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Journal of the Korean Physical Society, Vol. 56, No. 6, June 2010, pp. 20292031

Characteristics of a Discharge-plasma Waveguide for High-energyAccelerators and X-ray Light Source Applications

Seong Yong Oh, Min-Seok Kim, In Hyuk Nam and Hyyong Suk

Advanced Photonics Research Institute and Graduate Program of Photonics and Applied Physics,Gwangju Institute of Science and Technology, Gwangju 500-712

(Received 24 November 2009, in final form 3 May 2010)

Compared with conventional accelerators using RF (radio-frequency) or microwave, laser-drivenplasma wakefield accelerators can provide a much higher acceleration gradient of 100 GeV/m.However, diffraction of the focused laser beam limits the acceleration distance and energy to less

than a few mm and a few hundred MeV, respectively. To overcome these limitations, we developed adischarge-plasma waveguide, in which the focused intense laser beam is guided over a long distanceof a few cm due to the parabolic plasma density profile. In this paper, detailed experimental resultswith the developed plasma waveguide are reported.

PACS numbers: 41.75.Jv, 41.60.Cr, 41.60.ApKeywords: Advanced accelerator, Laser wakefield acceleration, X-ray light sourceDOI: 10.3938/jkps.56.2029

I. INTRODUCTION

In laser-driven plasma wakefield acceleration, an in-

tense laser pulse is sent through an underdense plasma,and a strong plasma wake wave is generated behindthe laser pulse. According to the 1-dimensional coldfluid theory [1], the generated longitudinal electric fieldof the plasma wake wave is approximately given byEz[V/m] = (mecp)/e 100

np[cm3], where me and

eare the electron mass and charge, respectively, c is thespeed of light in free space, and np is the plasma density.If the plasma density is 4 1018 cm3, for example,the electric field is calculated as 200 GV/m, which arethree orders of magnitude higher than that of microwave-based conventional accelerators. The plasma wave is sostrong that it is a highly nonlinear wave. As a result, the

so-called wave-breaking process can happen, and someplasma electrons of the dense plasma wave structure canbe self-injected into the acceleration phase and acceler-ated to high energies.

There are several ways for laser-driven plasma wake-field acceleration. One way is to use a supersonic gas jet[2], where an intense laser pulse is focused in the super-sonic gas flow and the strong interaction of the photo-ionized plasma and the laser pulse leads to production ofhigh energy electrons. This method was the most com-mon way in the past, but this method has one importantlimitation; i.e., we cannot avoid the diffraction problemof the focused laser beam. Hence, a new method using a

E-mail: hysuk@gist.ac.kr; Fax: +82-62-715-3350

plasma waveguide is now being pursued for higher energygeneration, where a few-centimeter-long capillary plasmawaveguide is used to optically guide the laser pulse overa long distance [3]. In other words, the capillary plasma

waveguide, which is produced by an electrical dischargeof a gas and can have a transversely parabolic densityprofile by thermal expansion, can focus the diverginglaser beam in the plasma waveguide. Hence, the di-verging diffraction effect can be compensated for by thefocusing effect of the plasma waveguide so that the laser-plasma interaction distance and the accelerated energycan be enhanced significantly. At GIST (Gwangju In-stitute of Science and Technology), we have an ongoingresearch program to produce GeV-level high-energy elec-trons by using a high-power laser beam and a cm-longcapillary plasma waveguide, where the generated elec-tron beams can be used for fs X-ray source applications

[4,5]. In the next section, we report the recent experi-mental results for the development of a capillary plasmawaveguide.

II. EXPERIMENTAL RESULTS OF THE

CAPILLARY PLASMA WAVEGUIDE

To avoid the diffraction problem of a focused laserbeam, we developed a gas-filled capillary plasma waveg-uide. For the capillary plasma waveguide, we machinedtwo pieces of sapphire plates to have a groove on thesurface by using the fs Ti:sapphire machining laser atAPRI (Advanced Photonics Research Institute), GIST.

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Fig. 1. (Color online) (Top) Schematic of the capillaryplasma source system at GIST, which consists of the plasmasource itself and the driving high-voltage circuit. (Bottomleft) Picture of the fabricated plasma source and (bottomright) picture of the discharge-produced plasma waveguidewith He gas.

Hence, a very small hole with a diameter of 200 300mis formed when the two sapphire plates are put together.The length of the capillary can be 7, 15, 33, or 50 mm,and we can choose the capillary length, depending onexperiment. One Cu electrode is placed at each end ofthe capillary hole, and a pulsed high voltage is appliedto these electrodes for electrical discharge. As shownin Fig. 1, we inject gas (helium or hydrogen) into thecapillary hole by using two slots in the capillary, so arather uniform density is produced along the capillaryhole. The gas is injected through solenoid valve that isopened for a short time (in ms range), which can limit thetotal amount of the gas to avoid any potential possibility

for explosion of hydrogen. The gas pressure is varied be-tween 150 mbar and 300 mbar, which can provide plasmaelectron densities in the range of 1018 cm3 for resonantexcitation of a plasma wake wave. The top figure of Fig.1 shows the overall schematic diagram of the system. Asshown in the figure, a capacitor of 1.7 nF is charged toa high voltage up to 30 kV by using a DC high voltagepower supply; then, the thyratron switch (made by E2V)is fired by using a triggering signal from the delay gen-erator (SRS DG535). The current through the resisterinduces a high voltage, and this high voltage is appliedto the two electrodes, which leads to an electrical break-down and plasma production in the capillary hole. The

plasma current increases sharply as a function of time.During this process, the plasma column is heated to high

Fig. 2. (Color online) (a) Discharge current profiles in thecapillary system for different charging voltages. Note that therise time is shorter and the peak current is higher for a highercharging voltage. (b) Discharge current profiles for differentpressures for a fixed charging voltage. In these data, heliumgas was used.

temperature, and the electrons move radially faster thanions. As a result, a kind of a parabolic density profileof np r2 is produced in the radial direction. This

parabolic electron density profile can have a focusing ef-fect on the diverging laser beam as the phase velocity ofa laser beam is given by ph = c/

1 (p/0)2, and it

has a higher phase velocity at an off-centered position.Here, p and 0 are the plasma oscillation frequencyand the laser frequency, respectively. Figure 1 (bottom)shows a picture of the fabricated capillary system and theproduced plasma waveguide when a high-voltage pulse isapplied to the He-filled capillary.

Figure 2(a) shows the measured plasma current pro-files in the capillary test, where helium gas was used.As shown in the figure, a higher voltage can producea higher peak current and a shorter rise time. For ex-

ample, the 30 kV charging voltage can produce a peakcurrent of about 270 A, which is expected to produce a

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Characteristics of a Discharge-plasma Waveguide for High-energy Accelerators and X-ray Seong Yong Oh et al. -2031-

Fig. 3. Measured electron density in the plasma waveguide

as a function of time.

Fig. 4. Schematic of an FEL light source based onthe laser-driven plasma accelerator using a capillary plasmawaveguide.

partially ionized plasma (ionization ratio >20%). In thecase of hydrogen, ionization ratio increases significantlybecause the ionization energy of hydrogen (13.6 eV) isabout 4 times smaller than the second ionization energyof helium. For this reason, we are going to use hydrogengas for a real laser-plasma acceleration experiment later.Figure 2(b) shows discharge current profiles when thepressure is changed; the discharge voltage is fixed at 25kV. The figure indicates that the peak discharge currentincreases as the pressure decreases in the range of100mbar.

When the electrical discharge occurs in the gas, plasmais produced, and it emits light with information on someplasma parameters. This light was collected through anoptical fiber and analyzed using a spectrometer with a1024 256 intensified charge-coupled device (ICCD) toget the time-resolved electron density profile. Figure 3shows the time-resolved electron density profile in theplasma waveguide using the spectral line for He I (587.5nm) (for details, see Ref. 6). The result indicates thatthe plasmas electron density reaches an order of 1018

cm3 within 100 ns after starting the discharge, whichis close to the required range.

We are going to use the developed plasma waveguidesoon for GeV-range electron beam generation at GIST,

where a 100-TW/35-fs Ti:sapphire laser system is avail-able. If GeV-level high-energy electron beams can begenerated, the beams can be used as a compact (table-top size) fs synchrotron radiation source in the X-ray

range (soft or hard X-rays, depending on the beam en-ergy). If the beam quality can somehow be enhancedsignificantly, it could be used for a FEL (free-electronlaser) in the future. Figure 4 shows a schematic dia-gram for such an FEL experiment. The main advantageof FEL radiation is that the light is coherent and muchmore intense compared with incoherent synchrotron ra-diation.

III. CONCLUSIONS

A capillary plasma source was developed at GIST forGeV-range plasma wakefield acceleration. The test re-sult shows that the measured plasma density is close towhat is required for laser wakefield acceleration experi-ments. If the source can produce GeV-level high-energyelectron beams, it can be used as a fs-duration X-raylight source with a table-top size. Depending on thebeam energy, the water window region or shorter wave-length region may be covered. If the beam quality is goodenough, even FEL operation can be expected. This kindof source may be a new, compact, very economical next-generation fs X-ray light source, which can be used in awide range of applications in diverse research fields.

ACKNOWLEDGMENTS

This work was financially supported by the ChallengeProject and the Basic Research Project of National Re-seach Foundation of Korea. It was also partially sup-ported by the National Core Research Center and TopBrand projects.

REFERENCES

[1] T. Tajima and J. M. Dawson, Phys. Rev. Lett. 43, 267(1979).

[2] J. Faure et al., Nature 431, 541 (2004); C. G. R. Geddeset al., ibid. 431, 538 (2004); S. P. D. Mangles et al., ibid.431, 535 (2004).

[3] W. P. Leemans et al., Nature-physics 2, 696 (2006).[4] K. Nakajima, Nature-physics 4, 92 (2008).[5] H. P. Schlenvoigt et al., Nat. phys. 4, 130 (2008).[6] S. Y. Oh, Han S. Uhm, H. Kang, I. W. Lee and H. Suk,

J. Appl. Physics (to be published).