Plasma Physics and Technology 3(1):9–11, 2016 © Department of Physics, FEE CTU in Prague, 2016

SEMICONDUCTOR DETECTORS FOR OBSERVATION OF MULTI-MEVPROTONS AND IONS PRODUCED BY LASERS

Krása J.a,∗, Klír D.b, De Marco M.a,e, Cikhardt J.b, Velyhana,Řezáč K.b,c, Pfeifer M.a, Krouský E.a, Ryć L.d, Dostál J.c,

Kaufman J.a, Ullschmied J.c, Limpouch J.e

a Institute of Physics of the CAS, Na Slovance 2, 121 21 Prague, Czech Republicb Faculty of Electrical Engineering, CTU in Prague, Technická 2, 166 27 Prague, Czech Republicc Institute of Plasma Physics of the CAS, Za Slovankou 3, 182 00 Prague, Czech Republicd Institute of Plasma Physics and Laser Microfusion, EURATOM Association, Warsaw, Polande Faculty of Nuclear Sciences and Physical Engineering, CTU in Prague, Břehová 2, 115 19 Prague,Czech Republic

∗ krasa@fzu.cz

Abstract. The application of time-of-flight Faraday cups and SiC detectors for the measurement ofcurrents of fast ions emitted by laser-produced plasmas is reported. Presented analysis of signals of iondetectors reflects the design and construction of the detector used. A similarity relation between outputsignals of ion collectors and semiconductor detectors is established. Optimization of the diagnosticsystem is discussed with respect to the emission time of electromagnetic pulses interfering with signalsinduced by the fastest ions accelerated up to velocities of 107 m/s. The experimental campaign onlaser-driven ion acceleration was performed at the PALS facility in Prague.

Keywords: laser-accelerated ions, ion collectors, SiC detectors, similarity relations, electromagneticpulse. .

1. IntroductionLaser-produced plasmas are employed as secondarysources of X-ray radiation, beams of electrons aswell as ions capable for various applications. Laser-acceleration of ions represents an effective techniquethat allows accelerating of all ionized elements forvarious applications including acceleration of ions en-tering to fusion reactions with chosen elements in apitcher-catcher target scheme [1]. A stream of fastions expanding into an interaction chamber to impacton a secondary target should have optimal energydistribution with respect, for example, to the cross-section of required fusion reactions. The ion expansionis detected with the use of a variety of detectors whosesignals are proportional to the number of impactedparticles, or their velocity, eventually deposited en-ergy into the sensitive detector’s bulk [2]. Due to theplasma rarefaction caused by its expansion, the ionsenter to a phase, where no collisions between ionsoccur and their charge-states are frozen. Then thetime-of-flight spectra of ions exhibit a well-defineddependence on the detector’s distance from a target,which allows us rescaling the ion current observed ata distance L2 from a target to another distance L1.The procedure needed for the ion current rescalingfrom one to another distance is based on similarityrelations [2]:

tL1 = (L1/L2)× tL2 (1)

andjL1 = (L2/L1)3 × jL2. (2)

The last relationship for currents allows us to rescalethe signals of ion detectors to the same distance andcollecting area and verify whether the time-resolvedsignals of ion collectors are identical. If the signalsmatch themselves then recombination phenomena canbe neglected at the chosen flight distances of observa-tion and the analysis of ion currents based on a shiftedMaxwell–Boltzmann velocity distribution can be ac-complished [3, 4]. Figure 1 shows a schematic diagramof the experiment arrangement for cross-calibrationof various ion detectors by a stream of ions emittedby a laser-produced plasma. A ring detector has aninner hole allowing the passage of ions to the seconddetector. This configuration overwhelms an undesir-able effect of the anisotropy in ion emission [5] andmakes an application of the similarity relationship 2possible.This contribution is devoted to the application of

silicon carbide semiconductors as detectors of multi-MeV protons and deuterons which have sufficient ki-netic energy to enter fusion reactions as, for example,2D(d;n)3He, 7Li(d; n)8Be, and 11B(p;α)2α, which aredriven by lasers as well as z-pinches [1, 6, 7].

2. Generation and detection of ionsDeuteron-producing experiments were carried out onthe PALS at the Institute of Plasma Physics in Prague.

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Krása J., Klír D., De Marco M. et al. Plasma Physics and Technology

Figure 1. Schematic diagram of the experiment andan example of ion current densities j measured withion detectors IC1 (ring ion collector) and IC2 locatedat distances L1 and L2 from the target, respectively

At the fundamental wavelength of 1.315 µm, the PALSlaser was capable of delivering the pulse with a dura-tion of 300 ps (FWHM) and an energy of about 500 Jon a target. The corresponding intensity producingthe plasma on deuterated-polyethylene targets wasIL ≈ 3× 1016 W/cm2. The thickness of targets variedfrom 200 to 500 µm.The emission of ions was observed with the use

of ion collectors (ICs), silicon-carbide (SiC) time-of-flight detector positioned in the far expansion zone,i.e., outside the recombination zone. The ion collectorpositioned 1.51 m far from the target was screenedwith an Al foil of 6 µm in thickness to suppress a photo-peak in the IC signal induced by XUV radiation. Thesuppression of the photopeak of a 150 ns durationallowed us to detect MeV protons reaching the IC50 ns after the laser pulse [8]. On the other hand,the 6-µm Al foil absorbed slower ions and, thus, lim-ited the range of energy spectra to energies higherthan 500 keV and 600 keV for protons and deuterons,respectively.

The SiC detectors are blind to visible and infraredradiation. This characteristics is advantageous be-cause the laser-produced plasma emits very intenseradiation with wavelength λ > 380 nm. Distanceof the SiC detector to the target was 1.54 m. Theinternal structure of the SiC detector and the geom-etry of its front electrode is shown elsewhere [9, 10].The SiC detector is composed of 4 layers: Ni2Si con-tact (200-nm thick front electrode), 100-µm epitaxiallayer, 150-nm n+buffer layer localized above the ac-tive n++4H-SiC layer of 430 µm in thickness, and thelast layer is Ni-ohmic-contact. The used SiC detectorwas biased at 300 V.

3. Results and discussionThe signal Sx of a time-of-flight detector measuringthe impacted ions in x-direction depends on its re-sponse to their number, velocity or energy, which

Figure 2. Signals of SiC and IC detectors related to thedistance of 1.5 m to the target. Ions were emitted bya CD2 target exposed to the laser intensity of 3 × 1016

W/cm2.

can be expressed as follows: Sx(L,t, α)∝ vαf(v)dv ,where v is the velocity of ions in the direction of obser-vation, f(v) is the 3-dimensional velocity distributionfunction and α = 1 if the response is proportional tothe stream (current) of incident particles – applicableto ion collectors, and α = 2 if the response is propor-tional to the deposited energy by particles – applicableto semiconductor detectors [2]. Both the types of iondetectors give information on charge density carriedby ions if their signals are rescaled to a unit of sensi-tive area of detectors and to the same distance fromthe irradiated target. Simplifying the equations usedto model signals of detectors, we obtain simple rela-tionships between the charge density carried by ions,nQ, and output signal of ion collectors, SIC, as wellas SiC detectors, SSiC:

nQ(L, t) ≈ SIC(L, t)t/L ≈ SSiC(t)t2/L2, (3)

where L/t = v(t) is the velocity of ions passing thedistance L at a time t. Relationship 3 establishesa similarity relation between signals of ion collectorsand semiconductor detectors.Figure 2 shows a comparison of signals SIC(t) and

SSiC(t), where SiC detector is placed in the IC2 place.The reference distance from the target is 1.5 m. It isevident that the shape of the signal SIC(t) differs fromthe signal SSiC(t) because SIC(t) is proportional to thevelocity and charge of detected ions while the signalSSiC(t) is proportional to the energy of ions. The ICdetected only protons, deuterons and carbon ionsbeing able to pass through the Al foil placed in frontof the IC. Thus, the IC signal turns down at about150 ns for ionized species emitted by the CD2 plasma.Corresponding kinetic energies of ions stopped in theAl foil were calculated using the Stopping and Rangeof Ions in Matter (SRIM) code[11].

The TOF signals allow us to determine the fastestvelocity of detected ions if the electromagnetic pulse

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Figure 3. Comparison of output signals of SiC detectorand ion collector, which were rescaled to ion chargedensity using relationship 3, see Figure 2.

(EMP) does not strongly interfere with the detector’ssignal, as the row signal (blue line) SIC(t) shows inFigure 2; the black line is the smoothed SIC(t) signal.The EMP is produced in the interaction chamberduring generation and expansion of the laser-producedplasma [12]. Thus, in this laser-plasma shot, theprotons, which are the fastest of all ions emitted bythe laser-produced plasma, reached energy of 2.5MeV,as determined from SSiC(t).

The rescaling SSiC(t) and SIC(t) to the charge den-sity of ions with the use of (1) is shown in Figure 3.Since the rescaled signals match themselves, Figure 3confirms that the SiC detector can be successfullyapplied as an ion detector in plasma experiments. Wenote that the rescaling of SIC(t) and SSiC(t) to nQ(t)reduces the noise which occurs on detector signalsdue to the interference with an electromagnetic pulse(EMP) during the first period of a few hundreds ofnanoseconds of the observation of ions.

4. ConclusionsAlthough the SiC detector used in the experiment arenot fully valuable for the measurement of the low-energy protons and ions as the IC, as they cannotpenetrate through the input 200-nm Ni2Si layer, thisSiC detector is a valuable tool for detection of multi-MeV protons. The advantage of a SiC detector isthat it is more sensitive for fast ions than the ioncollector [8–10].

AcknowledgementsThe research leading to these results has receivedfunding from the Czech Science Foundation (GrantNo. 16-07036S), European Regional Development—the project ELI: Extreme Light Infrastructure(CZ.02.1.01/0.0/0.0/15_008/0000162), and the CzechRepublic’s Ministry of Education, Youth and Sports(LD14089).

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