| 摘要: | 相 對 於 傳 統 的 直 線 電 子 加 速 器 ， 雷 射 尾 波 場 加 速 器 (Laser Wakefield
Accelerator)具有更高的加速梯度，甚至可以到達每公厘數百 MeV。如果能克服
一些挑戰如：穩定度不夠高、電子能散不夠低、發射度不夠小，它將是一個良好
的桌上型自由電子雷射的電子源。
當高強度雷射打在氣體靶材上，氣體會被游離成電子和離子，這些電子會被
雷射的有質動力推向外側，再被原本的離子吸引回來，這樣會在雷射後面形成一
個離子腔的結構，也稱作「泡泡(bubble)」，當電子進入泡泡中，便有機會被加速，
而控制電子進入加速場的方式稱為注入機制，而注入法會對產生的電子表現有很
大的影響。
我們使用中央大學強場物理實驗室的高功率飛秒雷射，我們在實驗室中進行
了三種不同注入法的雷射尾波場加速實驗：第一種是游離注入法，利用高原子序
的氣體如氮分子的內外層電子的游離能差距，因為內層電子游離的位置更接近雷
射中央，其有較大的機會注入，利用其容易注入的特性，在實驗上可以更快幫助
我們找到實驗條件如雷射聚焦位置和雷射的群延遲色散(group delay dispersion)。
第二種是自注入法，當雷射超過臨界強度時，雷射尾波變得非線性，便會導致波
崩，游離的電子軌跡變得不穩定，隨時都有能自己進入加速區而注入。第三種是
震波前注入法，在氣體噴嘴上方插入刀片後，噴氣時產生的震波前是一個短距離、
高梯度的密度分布，當雷射經過這個震波前時，泡泡會瞬間被拉長，使原本在泡
泡後面的電子被注入，因為注入發生的時間很短，產生的電子能量會很集中，所
以震波前注入法是我們實驗的主要目地。
最後我們成功藉由震波前注入法產生單能的電子，其鋒值能量到達 125 MeV，
能散卻只有 7.5 %，雖然電子電量僅有 5.2 pC，其發散度也只有 2.5 mrad，我們
想進一步優化氣體的噴嘴和雷射的品質，來產生更穩定的單能電子。;Compare to traditional linear electron accelerator, Laser Wakefield Accelera-
tor (LWFA) have higher acceleration gradient which is about hundreds of MeV
per millimeter. If we can overcome the challenge of high stability, low energy
spread, and low emittance, it is a good electron source to construct laboratory-
scale Free Electron laser (FEL).
The wakefield is generated by the ionization of the gas by a high intensity
laser. The ionized electrons are expelled by the ponderomotive force and then
attracted by the ions. Thus, there will form an ion cavity which is called a bub-
ble behind the laser. The bubble can trap electrons and accelerate them, so the
way to make the electrons move into the bubble is called injection method. We
conducted the experiment of LWFA by three different injection methods: ioniza-
tion injection, self-injection, and shock-front injection with 100 TW laser system
in NCU. The ionization injection is a simple way to generate electron beam by
a gas target with high atomic number. The electrons at inner shell are only ion-
ized at the center of the laser beam, so these electrons are easy to be injected.
This property can help us to find the preliminary shooting parameters, such like
laser focus position and group delay dispersion (GDD).
Self-injection occurs when the laser intensity is high enough, the plasma wave
may become broken. Thus, the electrons may be injected by itself throughout the
plasma channel. The shock-front injection happens when the laser pulse passes
through a shock front with high density gradient in micrometer scale. The sud-
denly dropped density makes the bubble length increase, so the electrons at the
back of the bubble may be injected. If the self-injection doesn’t occur, the in-
jection volume is only near the shock front. As a result, the energy spread is
the smallest among these three injection meachanisms although the aceleration
length is reduced by the shock front position.
In our results, the monoenergetic electron beams with the peak energy of
125 ± 12.2 MeV, FWHM energy spread are only 7.5 ± 1.68%, charge of 5.2 ±
2.2 pC, FWHM divergence of 2.5 ± 0.8 mrad, are generated by the shock-front
injection. However, the probability of these monoenergetic shots is only 15 %.
The stability of the electron means should be further improved by improving
the gas jet design and the laser quality. |
