以對波導結構進行改變來在電漿波導式雷射電漿波電子加速器中增強電子振盪產生X光脈衝的效率

桌上型硬X光脈衝是利用雷射脈衝在電漿中激發的電漿波來加速電子，並藉由高能電子在電漿中進行橫向振盪產生飛秒(〖10〗^(-15) 秒) X光光源。實驗將會使用本實驗室在國立中央大學所建立的20兆瓦(TW)與100兆瓦(TW)雷射系統，在實驗中將會同時控制五道雷射光。首先運用點火與加熱的雷射脈衝形成長約10毫米(mm)的電漿波導，1.9奈秒(ns)後主脈衝雷射被聚焦成10微米(μm)並打入上述的電漿波導中，主脈衝雷射將會在電漿波導中產生電漿波，搭配第一道橫向加熱脈衝聚焦在電漿波導上，此時電漿波導的結構將會被改變，使得電子更容易被電漿波束縛住並加速電子，當電子被電漿波加速到能量約200 MeV以上時，這些高能電子同時會在電漿波導的通道中一邊往前傳播一邊進行橫向的振盪運動，此時第二道橫向加熱脈衝也會聚焦在後方一小段電漿波導上，再次加工電漿密度結構，因此可以在電子束傳播的路徑上製造出一段低密度區域(drift space)。當電子束經過此低密度區時橫向振盪的振幅會增加，因而產生高能量、高強度的X光脈衝。而這一切事情的經過都會在遠小於1秒內完成。　　藉由此種方式，產生的X光能量約為1∼10 keV相當於波長0.1奈米(nm)。對比於同步輻射中心需要大量的空間與昂貴建造價格，此種方式可在一般實驗室內產生且不用昂貴的建造經費，就可以得到桌上型的超短脈衝X光源。除此之外，此X光源更有脈衝長度短(達到飛秒等級)的優勢，相較於其他X光源有很高的時間解析能力，因此在瞬態分析上獨具優勢。最後，產生的X光將會傳播至實驗站後方的診斷系統對其做完全診斷，如此一來，即可得知此X光源的光子數量、發散角等資訊。而此種產生桌上型硬X光脈衝的方式稱為Betatron Radiation。

關鍵字

電漿波導式雷射電漿波電子加速器； X光脈衝

並列摘要

Here we demonstrate the use of laser-driven plasma accelerators, which accelerate high-charge electron beams to high energy in short distances. The particles being accelerated in the plasma accelerator also undergo transverse (betatron) oscillations to intrinsically ultrafast beams of hard X-rays. The experiment was performed at National Central University, Taiwan, using a 100-TW-class Ti:sapphire laser system with 10-Hz pulse repetition rate. Five laser beams from this system were used for the experiment. The axicon ignitor and heater pulses were used to fabricate a ∼ 1-cm length in full width at half maximum (FWHM) uniform plasma waveguide in a gas jet. The 1.1-J, 40-fs pump pulse was coupled into the plasma waveguide at a delay of 1.9 ns with respect to the axicon heater pulse to excite a plasma wave (plasma bubble) which can accelerate electrons. The focal spot size was 10 μm in FWHM. By adding a transverse heater 1 pulse into the axicon ignitor-heater scheme for producing a plasma waveguide, a variable three-dimensionally structured plasma waveguide can be fabricated. With this technique, electron injection in a plasma-waveguide-based laser wakeﬁeld accelerator was achieved and resulted in production of electron beam energy large than 200 MeV. Then a 116-mJ, 210-ps transverse heater 2 pulse was used to introduce lower density spatial gaps between uniform plasma density sections that behind the fabricated plasma waveguide. When accelerated electrons that enter the depression at the proper phase in their betatron oscillation will increase their transverse displacement, thus exiting the depression with larger betatron amplitude. This technique opens a route to a compact hard-X-ray pulse source. We could produce x-ray with photon energies in the range of 1–10 keV by using lower density spatial gaps between uniform plasma density sections. If we compare laser- driven betatron-radiation x-ray source with conventional particle accelerator facilities, we find the size is significantly smaller and the cost is much cheaper. This reduces the size of the synchrotron source from the tens of meters to the centimeter scale, simultaneously accelerating and wiggling the electron beam. Then the betatron radiation has intrinsically striking features for ultra-fast imaging. Therefore laser- driven betatron-radiation x-ray source has the potential to be a table top hard-X-ray pulse source.

並列關鍵字

betatron oscillation ； X-ray pulse

參考文獻

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[2] C.E. Clayton, C. Joshi, C. Darrow, and D. Umstadter.“Relativistic plasma wave excitation by collinear optical mixing” Phy. Rev. Lett.,54,2343 (1985).

[3] D. Strickland and G. Mourou “Compression of amplified chirped optical pulses. Opt. Commun.,56,219 (1985).

[4] E. Esarey, P. Sprangle, J. Kral, and A. Ting. “Overview of plasma-based accelerator concepts.” IEEE Trans. Plasma Sci., 24,252 (1993).

[5] K.Y. Kim,I. Alexeev, and H. M. Milchberg.”Single-shot super continuum spectral interferometry”. Appl. Phys. Lett 81, 4124 (2002).

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以對波導結構進行改變來在電漿波導式雷射電漿波電子加速器中增強電子振盪產生X光脈衝的效率

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