 (2014)

IB & 7.5kA

> Wakeﬁeld-induced ionization injection A. Martinez de la Ossa et al., Physical Review Letters 111, 245003 (2013)

IB & 10 kA

Jens Osterhoﬀ | plasma.desy.de | LA3NET School, Salamanca | Oct 1, 2014 | Page 003 Access to novel in-plasma beam-generation techniques requires control over plasma proﬁle in LWFA/PWFA

> Density down-ramp injection J. Grebenyuk et al., NIM A 740, 246 (2014)

n0 = 1.2 x 1018 cm-3 IB & 1kA

> Laser-induced ionization injection (Trojan Horse injection) B. Hidding et al., Physical Review Letters 108, 035001 (2012) Driver: Eb = 1 GeV, Ib = 10 kA, Qb = 574 pC σz = 7 μm, σx,y = 4 μm, εx,y = 1 μm IB & 5kA

injection > Beam-induced ionization injection A. Martinez de la Ossa et al., NIM A 740, 231 (2014) acceleration IB & 7.5kA

> Wakeﬁeld-induced ionization injection A. Martinez de la Ossa et al., Physical Review Letters 111, 245003 (2013)

IB & 10 kA

Jens Osterhoﬀ | plasma.desy.de | LA3NET School, Salamanca | Oct 1, 2014 | Page 003 Laser diffraction mitigation in LWFA by transverse plasma density tailoring (plasma channel)

2 ZR

Laser

Jens Osterhoﬀ | plasma.desy.de | LA3NET School, Salamanca | Oct 1, 2014 | Page 004 Laser diffraction mitigation in LWFA by transverse plasma density tailoring (plasma channel)

2 ZR

Laser

Plasma waveguide

Jens Osterhoﬀ | plasma.desy.de | LA3NET School, Salamanca | Oct 1, 2014 | Page 004 Laser diffraction mitigation in LWFA by transverse plasma density tailoring (plasma channel)

2 ZR Capillary discharge plasma waveguides •Plasma fully ionized for t > 50 ns Laser •After t ~ 80 ns plasma is in quasi-equilibrium: Ohmic heating is balanced by conduction of Plasma waveguide heat to wall •Ablation rate small: cap. lasts for >106 shots 17 19 -3 •np ≈ 10 - 10 cm

Jens Osterhoﬀ | plasma.desy.de | LA3NET School, Salamanca | Oct 1, 2014 | Page 004 Laser diffraction mitigation in LWFA by transverse plasma density tailoring (plasma channel)

In this example: ZR = 2 mm, guiding over 16 mm, guiding eﬃciency > 90 %

Karsch, Osterhoﬀ et al., New J. Phys. 9, 415 (2007)

Jens Osterhoﬀ | plasma.desy.de | LA3NET School, Salamanca | Oct 1, 2014 | Page 004 Electron-laser dephasing mitigation in LWFA by longitudinal plasma density tailoring (plasma taper)

Jens Osterhoﬀ | plasma.desy.de | LA3NET School, Salamanca | Oct 1, 2014 | Page 005 Electron-laser dephasing mitigation in LWFA by longitudinal plasma density tailoring (plasma taper)

Constant density plasma

Laser pulse, plasma wave travel with vwave = vg < c Electrons travel with ve ≈ c > vwave ⇒ they outrun the accelerating ﬁeld structure Jens Osterhoﬀ | plasma.desy.de | LA3NET School, Salamanca | Oct 1, 2014 | Page 005 0.20

0.15 E 0.10

0.05 Electron-laser dephasing mitigation in LWFA by 0.00 longitudinal plasma density tailoring (plasma0.0 0.2 taper)0.4 0.6 0.8 1.0 1.2 ⇥ z Eˆ z

800 Ideal taper Linear taper 600 No taper

400 γ Energy gain gain Energy 200

0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 ⇥ Normalized propagationz distance z

Constant density plasma RisingEˆ density plasma z Laser pulse, plasma wave travel with vwave = vg < c Plasma wave phase velocity vwave may be set to ve ⇒ electrons can be phase locked Electrons travel with ve ≈ c > vwave 13 1 18 3 Np =1 a0 =0.5 ⇤i =5.64 10 n0 = 10 ⇒ they outrun the accelerating ﬁeld structure → Rittershofer15 1 et al., Phys. Plasmas 17, 0631044 (2010) ⇤ =2Jens.36 Osterho10ﬀ | plasma.desy.de⇥ = | LA3NET 800 School, rSalamanca= 10 | Oct 1, 2014s | =2 Page .00355 N l l l l Interferometry

>The instrument: M1 compensation plate M2 >Two wavefronts are overlapped and BS interference pattern resulting form phase difference is detected. Source

>When can two waves interfere?

x >path difference arms < coherence length ≈ λcental²/Δλ Detector

>fs laser: λ = 800nm, Δλ = 80nm -> few µm! I

x Jens Osterhoﬀ | plasma.desy.de | LA3NET School, Salamanca | Oct 1, 2014 | Page 006 Interferometry

>Path difference: M1 compensation

plate M2 BS

Source

typical: vacuum as reference and thus Nref=1 z1 z2

>Crucial for interferometers: Stability Detector

>systems with sophisticated vibration damping, Δ� ≈ 10-5 more typical: Δ� ≈ 10-1 detection circuit and initial phase control resolve B. V. Weber and S. F. Fulghum, “A high-sensitivity two- color interferometer for pulsed power plasmas,” Rev. Sci. Instrum.,Vol. 68, pp 1227-1232, Feb. 1997

Jens Osterhoﬀ | plasma.desy.de | LA3NET School, Salamanca | Oct 1, 2014 | Page 007 Interferometry

In Gases: Index of refraction varies 1"E+1" linearly with density (until N=Nvac) Thus when Nref =1

1"E+0" fringe'shi*' Example: Hydrogen 1"E$1" N-1 (at atmospheric): 1.3991x10-4

interaction length: 1mm 1"E$2" light source: NdYAG 532nm 1"E+17" 1"E+18" 1"E+19" 1"E+20" hydrogen'density'/'1/cm³'

Jens Osterhoﬀ | plasma.desy.de | LA3NET School, Salamanca | Oct 1, 2014 | Page 008 Refractive index in plasma

Assumptions: cold, non-magnetised, collision free

Dispersion relation in plasma:

Refractive index:

Jens Osterhoﬀ | plasma.desy.de | LA3NET School, Salamanca | Oct 1, 2014 | Page 009 Summary on interferometry

Total phase shift: Δ�

Gladstone-Dale constant

-15 2 Usually: c1 << c2 ≈ 5x10 m >Densities which can be measured depend on interaction length and wavelength used >Even in weakly ionised plasmas neutral background can be neglected >No calibration required >Pulsed lasers allow for high time resolution >Usually limited to 1018 cm-3 at plasma dimensions

Measured interferogram FFT of interferogram interferogram of FFT interferogram Measured

Interferometry measure? we did What FFT FFT reference (no plasma)interferogram reference with plasma

iFFT iFFT

phase distribution discontinuousretrieveddistribution phasephase phase distributiondiscontinuous distribution phase

interferogram “data” n

unwrapn density critical = c n unwrap density electron = e n refraction of index = η vector wave = k shift phase = ϕ Δ

M. Takeda, H. Ina and S. Kobayashi, Fourier – Transform Method of Fringe – Paern Analysis for Computer – Based Topography and Interferometry, J. Opt. Soc. Am., Vol. shift Jens Osterhophase ﬀ | plasma.desy.de | LA3NET School, Salamanca | Oct 1, 2014 | Page 0011 72, 1, 156, 1981 Howshift? do wephase determinethe thedetermine phasewe do shift?HowHow shift? do wephase determinethe shift? thedetermine phasephase we the do shift?How measure we did Why

Measured interferograminterferogram of FFT FFTMeasured of interferogram interferograminterferogram interferogram of Measured FFT FFT of interferograminterferogram Measured

Interferometry measure? we did What FFTFFT FFTFFT reference (no plasma)interferogram reference with plasma

iFFT iFFT iFFT iFFT

phasedistribution distributionphase discontinuous discontinuousphaseretrieved distribution distribution phasephase phasedistribution distribution phase discontinuous discontinuous phasedistribution distributionphase

interferogram “data” n n n

unwrap unwrapn density critical = c n unwrap unwrapdensity electron = e n refraction of index = η vector wave = k shift phase = ϕ Δ

M. Takeda, H. Ina and S. Kobayashi, Fourier – Transform Method of Fringe – Paern Analysis for Computer – Based Topography and Interferometry, J. Opt. Soc. Am., Vol. shift Jens Osterhophase ﬀ | plasma.desy.de | LA3NET School, Salamanca | Oct 1, 2014 | Page 0011 72, 1, 156, 1981 How does the electron density determinate from the phase shift?

z Phase Distribution Lineout f0r zm

Δφ(y,zo) How to retrieve space resolved density

>known: problem is radially symmetric Fitted function zm >“unwrap” density proﬁle (Abel inversion) y y y y r symmetry axis Density Profile [m-3] x Δ�

Jens Osterhoﬀ | plasma.desy.de | LA3NET School, Salamanca | Oct 1, 2014 | Page 0012 Absorption spectroscopy

>Transmission of light through a medium is measured to calculate the line of sight integrated density. >Plasma emission (decreases by 1/r2 with distance) can usually be neglected

I >Lambert-Beer law applies:

>In case of little absorption �

>Here �’� also includes effects from induced emission and thus can be negative

Source Detector

Jens Osterhoﬀ | plasma.desy.de | LA3NET School, Salamanca | Oct 1, 2014 | Page 0013 Absorption spectroscopy

E2 B12 B21 >�’� in the two level system:

E1 Assume: Population density thermal, line proﬁles g(*)(�) are the same and

degeneracy

>Find, that induced emission is negligible for E2-E1>>kBT and intensity which is sufﬁciently small if intensity too high: Saturation effects (ratio max absorption to FWHM decreases) >Possible in gases and plasmas, temporal resolution linked to radiative lifetime >Problems: Usually hard to detect (signal to noise), often requires a wavelength tunable light source

Jens Osterhoﬀ | plasma.desy.de | LA3NET School, Salamanca | Oct 1, 2014 | Page 0014 Scattering on bound electrons

>scattering of (laser) light on bound electrons (gas, plasma) can also be used as density diagnostic >two cases occur:

Elastic scattering (Rayleigh) Inelastic scattering (Raman) leaves species in same quantum state changes the quantum state of the species scattered photons of same energy energy difference excitation to emission

Energy levels excitation Stokes- excitation Rayleigh RamanStokes Raman Rayleighscattering scattering

Rot. / vib. mode

Jens Osterhoﬀ | plasma.desy.de | LA3NET School, Salamanca | Oct 1, 2014 | Page 0015 Scattering on bound electrons

>scattering of (laser) light on bound electrons (gas, plasma) can also be used as density diagnostic >two cases occur:

Energy levels excitation Stokes- excitation Rayleigh RamanStokes Raman Rayleighscattering scattering

Rot. / vib. mode species speciﬁc

Jens Osterhoﬀ | plasma.desy.de | LA3NET School, Salamanca | Oct 1, 2014 | Page 0015 Rayleigh Scattering

>Dipole scattering, depends on the polarizability

number of scatterers polarizability angle in respect to incoming beam

distance scatterer-detector

differential scattering cross section has been measured accurately for various species

>Allows for measuring species densities but usually faces the problem of other scattered light >In spectroscopic analysis: velocity distribution of scatterer induces broadened line profile: Temperature and flow fields can be measured! >Problems: not species sensitive (gases, plasmas), parasitic stray light can influence detection

Jens Osterhoﬀ | plasma.desy.de | LA3NET School, Salamanca | Oct 1, 2014 | Page 0016 Raman Scattering

>Inelastic process in which energy can be transferred to or from the scatterer

>When energy is transferred to the scatterer: Stokes lines, �s=�0-�t

>When energy is transferred from the scatterer: Anti-Stokes lines, �as=�0+�t >Always: Raman scattering stronger than Rayleigh scattering

Ωeff: Optics and detector efﬁciency T. Weineisen et al. Phys. Rev. ST Sccel. Beams 14, 050705 >Requirement for a species to be Raman active: Polarizability must be anisentropic (rotational Raman scattering) or change (vibrational Raman scattering) Since polarisability always isentropic -> no rotational Raman! >Upper state can be virtual or real electronic transition (resonance raman spectroscopy)

Jens Osterhoﬀ | plasma.desy.de | LA3NET School, Salamanca | Oct 1, 2014 | Page 0017 045107-3 Scannell et al. 045107-3 Scannell et al. Rev. Sci. Instrum. 81,045107Rev.͑2010 Sci.͒ Instrum. 81,045107͑2010͒

TABLE I. Total number of detectedTABLE photoelectrons I. Total number from Raman of detected scattering photoelectronsof the from scattered Raman scattering photons forof the the different scattered gases photons and for lasers the are different gases and lasers are integrated over the entire spectrumintegrated at a gas over pressure the entire of 10 spectrum mbar. Optical at a gas pressureshown of in 10 Fig. mbar.1. Optical Oxygen providesshown in a Fig. greater1. Oxygen number provides of scat- a greater number of scat- transmission of 10%, f/12, scatteringtransmission length of of 67 10%, mm, f/12, and EQEscattering of 4%. length of 67 mm, and EQE of 4%. tered photons than nitrogen,tered although photons both than elements nitrogen, produce although both elements produce scattered spectra close to the laser wavelength. To obtain a H2 D2 N2 H2 O2 D2 N2 O2 scattered spectra close to the laser wavelength. To obtain a random error of less than 1%,random at least error 10 of 000 less photo than electrons 1%, at least 10 000 photo electrons 1.25 J, ␭ =532 nm 124 141 836 2135 0 1.25 J, ␭0 =532 nm 124 141should 836 be detected 2135 from Ramanshould scattering be detected at fromeach gas Raman pres- scattering at each gas pres- 3.0 J, ␭ =694.3 nm 129 151 902 2303 0 3.0 J, ␭0 =694.3 nm 129 151sure. The 902 number 2303 of lasersure. pulses The required number to of achieve laser pulses this required to achieve this 2.5 J, ␭0 =1064 nm 282.5 J, ␭ =1064 34 nm 208 28 536 34 208 536 0 accuracy at the center ofaccuracy plasma at may the be center deduced of plasma from may be deduced from Table I. Table I. not necessarily in the infrared.not necessarily The possible in the detectors infrared. avail- The possible detectors avail- able for the ITER LIDARable system, for the and ITER their LIDAR EQE are system, dis- and their EQE are dis- cussed in detail in Ref. 8. 10cussed mbar in is detail a low in gas Ref. pressure8. 10 mbar and is a low gas pressure and V. FILTER WAVELENGTHSV. FILTER FOR RAMAN WAVELENGTHS SCATTERING FOR RAMAN SCATTERING so should be acceptable fromso should a safety be perspective. acceptable If from a higher a safety perspective. If a higher gas pressure is allowable,gas the pressure number of is allowable, scattered photons the number ofFour scattered different photons filter transmissionFour different bands are filter considered transmission in bands are considered in may be scaled linearly. Themay table be scaledshows linearly. that there The are table sig- showsdetail that in there the following are sig- sectionsdetail of in this the paper. following These sections transmis- of this paper. These transmis- nificantly more scattered photonsnificantly from more the scattered high Z elements photons fromsion the bands high Z are elements summarizedsion in Fig. bands2͑a are͒. To summarized capture nitrogen in Fig. 2͑a͒. To capture nitrogen and significantly more scatteredand significantly photons from more the scattered lower photonsanti-Stokes from lines the lower from a 1064anti-Stokes nm laser, lines a filter from of a bandwidth 1064 nm laser, a filter of bandwidth wavelength lasers. The tablewavelength also shows lasers. that scattering The table fromalso shows20 that nm scattering transmitting from from20 1041.5–1061.5 nm transmitting nm from was chosen. 1041.5–1061.5 nm was chosen. 10 mbar of nitrogen using a10 1064 mbar nm of laser nitrogen produces using a a similar 1064 nm laserThis produces filter will a capture similar a veryThis large filter fraction will capture of the a anti-Stokes very large fraction of the anti-Stokes Rotationalnumber Raman of photons Scattering as scatteringnumber from of photons 10 mbar as of scattering hydrogen fromspectrum, 10 mbar of missing hydrogen only thosespectrum, lines very missing close only to those the laser lines very close to the laser using a 532 nm laser. Thisusing number a 532 of photons nm laser. is This measurable; number of photonswavelength. is measurable; As shown inwavelength. Fig. 2͑b͒, this As filter shown will in provide Fig. 2͑b͒, this filter will provide scattering from 10 mbar ofscattering nitrogen from is routinely 10 mbar performed of nitrogen isuseful routinely Thomson performed scatteringuseful measurements Thomson down scattering to low measurements elec- down to low elec- >polarizabilityon MAST. tensor15 is Althoughanisentropic, in �on the‖≠� MAST. MAST⊥ 15 caseAlthough this is in achieved the MASTtron case temperature. this is achieved tron temperature. as the moleculewith higher rotates etendue the presented collectionwith polarizability higher optics, etendue it changes: is performed collection with optics, it isThe performed filter considered with for capturingThe filter nitrogen considered Stokes for capturinglines nitrogen Stokes lines ->Induced dipole is modulated by rotation -> results in rotational transitions lower laser energy and muchlower shorter laser scattering energy and length much than shorterfrom scattering a 1064 length nm than laser isfrom also a 20 1064 nm and nm laser transmits is also from 20 nm and transmits from the system used to compileStokesthe Table systemI. The used spectral to compile distributions Table I. The1070.25–1090.25 spectral distributions nm. A larger1070.25–1090.25 gap is required nm. to A the larger laser gap is required to the laser >Selection rule: ΔJ=0,±2 J: total angular momentum

20 20 20 20 20 20 20 20 Rayleigh Anti-Stokes

H2 D H2 N D O N O

15 15 15 2 15 15 2 15 2 >Assume rigid rotator with inertia15 I 15 2 2 2 /sr) /sr) 2 2 532nm 532nm (cm (cm ) ) r r) r /s /s /s ) ) 2 2 2 r r) r m m m (c (c (c /s /s /s 2 2 2 1 1 1 1 1 3 m m m - -3 -3 x x x1 0 0 0

(c (c 10 (c 10 10 1 1 1 1 1 3 - -3 -3 x x x1 W

0 10 0 10 0 10 dW dW / /d / W dW dW ds ds ds / /d / ds ds ds -31 -31 10 10 Stokes Stokes spectrum spectrum x10 for ΔJ=2: x10 Ω Ω anti-Stokes anti-Stokes /d /d

5 5 5

5 5 5 σ σ 5 5 spectrum spectrum d for ΔJ=-2: Δ�=-2B(2J+3) d

0 0 0 >(Anti-)Stokes lines have a constant0 separation of 4B 0 0 0 0 480 500 520 540 560 580 600 480 500480520500540520560540580560600580 600 525480 500530520 540535 560540580 600 525 525530 530535 535540 540 525 530 535 540 wavelength (nm) wavelengthwavelength (nm) (nm) wavelengthwavelength (nm) (nm) wavelengthwavelength (nm) (nm) wavelength (nm)

10 10 10

>Separation stokes to anti-stokes: 12B 10 10 10 10 10 R. Scannell et al. Rev. Sci. Instrum. 81, (2010) 045107 Jens Osterhoﬀ | plasma.desy.de | LA3NET School, Salamanca | Oct 1, 2014 | Page 0018

8 8 8 8 8 8 8 8 H2 D2 H2 N2 D2 O2 N2 O2 /sr) /sr) 2 2

6 6 6 6 6 6 6

6 (cm (cm 694.3nm 694.3nm ) ) r r) r /s /s /s 2 2 2 m m m (c (c (c 1 1 1 1 1 3 - -3 -3 x x x1 0 0 0 W dW dW / /d / ds ds ds ) ) r r) r /s /s /s 2 2 2 m m m (c (c (c 1 1 1 1 1 3 - -3 -3 x x x1 0 0 0 W dW dW / /d / ds ds ds -31 -31

4 4 4

4 4 4 x10 x10 4 4 Ω Ω /d /d σ σ

2 2 2

2 2 2 d d 2 2

0 0 0 0 0 0 0 0 650 700 750 650 650700 700750 750 685 650690 695700700 705750 685 685690 690695 695700 700705 705 685 690 695 700 705 wavelength (nm) wavelengthwavelength (nm) (nm) wavelengthwavelength (nm) (nm) wavelengthwavelength (nm) (nm) wavelength (nm)

20. 20. 20. 2.0 20. 2.0 20. 20.

H H2 D N O 2 D 15. N 2 15. O 2 15. 2 1.5 15. 1.5 2 15. 2 15. 2 /sr) /sr) 2 2

(cm 1064nm (cm 1064nm ) ) r) r r s / /s /s 2 2 2 m c m m ( (c (c 1 1 1 3 3 -3 - - x1 x1 0

0 1.0 0 1.0 1.0 x1 W dW dW / / /d s ds ds d ) ) r) r r s / /s /s 2 2 2 m c m m ( (c (c 1 1 1 3 3 -3 - - x1 x1 0

0 1.0 0 1.0 1.0 x1 W dW dW / / /d s ds ds d -31 -31 1.0 1.0 x10 x10 Ω Ω /d /d

05. 05. 05.

05. 05. 05. σ σ 0.5 0.5 d d

00. 00. 00. 0.0 00. 0.0 00. 00. 1000 1050 1100 1150 1200 1000100010501050110011001150115012001200 10501000106010501070110010801150 1200 10501050106010601070107010801080 1050 1060 1070 1080 wavelength (nm) wavelengthwavelength (nm) (nm) wavelengthwavelength (nm) (nm) wavelengthwavelength (nm) (nm) wavelength (nm)

FIG. 1. ͑Color online͒ Raman spectraFIG. 1. for͑Color scattering online from͒ Raman various spectra laser for wavelengths scattering and from gases various at 298 laser K. wavelengths Note that each and row gases of at plots 298 has K. different Note that intensity each row of plots has different intensity scales. scales.

Downloaded 14 Jan 2013 to 131.169.205.89.Downloaded Redistribution 14 Jan subject 2013 to to 131.169.205.89. AIP license or copyright; Redistribution see http://rsi.aip.org/about/rights_and_permissions subject to AIP license or copyright; see http://rsi.aip.org/about/rights_and_permissions Vibrational Raman Scattering

>polarizability changes during vibration >As before: ΔJ=0,±2 Stokes >In addition: Δv=±1 Δv=-1 is usually not observed since v>0 weakly populated

Anti-Stokes >Result is the same as before, just with another superimposed shift which is constant for all J For ΔJ=0 lines not separated (Q-Branch), ΔJ=2 (S-Branch) and ΔJ=-2 (O-Branch)

3 2 1 J’= 0 v=1 3 2 1 J= 0 v=0 Δv=1 Δv=1 Δv=1 ΔJ=2 ΔJ=0 ΔJ=-2

S-Branch Q-Branch O-Branch Jens Osterhoﬀ | plasma.desy.de | LA3NET School, Salamanca | Oct 1, 2014 | Page 0019 Scattering on Bound Electrons comparison

Molecular nitrogen T=300K n=1025 cm-3 ϴ= 90o vibrational separation: few hundred to few thousand cm-1

rotational separation of lines: Also:B is proportional -1 -1 B is usually a few cm (N2:1.99cm ) to molecular weight

Rayleigh

R. B. Miles et al. Meas. Sci. Technol. 12, (2001) R33-R51 Jens Osterhoﬀ | plasma.desy.de | LA3NET School, Salamanca | Oct 1, 2014 | Page 0020 Scattering on Bound Electrons Summary

>Rayleigh and Raman scattering both are suitable as density (proﬁle) diagnostic has been proven for plasma acceleration gas targets at densities down to few 1017 cm-3 >Weak signals usually need long interaction times, thus poor time resolution >Experimentally challenging to take into account for optics and detector efﬁciencies, as well as spatial intensity distribution (e.g. due to focussing) thus most times better results with calibration >Raman scattering allows for species discrimination (can be important for some injection mechanisms for plasma acceleration) >Raman scattering only allows to observe molecules, no atoms, thus not suitable for diagnostics in plasmas with a high degree of dissociation / ionisation

Jens Osterhoﬀ | plasma.desy.de | LA3NET School, Salamanca | Oct 1, 2014 | Page 0021 Laser Induced Fluorescence

>Concept: Via (Laser) radiation a specific quantum state of an atom (molecule) is excited to a different (higher energetic) quantum state. The emitted fluorescence from the higher quantum state allows to deduce information about the initial quantum state. >Usually: Observation of an indirect fluorescence decay path

E3 Laser B13 B31 A31 A32

E1 excitation stimulated spontaneous Detector emission emission Target

Jens Osterhoﬀ | plasma.desy.de | LA3NET School, Salamanca | Oct 1, 2014 | Page 0022 Laser Induced Fluorescence

>Advantages: High spatial resolution (overlap of laser excitation and imaging system) Time resolution on the order of excitation No intrinsic radiation background (3 level system) -> very sensitive Ground state accessible! >Disadvantages: Possible plasma radiation at detection wavelength various non-linearities >What can be measured? Velocities (temperatures) Densities Electric ﬁeld >Requirements: Optically thin target medium (no reabsorption of emitted photons) Deexcitation purely by ﬂuorescence emission Particle loss out of excitation volume negligible If at all only small inhomogeneities in excitation volume

Jens Osterhoﬀ | plasma.desy.de | LA3NET School, Salamanca | Oct 1, 2014 | Page 0023 Laser Induced Fluorescence

>Measuring velocity (temperature) distribution is comparably easy since only the line profile but not the absolute intensity is important >What contributes? >The natural line width of the species under investigation >The (mean) particle velocity and temperature distribution >Doppler effect: particles moving in laser direction see different frequency resulting in shift for mean particle velocity spectral broadening for temperature distribution >The laser profile (must be small compared to the doppler broadened profile) >Careful: Line width liked to intensity in case of saturation Requires a good spectrometer

Jens Osterhoﬀ | plasma.desy.de | LA3NET School, Salamanca | Oct 1, 2014 | Page 0024 Laser Induced Fluorescence

>What contributes to the LIF signal detected?

differential scattering cross section ﬂuorescence cross section branching ratio

polarisation dependence degeneracy

C: properties of the detection system: quantum efﬁciency, gain, measurement resistance, transmission, accepted solid angle, interaction volume.

Jens Osterhoﬀ | plasma.desy.de | LA3NET School, Salamanca | Oct 1, 2014 | Page 0025 Laser Induced Fluorescence

>Detected LIF signal is inﬂuenced by numerous parameters which are not necessarily known precisely, thus measuring absolute densities is hard. >How to overcome this problem? >Different options possible: >1. Fill probe volume with a known density to calibrate setup Possible for gases but not necessarily for plasma generated species, e.g. atomic hydrogen >2. Use Rayleigh scattering of a well characterised species to calibrate result depends on knowledge of the cross sections!

Jens Osterhoﬀ | plasma.desy.de | LA3NET School, Salamanca | Oct 1, 2014 | Page 0026 Laser Induced Fluorescence Summary

>LIF is a very sensitive technique, especially in 3 (or more) level systems >Calibration of LIF signal can be tricky >Once calibrated possible to measure densities at 1014 cm-1 level e.g. H.F. Döbele et al. Plasma Sources Sci. Technol. 14 (2005) S31–S41 >High spatial resolution, good temporal resolution >Small probed volume -> needs a stable or periodic source if density maps are desired >Especially for hydrogen: Problem of excitation! >Hydrogen ground state excitation: Lyman series excitation to ﬁrst level needs 121,5nm radiation >Ways to overcome: Using two photons to excite simultaneously (TALIF) >Smaller cross section, even harder to calibrate

Jens Osterhoﬀ | plasma.desy.de | LA3NET School, Salamanca | Oct 1, 2014 | Page 0027 Electron Diffraction Experiment

REGAE(RelativisticElectronGunforAtomicExploration) FemtosecondElectronDiffraction(FED):Fundamentalinformationonultra fastatomic processes or makingExternalamolecularmovie injection, experiments planned at REGAE How to solve Spacechargeproblem? reversibleprocess Timeresolution 1) Numberof electronsperbunch RF-accelerator with Spatialresolution irreversibleprocess E = 5 MeV Relativistic Electron Gun for Atomic Exploration Timeresolution 2) relativistic electronbeam ΔE = 33 keV bunchcurrent τ = 14 fs RMS magnetic lens εn = 0.3 mm mrad σtrans = 8.5 µm RMS Q = 1 pC Electron probe sample beam synch’ed to a Ti:Sa laser

photocathode

Laserpump beam Courtesyof G.Sciaini(of Toronto)

DPG/Karlsruhe /March 2011 Shima Bayesteh/DESY 2

Courtesy of G. Sciaini, K. Flöttmann, and D. Miller Jens Osterhoﬀ | plasma.desy.de | LA3NET School, Salamanca | Oct 1, 2014 | Page 0028 Electron Diffraction Experiment

REGAE(RelativisticElectronGunforAtomicExploration) FemtosecondElectronDiffraction(FED):Fundamentalinformationonultra fastatomic processes or makingExternalamolecularmovie injection, experiments planned at REGAE How to solve Spacechargeproblem? reversibleprocess Timeresolution 1) Numberof electronsperbunch RF-accelerator with Spatialresolution irreversibleprocess E = 5 MeV Relativistic Electron Gun for Atomic Exploration Timeresolution 2) relativistic electronbeam ΔE = 33 keV bunchcurrent τ = 14 fs RMS magnetic lens εn = 0.3 mm mrad mirror with hole σtrans = 8.5 µm RMS Q = 1 pC Electron probe sample beam synch’ed to a Ti:Sa laser

photocathode

Laserpump beam Courtesyof G.Sciaini(of Toronto) high-intensity laser pulse

DPG/Karlsruhe /March 2011 Shima Bayesteh/DESY 2

Courtesy of G. Sciaini, K. Flöttmann, and D. Miller Jens Osterhoﬀ | plasma.desy.de | LA3NET School, Salamanca | Oct 1, 2014 | Page 0028 Electron Diffraction Experiment

REGAE(RelativisticElectronGunforAtomicExploration) FemtosecondElectronDiffraction(FED):Fundamentalinformationonultra fastatomic processes or makingExternalamolecularmovie injection, experiments planned at REGAE How to solve Spacechargeproblem? reversibleprocess Timeresolution 1) Numberof electronsperbunch RF-accelerator with Spatialresolution irreversibleprocess E = 5 MeV Relativistic Electron Gun for Atomic Exploration Timeresolution 2) relativistic electronbeam ΔE = 33 keV bunchcurrent τ = 14 fs RMS magnetic lens εn = 0.3 mm mrad mirror with hole σtrans = 8.5 µm RMS Q = 1 pC Electron probe sample beam synch’ed to a Ti:Sa laser

photocathode Gas target Laserpump beam Courtesyof G.Sciaini(of Toronto) high-intensity laser pulse

DPG/Karlsruhe /March 2011 Shima Bayesteh/DESY 2

Courtesy of G. Sciaini, K. Flöttmann, and D. Miller Jens Osterhoﬀ | plasma.desy.de | LA3NET School, Salamanca | Oct 1, 2014 | Page 0028 Electron Diffraction Experiment

REGAE(RelativisticElectronGunforAtomicExploration) FemtosecondElectronDiffraction(FED):Fundamentalinformationonultra fastatomic processes or makingExternalamolecularmovie injection, experiments planned at REGAE How to solve Spacechargeproblem? reversibleprocess Timeresolution 1) Numberof electronsperbunch RF-accelerator with Spatialresolution irreversibleprocess E = 5 MeV Relativistic Electron Gun for Atomic Exploration Timeresolution 2) relativistic electronbeam ΔE = 33 keV bunchcurrent τ = 14 fs RMS magnetic lens εn = 0.3 mm mrad mirror with hole σtrans = 8.5 µm RMS Q = 1 pC Electron probe sample beam synch’ed to a Ti:Sa laser

photocathode Gas target Laserpump beam Courtesyof G.Sciaini(of Toronto) high-intensity laser pulse

DPG/Karlsruhe /March 2011 Shima Bayesteh/DESY Accelerate electron bunch to >> 5 MeV 2

Courtesy of G. Sciaini, K. Flöttmann, and D. Miller Jens Osterhoﬀ | plasma.desy.de | LA3NET School, Salamanca | Oct 1, 2014 | Page 0028 Plasma acceleration at REGAE will allow for novel studies: temporal bunch compression

Bunch is injected close to the zero crossing of the longitudinal electric ﬁeld, front of the bunch becomes slower than the back

Jens Osterhoﬀ | plasma.desy.de | LA3NET School, Salamanca | Oct 1, 2014 | Page 0029 Plasma acceleration at REGAE will allow for novel studies: temporal bunch compression

Bunch is injected close to the zero crossing of the longitudinal electric ﬁeld, front of the bunch becomes slower than the back

Jens Osterhoﬀ | plasma.desy.de | LA3NET School, Salamanca | Oct 1, 2014 | Page 0029 Plasma acceleration at REGAE will allow for novel studies: temporal bunch compression

Bunch is injected close to the zero crossing of the longitudinal electric ﬁeld, front of the bunch becomes slower than the back

35003.5

a0=5, 9900x200 30003.0

25002.5 Bunch length [as]

20002.0

1.5

Bunch length [fs] 1500

10001.0

0.5500 70 75 80 85 90 95 100 Injection offset [fs]

→ REGAE bunch is compressed from 10 fs to ~1.5 fs → Laser to electron-beam jitter must be ≲10 fs

Jens Osterhoﬀ | plasma.desy.de | LA3NET School, Salamanca | Oct 1, 2014 | Page 0029 VENTEON Pulse One PE oscillator for repetition rate stabilization and low phase jitter

VENTEONLaserTechnologiesGmbH VENTEON Laser Technologies GmbH, Hertzstr. 1B, 30827 Garbsen, Germany Hertzstr.1b,30827Garbsen Germany Matthias Schnepp Accelerator Physics Group, laser engineer Tel.+49(0)511|76217219 Notkestrasse 85 Fax+49(0)511|76217220 D-22607 Hamburg [email protected] > Repetition rate: 83.2 MHz (REGAE RF frequency at ~3 GHz is 36th harmonic)

> Locked by two piezos with different resonanceQuotation No.: frequencies An-VP1-1202-3R of 4 kHz and > 50 kHz > Pump laser: low-phase jitter CoherentDate: Verdi G5 CEP29.02.2012 Quotation Pos. Qty. Description Unit Price / € JensPrice Osterho ﬀ/ €| plasma.desy.de | LA3NET School, Salamanca | Oct 1, 2014 | Page 0030

01 1 VENTEON | PULSE: ONE PE - IP 85,900.00 € Ultrafast Ti:Sapphire laser oscillator - High PowerEdition with integrated DPSS pump laser

Technical Specifications: Pulse duration < 15 fs (FWHM) / Repetition rate: 83.2 MHz Spectral bandwidth (FWHM) >100 nm Pulse energy > 6 nJ @ 83.2 MHz repetition rate (average output power 500 mW @ 5W pump power) Center wavelength 800 nm

Extended Breadboard Option Extended breadboard size to include pump laser and additional optics within the laser enclosure. Water-cooled Breadboard design Footprint: 525 mm x 600 mm

Verdi G5 CEP OPSLaser-Diode System Diode-Pumped Solid-State Lasers, High-Power 5 W CW Output at 532 nm

Chiller for VENTEON | PULSE: ONE Suitable Water Chiller for IP system Working temperature range -20 °C ... 40 °C

Additional purging cover and filter unit included

02 1 Preparation for Repetition Rate Stabilization 9,900.00 € Allows for phase-locking the repetition frequency to an external microwave reference (reference and locking electronics not included) Includes 2 low voltage piezos with high resonance frequencies (piezo 1: travel: 12 m / fres: > 4 kHz (loaded); piezo 2: travel: 2 m / fres: > 50 kHz (loaded)) and one DC stepper motor for long-term operation / slow feedback

1/2 Challenges for system integration

> Synchronization > Hardware interfacing > Software interfacing ■ Temporal stability ■ Vacuum compatibility ■ Integration of laser controls and data ■ Pointing stability → vibrations ■ EM noise acquisition into accelerator control

Jens Osterhoﬀ | plasma.desy.de | LA3NET School, Salamanca | Oct 1, 2014 | Page 0031 Challenges for system integration

> Synchronization ■ Temporal stability ■ Pointing stability → vibrations

Amplitude spectrum (µm/s) Underground bunker > Better than VC-G norm THALES request VC-E (3 µm/s) for < 3 µrad pointing

Frequency (Hz)

Jens Osterhoﬀ | plasma.desy.de | LA3NET School, Salamanca | Oct 1, 2014 | Page 0031 Basic REGAE synchronization scheme by courtesy of H.Schlarb

Master clock 200 TW Laser Direc&onal+ option 1 Laser$ coupler+

Laser pulse Laser pulse RF+window+

Master clock option 2 LLRF$ klystron$

Analog$ Feedback$ ;1.3$dB$ ;6.0$dB$ Phase$ shi:er$

Gun$ Buncher$ Plasma Electron bunch Fundamental frequency: 3 GHz (S-Band) Jens Osterhoﬀ | plasma.desy.de | LA3NET School, Salamanca | Oct 1, 2014 | Page 0032 Two methods for laser-accelerator synchronization by courtesy of S.Schulz

RF cavities Synchronization to the accelerator: requires temporal synchronization of photocathode laser high-power laser two independent laser oscillators (master clock) (slave oscillator)

> Radio-frequency (RF) scheme (standard) ■ repetition rates must be locked ■ RF signals generated from pulse trains ■ electronic phase-lock loop (PLL) keeps laser phases locked by adjusting piezo-controlled cavity length

Jens Osterhoﬀ | plasma.desy.de | LA3NET School, Salamanca | Oct 1, 2014 | Page 0033 Standard synchronization technique: RF mixing by courtesy of S.Schulz

>constraints: photo-detection (AM-to-PM), temperature dep. of RF parts optical reference BPF 3.0 GHz phase shifter mixer ampliﬁer BPF 3.0 GHz photodiode

low-pass ﬁlter sync signal to RF accelerator electronics

> Generation of RF signals from optical pulse trains (using harmonics of the accelerator frequency) > Mixing and low-pass ﬁltering results in phase-difference signal > Digital controller to phase-lock mode-locked laser to reference signal

Jens Osterhoﬀ | plasma.desy.de | LA3NET School, Salamanca | Oct 1, 2014 | Page 0034 Two methods for laser-accelerator synchronization by courtesy of S.Schulz

RF cavities Synchronization to the accelerator: requires temporal synchronization of photocathode laser high-power laser two independent laser oscillators (master clock) (slave oscillator)

Jens Osterhoﬀ | plasma.desy.de | LA3NET School, Salamanca | Oct 1, 2014 | Page 0035 Two methods for laser-accelerator synchronization by courtesy of S.Schulz

RF cavities Synchronization to the accelerator: requires temporal synchronization of photocathode laser high-power laser two independent laser oscillators (master clock) (slave oscillator)

> Optical cross-correlation ■ utilizes nonlinear optical eﬀect (SHG) ■ balanced scheme (not dependent on laser intensity) ■ creates error signal for PLL

Jens Osterhoﬀ | plasma.desy.de | LA3NET School, Salamanca | Oct 1, 2014 | Page 0035 Advanced synchronization technique: balanced optical cross-correlation by courtesy of S.Schulz

> concept by F. X. Kärtner master laser oscillator (formerly MIT, now DESY) (horizontal polarization)

slave laser oscillator end mirror polarization-dependent (vertical polarization) beam combiner

group delay element SHG ﬁlter SHG SHG crystal detector 2 detector 1 reﬂected signal forward signal

HT at second harmonic HT at second harmonic HR at fundamental HR at fundamental > Nonlinear optical crystal (PPKTP) for sum-frequency generation > Difference of detector signals is applied as error signal for PLL

Jens Osterhoﬀ | plasma.desy.de | LA3NET School, Salamanca | Oct 1, 2014 | Page 0036 Advanced synchronization technique: balanced optical cross-correlation by courtesy of S.Schulz

> difference of detector signals is applied as error signal for control loop data file: 2012−05−28T204801−ScanLINK69.mat 8000 measured data 6000 calib = 0.014 fs/mV 4000

2000

OXC signal (mV) −2000

−4000

−6000 0 500 1000 1500 2000 2500 3000 time (fs)

> Scan100 pulse delay using optical delay stage > Linear80 ﬁt around zero-crossing → calibration factor < 20 as / mV

> Fixing60 OXC signal at zero-crossing allows for intensity-independent phase locking

standard devaition (mV) Jens Osterhoﬀ | plasma.desy.de | LA3NET School, Salamanca | Oct 1, 2014 | Page 0037

0 0 500 1000 1500 2000 2500 3000 time (fs) Comparison of measurement results depending on method by courtesy of S.Schulz

RF cavities

photocathode laser high-power laser (master clock) (slave oscillator)

RF mixing Optical x-correlator

> Stable lock for > 30 h > Stable lock for > 30 h

> Shot-to-shot rms timing jitter 31.7 fs > Shot-to-shot rms timing jitter 1.2 fs ■ out-of-loop measurement ■ out-of-loop measurement

> Long-term peak-to-peak drift 686 fs > Long-term peak-to-peak drift 6.1 fs ■ calculated from 30 minute moving average ■ calculated from 30 minute moving average Locked oscillators: (out-of-loop) (out-of-loop) two independent oneﬁve ORIGAMI

Jens Osterhoﬀ | plasma.desy.de | LA3NET School, Salamanca | Oct 1, 2014 | Page 0038 Comparison of measurement results depending on method by courtesy of S.Schulz

RF cavities

photocathode laser high-power laser (master clock) (slave oscillator)

RF mixing Optical x-correlator

> Stable lock for > 30 h > Stable lock for > 30 h

> Shot-to-shot rms timing jitter 31.7 fs > Shot-to-shot rms timing jitter 1.2 fs ■ out-of-loop measurement ■ out-of-loop measurement

Peak-to-peak drift from sensitivity of electronics and lasers on environmental changes…

Jens Osterhoﬀ | plasma.desy.de | LA3NET School, Salamanca | Oct 1, 2014 | Page 0038 Inﬂuence of environmental changes on laser delay by courtesy of S.Düsterer

Drifts for 800 nm lasers in air > Temperature (change of refractive index ~1.0×10-6 / K) ■ delay on 1 m beam path / K ≙ 3 fs → ~300 fs delay per Kelvin change (100 m laser path) > Humidity (change of refractive index ~1.0×10-8 / % humidity) ■ delay on 1 m beam path / % ≙ 0.03 fs → ~30 fs delay per 10% change (100 m laser path) > Pressure (change of refractive index ~0.3×10-6 / mbar) ■ delay on 1 m beam path / mbar ≙ 1 fs → ~2 ps delay per 20 mbar change (100 m laser path)

Source: http://emtoolbox.nist.gov/Wavelength/Ciddor.asp

Jens Osterhoﬀ | plasma.desy.de | LA3NET School, Salamanca | Oct 1, 2014 | Page 0039 Inﬂuence of environmental changes on laser delay by courtesy of S.Düsterer

Can be mitigated by good AC system REGAE will be stabilized to within > 0.1 K peak-to-peak > 1% humidity in air Drifts for 800 nm lasers in air > Temperature (change of refractive index ~1.0×10-6 / K) ■ delay on 1 m beam path / K ≙ 3 fs → ~300 fs delay per Kelvin change (100 m laser path) > Humidity (change of refractive index ~1.0×10-8 / % humidity) ■ delay on 1 m beam path / % ≙ 0.03 fs → ~30 fs delay per 10% change (100 m laser path) > Pressure (change of refractive index ~0.3×10-6 / mbar) ■ delay on 1 m beam path / mbar ≙ 1 fs → ~2 ps delay per 20 mbar change (100 m laser path)

Source: http://emtoolbox.nist.gov/Wavelength/Ciddor.asp

Jens Osterhoﬀ | plasma.desy.de | LA3NET School, Salamanca | Oct 1, 2014 | Page 0039 Inﬂuence of environmental changes on laser delay by courtesy of S.Düsterer

Source: http://emtoolbox.nist.gov/Wavelength/Ciddor.asp

Must be actively compensated for by delay line

Jens Osterhoﬀ | plasma.desy.de | LA3NET School, Salamanca | Oct 1, 2014 | Page 0039 2.3 Temperatures – Ceiling 2

2.3.1 30.06.2011 13:15 - 30.06.2011 18:00

.RegaeTempGrp1/T_Decke_0.2 0moving0avg0(100time0steps0=017.50min) 24,60 24,59

24,58 24,58

24,57 24,56

24,56 24,54 24,55 24,52 24,54

Temp.(°C) 24,50 Temp0(°C) 24,53

24,48 24,52

24,46 24,51

24,44 24,50 0 1 2 3 4 5 0 1 2 3 4 5 Time.(hours) Time0(hours)

Temperature (°C) Description 24.532 Mean 0.03 RMS -0.077 Largest Deviation from the Mean -0.01 Smallest Deviation from the Mean

2.3.2 30.06.2011 20:30 - 01.07.2011 09:00 Inﬂuence of environmental changes on laser delay by courtesy of S.Düsterer

-RegaeTempGrp1/T_Decke_0.2 -moving-avg-(10-time-steps-=-17.5-min)Can be mitigated by good AC system -moving-avg-(10% -of-total-time-steps-=-133-min) 24,60 REGAE will be stabilized to within > 0.1 K peak-to-peak 24,58 24,54 > 1% humidity in air

24,56 Drifts for 800 nm lasers in air > Temperature (change of refractive index ~1.0×10-6 / K) 24,54 ■ delay on 1 m beam path / K ≙ 3 fs → ~300 fs delay per Kelvin change (100 m laser path) > Humidity (change of refractive index ~1.0×10-8 / % humidity) 24,52 24,52 ■ delay on 1 m beam path / % ≙ 0.03 fs → ~30 fs delay per 10% change (100 m laser path) > Pressure (change of refractive index ~0.3×10-6 / mbar) Temp-(°C) 24,50 Temp-(°C) ■ delay on 1 m beam path / mbar ≙ 1 fs → ~2 ps delay per 20 mbar change (100 m laser path)

24,48 Source: http://emtoolbox.nist.gov/Wavelength/Ciddor.asp

24,50 24,46 Must be actively compensated for by delay line

24,44 0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14

Time-(hours) Time-(hours) Jens Osterhoﬀ | plasma.desy.de | LA3NET School, Salamanca | Oct 1, 2014 | Page 0039

Temperature (°C) Description 24.52 Mean 0.015 RMS 0.069 Largest Deviation from the Mean 0.002 Smallest Deviation from the Mean

9! ! ! Summary

> Characterisation of the plasma density distribution is crucial for control over processes in laser- and beam-driven plasma wakeﬁeld accelerators > Interferometry is the main technique used today, problematic with usual lasers for densities below 1018 cm-3 > Other techniques (e.g. Raman scattering) promise better signal-to-noise at lower densities, but need development > When linking conventional accelerators to plasma accelerators, synchronisation on a level much below the inverse plasma frequency is crucial (~1 to ~100 fs) > First experiments in this direction are underway, e.g. at DESY

by S.Schulz

Jens Osterhoﬀ | plasma.desy.de | LA3NET School, Salamanca | Oct 1, 2014 | Page 0040 Thank you for your attention!

Jens Osterhoﬀ | plasma.desy.de | LA3NET School, Salamanca | Oct 1, 2014 | Page 0041