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Fast Z-pinch optical guiding for laser wakefield acceleration
Nuclear Instruments and Methods in Physics Research A 455 (2000) 155}160
Fast Z-pinch optical guiding for laser wake"eld acceleration Tomonao Hosokai *, Masaki Kando , Hideki Dewa , Hideyuki Kotaki , Syuji Kondo , Noboru Hasegawa , Kazuhiko Horioka, Kazuhisa Nakajima Advanced Photon Research Center, Kansai Research Establishment, Japan Atomic Energy Research Institute, 8-1 Umemidai, Kizu-chyo, Souraku-gun, Kyoto-fu 619-0215, Japan Department of Energy Sciences, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan High Energy Accelerator Research Organization (KEK), 1-1 Oho, Tsukuba-shi, Ibaraki-ken 305-0801, Japan
Abstract We have studied optical guiding of high-intensity laser pulses using fast Z-pinch for channel guided laser wake"eld acceleration (LWFA). A fast Z-pinch discharge can produce a long stable plasma channel which has a concave electron density pro"le in its core. We experimentally demonstrated the feasibility of optical guiding of high-intensity laser pulses ('10 W/cm) using Z-pinch discharge channel. A ray-trace calculation of the Ti:Sapphire laser pulse in the channel has been done, which could well explain the experimental results. 2000 Elsevier Science B.V. All rights reserved.
1. Introduction In order to increase the energy gain of electrons accelerated by the laser wake"elds, it is a critical issue to propagate a high-intensity short laser pulse in a plasma larger than the vacuum Rayleigh length limited by di!raction. [1] Several methods of optical guiding for extending the propagation distances of intense laser pulses have been proposed: relativistic self-guiding in a plasma [2] and guiding in preformed plasma channels generated by a focused laser pulse [3] or by slow discharge through a capillary in vacuum [4]. We have presented and studied the optical guiding of high-intensity laser pulses through a plasma channel produced by a fast Z-pinch discharge [5]. For the optical guiding of laser pulses, the electron density pro"le must be symmetric in the radial direction and have a min-
* Corresponding author.
imum on the axis, causing the wavefront to curve inward and the laser beam to converge. When this focusing force is strong enough to counteract the di!raction of the beam, the laser pulse can propagate over a long distance and maintain a small beam spot size in a plasma. For the guiding of intense ultra-short laser pulses shorter than the plasma wavelength, it is predicted that the relativistic self-channeling is ine!ective in preventing diffraction, but the preformed plasma channel can provide a robust optical guiding [6].
2. Fast Z-pinch discharge A high current fast Z-pinch discharge generates strong azimuthal magnetic "eld, which contracts the plasma radially inward down to &100 lm in diameter. The imploding current sheet drives the converging shock wave ahead of it, producing a concave electron density pro"le in the radial
0168-9002/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 0 ) 0 0 7 2 3 - 3
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direction just before the stagnation phase. The concave pro"le is approximately parabolic out to a radius of &50 lm, beyond which the density falls o!. In the research of capillary discharge pumped X-ray laser, it has been shown that stable and reproducible channel can be produced by this scheme, and this scheme can be scalable to form longer and higher density channels by tailoring the implosion [7]. It was reported that a stable, liner plasma column over the length of 12 cm could be produced at an electron density of more than 10 cm\ [8]. For the laser wake"eld acceleration (LWFA), a plasma channel with a density in the order of 10 cm\ and a length of&10 cm will be required [1]. We have started a study of optical guiding in the density range of 10}10 cm\ based on the channel production techniques of the discharge pumped X-ray laser. Since the density pro"le in the
plasma channel depends strongly on the discharge process, we have experimentally investigated the discharge dynamics and plasma channel formation.
3. Optical guiding experiment The typical experimental setup is shown in Fig. 1. We have used a capillary with an inner diameter of 1 mm and a length of up to 2 cm which was bored at the central axis of an alumina (Al O ) rod of 50 mm in diameter. The capillary load was placed between two electrodes with an inner diameter of 400 lm which were made of molybdenum. The electrodes were cylindrically connected to a thyratron (EG&G HY-5) and four ceramic capacitors of 2 nF using four coaxial cables. The capacitors were charged up to 20 kV (1.6 J). With this con"guration, the discharge current which was monitored by a Rogowski coil having a peak of 4.8 kA with a rise
Fig. 1. Experimental setup of a fast Z-pinch discharge for an optical guiding. A typical electron density pro"le of the implosion phase of the discharge in it's core is illustrated in the circle.
T. Hosokai et al. / Nuclear Instruments and Methods in Physics Research A 455 (2000) 155}160
157
time of about 15 ns and a duration of 70 ns (FWMH). The capillary was "lled with helium, under di!erential pumping at an initial pressure which was varied from 0.5 to 5 Torr. A DC discharge circuit was used to form an uniformly preionized helium gas in the capillary. The discharge dynamics was observed with a streak camera (HAMAMATSU C-2830) placed on the capillary axis. The visible light emission from the capillary was imaged on the streak-camera slit through a telescope (;20). To investigate the guiding channel formation in the capillary, a He}Ne laser beam (j"632.8 nm ) was focused on the front edge of the capillary to a spot size 40 lm in diameter by means of a FI"20 lens. The transmitted He}Ne laser beam pro"le at the exit of the capillary was observed through a band pass "lter (*j"1 nm), with the streak camera. As a further investigation, a high-intensity Ti:Sapphire laser pulse (j"790 nm, 90 fs, '1;10 W/cm) was focused on the front edge of the capillary to a spot size 40 lm in diameter by means of a FI"12 o!-axis parabolic mirror. The transmitted laser beam pro"le at the exit of the capillary was observed through a band pass "lter (*j"10 nm), with a CCD camera.
4. Results Fig. 2 shows a typical streak image of the plasma emission for the initial pressure of 0.9 Torr, together with the intensity pro"le along the radial direction at t"8.5 ns. The emission only from the central portion of the capillary with a diameter less than 400 lm was imaged on the streak camera. It was found that a luminous region produced at t"2.5 ns, which exhibits good symmetry with respect to the axis. Then, the luminous region on the axis faded out. Later, a brighter luminous region driven by a larger current produced at t"8.5 ns. It still maintains good symmetry with respect to the axis. In addition, as shown in Fig. 2(b), it had a clear dip with a width of 70 lm on the axis. The column oscillated in the radial direction several times while the main power pulse was alive. We have observed the brightness and the radial pro"le of the transmitted He}Ne laser beam and
Fig. 2. (a) A typical streak image of the fast Z-pinch discharge at an initial pressure of 0.9 Torr He: t"0 corresponds to the beginning of the main discharge pulse. (b) The radial intensity pro"le of the plasma emission was t"8.5 ns.
high-intensity Ti:sapphire laser pulses through the capillary at the exit. A typical streak image of the transmitted He}Ne laser beam pro"le through the capillary discharge plasma can be seen in Fig. 3. Since the observed time window of Fig. 3 almost coincided with that of Fig. 2(a), the beam pro"le evolution could be correlated with the discharge dynamics. As shown in Fig. 3, the beam radius gradually contracted in the implosion phase and a brightly enhanced smaller beam spot could be observed at the time (t&8.5 ns) when the luminous pro"le exhibited double peaks in Fig. 2. The observed spot image size (diameter) was &40 lm at the exit of the capillary. These results suggest that the guiding channel was uniformly
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T. Hosokai et al. / Nuclear Instruments and Methods in Physics Research A 455 (2000) 155}160
formed in the core of the column at the implosion phase. Fig. 4 shows typical CCD images of the transmitted high-intensity Ti:sapphire laser pulse pro"le
Fig. 3. A typical streak image of the transmitted He}Ne laser beam through the channel at an initial pressure of 0.9 Torr He: t"0 corresponds to the beginning of the main discharge pulse.
through the capillary discharge plasma at t&8.5 ns (a) and at t"0 ns (b). These show clearly that a high-intensity laser pulse could be guided through the channel over a distance of 2 cm corresponding to&12.5 Z , where Z &1.6 mm is the 0 0 Rayleigh length of the Ti:Sapphire laser system. The experimental results of the channel formation process were corroborated with the results of 1D-MHD simulation using code MULTI-Z [5,7,9,10]. Fig. 5 shows the typical time}space distribution of the electron density obtained by the simulation at an initial pressure of 0.9 Torr He. The result indicated that a concave electron density pro"le could be produced at t"8}8.5 ns due to an imploding shock and a current sheet. The channel was almost parabolic in the radial direction and the channel was fully ionized [5]. The electron density was estimated on the axis to be 2.0;10 cm\, on the peaks of the channel edge to be 7.0; 10 cm\: the density gradient was deduced to be 1.4;10 cm\ [5]. The matched beam radius r was given by r "[r /(pr *n)] , where *n
was the channel depth and r "e/mc was the classical electron radius [6]. With r "35 lm ac cording to the observed dip width in Fig. 2(b) and *n"5.0;10 cm\, the matched beam radius is r "23 lm. The observed spot radius of 20 lm in
Fig. 3 was consistent with this value.
Fig. 4. Typical CCD images of the transmitted high intensity Ti:Sapphire laser pulse (&1;10 W/cm) through the plasma at an initial pressure of 0.9 Torr He. The capillary length ¸"2 cm. (a) t&8.5 ns, (b) t"0 ns.
T. Hosokai et al. / Nuclear Instruments and Methods in Physics Research A 455 (2000) 155}160
Fig. 6 shows a result of ray-trace calculation of the Ti:Sapphire laser pulse in the channel using the density pro"les obtained by the MHD simulation.
159
As shown in Fig. 4(a), the laser pro"le has luminous region both on the axis and at the edge with a ring shape. They are in good correspondence with the results of the ray-trace calculation.
5. Summary In summary, we have studied the optical guiding of high-intensity laser pulse using a fast Z-pinch discharge and experimentally investigated the guiding channel formation process. The results indicated that the fast Z-pinch discharge could uniformly produce a concave electron density pro"le in implosion phase in its core. The optical guiding of high-intensity laser pulses ('1;10 W/cm) was experimentally demonstrated in the imploding phase of the discharge over a distance of 2 cm. The guided laser pro"le were well explained by the ray-trace calculation of the Ti:Sapphire laser pulse in the channel.
Acknowledgements Fig. 5. Typical time}space distribution of electron density obtained by the MHD simulation at an initial pressure of 0.9 Torr He.
We gratefully acknowledge Drs. T. Arisawa, H. Takuma, and Y. Kato for their encouragement. We also would like to thank Prof. T. Aoki of Tokyo
Fig. 6. Typical result of ray-trace calculation of the Ti:Sapphire laser pulse in the channel. It corresponds to Fig. 4(a).
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T. Hosokai et al. / Nuclear Instruments and Methods in Physics Research A 455 (2000) 155}160
Institute of Technology for providing his 1D-MHD code `MULTI-Za. References [1] K. Nakajima, Phys. Plasmas 3 (1996) 2169. [2] K. Krushelnick, A. Ting, C.I. Moore, H.R. Burris, E. Esarey, P. Sprangle, M. Baine, Phys. Rev. Lett. 78 (1997) 4047. [3] C.G. Durfee III, H.M. Milchberg, Phys. Rev. Lett. 71 (1993) 2409. [4] Y. Ehrlich, C. Cohen, A. Zigler, J. Krall, P. Sprangle, E. Esarey, Phys. Rev. Lett. 77 (1996) 4186.
[5] T. Hosokai, M. Kando, H. Dewa, H. Kotaki, S. Kondo, N. Hasegawa, K. Horioka, K. Nakajima, Opt. Lett. 25 (2000) 10. [6] P. Sprangle, E. Esarey, J. Krall, G. Joyce, Phys. Rev. Lett. 69 (1992) 2200. [7] T. Hosokai, M. Nakajima, T. Aoki, M. Ogawa, K. Horioka, Jpn. J. Appl. Phys. 36 (1997) 2327. [8] J.J. Rocca, V.N. Shlyaptsev, F.G. Tomasel, O.D. Cortazar, D. Hartshorn, J.L.A. Chilla, Phys. Rev. Lett. 73 (1994) 2192. [9] R. Ramis, R. Schmalz, J. Meyer-Ter-Vehen, Comput. Phys. Commun. 49 (1988) 475. [10] T. Aoki, J. Meyer-Ter-Vehen, Phys. Plasmas 1 (1994) 1962.
Fast Z-pinch optical guiding for laser wake"eld acceleration Tomonao Hosokai *, Masaki Kando , Hideki Dewa , Hideyuki Kotaki , Syuji Kondo , Noboru Hasegawa , Kazuhiko Horioka, Kazuhisa Nakajima Advanced Photon Research Center, Kansai Research Establishment, Japan Atomic Energy Research Institute, 8-1 Umemidai, Kizu-chyo, Souraku-gun, Kyoto-fu 619-0215, Japan Department of Energy Sciences, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan High Energy Accelerator Research Organization (KEK), 1-1 Oho, Tsukuba-shi, Ibaraki-ken 305-0801, Japan
Abstract We have studied optical guiding of high-intensity laser pulses using fast Z-pinch for channel guided laser wake"eld acceleration (LWFA). A fast Z-pinch discharge can produce a long stable plasma channel which has a concave electron density pro"le in its core. We experimentally demonstrated the feasibility of optical guiding of high-intensity laser pulses ('10 W/cm) using Z-pinch discharge channel. A ray-trace calculation of the Ti:Sapphire laser pulse in the channel has been done, which could well explain the experimental results. 2000 Elsevier Science B.V. All rights reserved.
1. Introduction In order to increase the energy gain of electrons accelerated by the laser wake"elds, it is a critical issue to propagate a high-intensity short laser pulse in a plasma larger than the vacuum Rayleigh length limited by di!raction. [1] Several methods of optical guiding for extending the propagation distances of intense laser pulses have been proposed: relativistic self-guiding in a plasma [2] and guiding in preformed plasma channels generated by a focused laser pulse [3] or by slow discharge through a capillary in vacuum [4]. We have presented and studied the optical guiding of high-intensity laser pulses through a plasma channel produced by a fast Z-pinch discharge [5]. For the optical guiding of laser pulses, the electron density pro"le must be symmetric in the radial direction and have a min-
* Corresponding author.
imum on the axis, causing the wavefront to curve inward and the laser beam to converge. When this focusing force is strong enough to counteract the di!raction of the beam, the laser pulse can propagate over a long distance and maintain a small beam spot size in a plasma. For the guiding of intense ultra-short laser pulses shorter than the plasma wavelength, it is predicted that the relativistic self-channeling is ine!ective in preventing diffraction, but the preformed plasma channel can provide a robust optical guiding [6].
2. Fast Z-pinch discharge A high current fast Z-pinch discharge generates strong azimuthal magnetic "eld, which contracts the plasma radially inward down to &100 lm in diameter. The imploding current sheet drives the converging shock wave ahead of it, producing a concave electron density pro"le in the radial
0168-9002/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 0 ) 0 0 7 2 3 - 3
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T. Hosokai et al. / Nuclear Instruments and Methods in Physics Research A 455 (2000) 155}160
direction just before the stagnation phase. The concave pro"le is approximately parabolic out to a radius of &50 lm, beyond which the density falls o!. In the research of capillary discharge pumped X-ray laser, it has been shown that stable and reproducible channel can be produced by this scheme, and this scheme can be scalable to form longer and higher density channels by tailoring the implosion [7]. It was reported that a stable, liner plasma column over the length of 12 cm could be produced at an electron density of more than 10 cm\ [8]. For the laser wake"eld acceleration (LWFA), a plasma channel with a density in the order of 10 cm\ and a length of&10 cm will be required [1]. We have started a study of optical guiding in the density range of 10}10 cm\ based on the channel production techniques of the discharge pumped X-ray laser. Since the density pro"le in the
plasma channel depends strongly on the discharge process, we have experimentally investigated the discharge dynamics and plasma channel formation.
3. Optical guiding experiment The typical experimental setup is shown in Fig. 1. We have used a capillary with an inner diameter of 1 mm and a length of up to 2 cm which was bored at the central axis of an alumina (Al O ) rod of 50 mm in diameter. The capillary load was placed between two electrodes with an inner diameter of 400 lm which were made of molybdenum. The electrodes were cylindrically connected to a thyratron (EG&G HY-5) and four ceramic capacitors of 2 nF using four coaxial cables. The capacitors were charged up to 20 kV (1.6 J). With this con"guration, the discharge current which was monitored by a Rogowski coil having a peak of 4.8 kA with a rise
Fig. 1. Experimental setup of a fast Z-pinch discharge for an optical guiding. A typical electron density pro"le of the implosion phase of the discharge in it's core is illustrated in the circle.
T. Hosokai et al. / Nuclear Instruments and Methods in Physics Research A 455 (2000) 155}160
157
time of about 15 ns and a duration of 70 ns (FWMH). The capillary was "lled with helium, under di!erential pumping at an initial pressure which was varied from 0.5 to 5 Torr. A DC discharge circuit was used to form an uniformly preionized helium gas in the capillary. The discharge dynamics was observed with a streak camera (HAMAMATSU C-2830) placed on the capillary axis. The visible light emission from the capillary was imaged on the streak-camera slit through a telescope (;20). To investigate the guiding channel formation in the capillary, a He}Ne laser beam (j"632.8 nm ) was focused on the front edge of the capillary to a spot size 40 lm in diameter by means of a FI"20 lens. The transmitted He}Ne laser beam pro"le at the exit of the capillary was observed through a band pass "lter (*j"1 nm), with the streak camera. As a further investigation, a high-intensity Ti:Sapphire laser pulse (j"790 nm, 90 fs, '1;10 W/cm) was focused on the front edge of the capillary to a spot size 40 lm in diameter by means of a FI"12 o!-axis parabolic mirror. The transmitted laser beam pro"le at the exit of the capillary was observed through a band pass "lter (*j"10 nm), with a CCD camera.
4. Results Fig. 2 shows a typical streak image of the plasma emission for the initial pressure of 0.9 Torr, together with the intensity pro"le along the radial direction at t"8.5 ns. The emission only from the central portion of the capillary with a diameter less than 400 lm was imaged on the streak camera. It was found that a luminous region produced at t"2.5 ns, which exhibits good symmetry with respect to the axis. Then, the luminous region on the axis faded out. Later, a brighter luminous region driven by a larger current produced at t"8.5 ns. It still maintains good symmetry with respect to the axis. In addition, as shown in Fig. 2(b), it had a clear dip with a width of 70 lm on the axis. The column oscillated in the radial direction several times while the main power pulse was alive. We have observed the brightness and the radial pro"le of the transmitted He}Ne laser beam and
Fig. 2. (a) A typical streak image of the fast Z-pinch discharge at an initial pressure of 0.9 Torr He: t"0 corresponds to the beginning of the main discharge pulse. (b) The radial intensity pro"le of the plasma emission was t"8.5 ns.
high-intensity Ti:sapphire laser pulses through the capillary at the exit. A typical streak image of the transmitted He}Ne laser beam pro"le through the capillary discharge plasma can be seen in Fig. 3. Since the observed time window of Fig. 3 almost coincided with that of Fig. 2(a), the beam pro"le evolution could be correlated with the discharge dynamics. As shown in Fig. 3, the beam radius gradually contracted in the implosion phase and a brightly enhanced smaller beam spot could be observed at the time (t&8.5 ns) when the luminous pro"le exhibited double peaks in Fig. 2. The observed spot image size (diameter) was &40 lm at the exit of the capillary. These results suggest that the guiding channel was uniformly
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T. Hosokai et al. / Nuclear Instruments and Methods in Physics Research A 455 (2000) 155}160
formed in the core of the column at the implosion phase. Fig. 4 shows typical CCD images of the transmitted high-intensity Ti:sapphire laser pulse pro"le
Fig. 3. A typical streak image of the transmitted He}Ne laser beam through the channel at an initial pressure of 0.9 Torr He: t"0 corresponds to the beginning of the main discharge pulse.
through the capillary discharge plasma at t&8.5 ns (a) and at t"0 ns (b). These show clearly that a high-intensity laser pulse could be guided through the channel over a distance of 2 cm corresponding to&12.5 Z , where Z &1.6 mm is the 0 0 Rayleigh length of the Ti:Sapphire laser system. The experimental results of the channel formation process were corroborated with the results of 1D-MHD simulation using code MULTI-Z [5,7,9,10]. Fig. 5 shows the typical time}space distribution of the electron density obtained by the simulation at an initial pressure of 0.9 Torr He. The result indicated that a concave electron density pro"le could be produced at t"8}8.5 ns due to an imploding shock and a current sheet. The channel was almost parabolic in the radial direction and the channel was fully ionized [5]. The electron density was estimated on the axis to be 2.0;10 cm\, on the peaks of the channel edge to be 7.0; 10 cm\: the density gradient was deduced to be 1.4;10 cm\ [5]. The matched beam radius r was given by r "[r /(pr *n)] , where *n
was the channel depth and r "e/mc was the classical electron radius [6]. With r "35 lm ac cording to the observed dip width in Fig. 2(b) and *n"5.0;10 cm\, the matched beam radius is r "23 lm. The observed spot radius of 20 lm in
Fig. 3 was consistent with this value.
Fig. 4. Typical CCD images of the transmitted high intensity Ti:Sapphire laser pulse (&1;10 W/cm) through the plasma at an initial pressure of 0.9 Torr He. The capillary length ¸"2 cm. (a) t&8.5 ns, (b) t"0 ns.
T. Hosokai et al. / Nuclear Instruments and Methods in Physics Research A 455 (2000) 155}160
Fig. 6 shows a result of ray-trace calculation of the Ti:Sapphire laser pulse in the channel using the density pro"les obtained by the MHD simulation.
159
As shown in Fig. 4(a), the laser pro"le has luminous region both on the axis and at the edge with a ring shape. They are in good correspondence with the results of the ray-trace calculation.
5. Summary In summary, we have studied the optical guiding of high-intensity laser pulse using a fast Z-pinch discharge and experimentally investigated the guiding channel formation process. The results indicated that the fast Z-pinch discharge could uniformly produce a concave electron density pro"le in implosion phase in its core. The optical guiding of high-intensity laser pulses ('1;10 W/cm) was experimentally demonstrated in the imploding phase of the discharge over a distance of 2 cm. The guided laser pro"le were well explained by the ray-trace calculation of the Ti:Sapphire laser pulse in the channel.
Acknowledgements Fig. 5. Typical time}space distribution of electron density obtained by the MHD simulation at an initial pressure of 0.9 Torr He.
We gratefully acknowledge Drs. T. Arisawa, H. Takuma, and Y. Kato for their encouragement. We also would like to thank Prof. T. Aoki of Tokyo
Fig. 6. Typical result of ray-trace calculation of the Ti:Sapphire laser pulse in the channel. It corresponds to Fig. 4(a).
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T. Hosokai et al. / Nuclear Instruments and Methods in Physics Research A 455 (2000) 155}160
Institute of Technology for providing his 1D-MHD code `MULTI-Za. References [1] K. Nakajima, Phys. Plasmas 3 (1996) 2169. [2] K. Krushelnick, A. Ting, C.I. Moore, H.R. Burris, E. Esarey, P. Sprangle, M. Baine, Phys. Rev. Lett. 78 (1997) 4047. [3] C.G. Durfee III, H.M. Milchberg, Phys. Rev. Lett. 71 (1993) 2409. [4] Y. Ehrlich, C. Cohen, A. Zigler, J. Krall, P. Sprangle, E. Esarey, Phys. Rev. Lett. 77 (1996) 4186.
[5] T. Hosokai, M. Kando, H. Dewa, H. Kotaki, S. Kondo, N. Hasegawa, K. Horioka, K. Nakajima, Opt. Lett. 25 (2000) 10. [6] P. Sprangle, E. Esarey, J. Krall, G. Joyce, Phys. Rev. Lett. 69 (1992) 2200. [7] T. Hosokai, M. Nakajima, T. Aoki, M. Ogawa, K. Horioka, Jpn. J. Appl. Phys. 36 (1997) 2327. [8] J.J. Rocca, V.N. Shlyaptsev, F.G. Tomasel, O.D. Cortazar, D. Hartshorn, J.L.A. Chilla, Phys. Rev. Lett. 73 (1994) 2192. [9] R. Ramis, R. Schmalz, J. Meyer-Ter-Vehen, Comput. Phys. Commun. 49 (1988) 475. [10] T. Aoki, J. Meyer-Ter-Vehen, Phys. Plasmas 1 (1994) 1962.