Thursday, September 21, 2006

That's no laser, it's a particle accelerator

That's no laser, it's a particle accelerator

Geoff Brumfiel

Israeli physicists have turned a laser into a particle accelerator. Dubbed a paser — for particle acceleration by stimulated emission of radiation — the device accelerates bundles of electrons using the same principle as a laser.

At present it can only accelerate electrons by about 0.15% of their initial speed, but it could lead to compact particle accelerators and tabletop X-ray devices, according to Samer Banna of the Israel Institute of Technology in Haifa. He and his colleagues will publish their work in Physical Review Letters.

Conventional lasers exploit the quantum properties of atoms. An energy source is used to boost the electrons in a group of atoms into an elevated energy state. Passing light in the form of photons stimulates the atoms and causes the electrons to fall back to the lower energy level, emitting more photons in the process. These in turn stimulate more atoms and so on, so that a large number of photons are emitted. The photons are all identical, which makes the beam of light uniform.

Pasers work on a similar principle, but the output is accelerated electrons. Packets of electrons are fired into a cloud of excited carbon dioxide gas. As in a laser, the gas releases a large number of identical photons. But those photons are instantly absorbed by the passing electrons, which get an energetic kick, and leave the device moving more quickly than when they came in.

The fact that the paser uses atoms to speed up electrons sets it apart from other particle accelerators. "This is unlike anything that's come before," says Eric Colby of the Stanford Linear Accelerator Center in California. The unique mode of action makes the paser far more efficient than current machines, which achieve acceleration by generating enormous electric fields inside huge cavities. Colby is optimistic about the paser's potential. "It's a pretty small effect now," he says, "but there are strong technical reasons to believe that a very significant gain in acceleration is possible."

Levi Schächter, of the Israel Institute of Technology, believes that the paser could also make its mark as a source of X-rays. If the high-speed electrons have their paths bent after they leave the device, they will release a laser-like beam of X-rays that could be used for medical or nanotechnology applications. But Schächter is reluctant to guess exactly what may come of the technology. "In Hebrew we say, 'It's difficult to make predictions, particularly regarding the future'."

It wouldn't be possible to produce the exact equivalent of a laser beam with electrons — the Pauli exclusion principle states that electrons cannot exist in the same energy state at the same time. But laser equivalents can in theory be created for other types of particle, such as gravitons (which carry the gravitational force), phonons (packets of vibration) or some nuclei — if a system can be found that emits them. In June, for example, a group of researchers reported building a sound laser, or saser, that uses semiconductor technology to create a uniform beam of phonons (A. J. Kent et al. Phys. Rev. Lett. 96, 215504; 2006).

Colby says that these new systems show that it is easier than one might think to generate laser-like behaviour. "If you can store energy in a material," he says, "a great many things can be done."

1 comment:

Anonymous said...

作者:Geoff Brumfiel 文 葛韶锋 译
注释:from Nature
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以色列的物理学家们成功地将激光器改装成一台粒子加速器,并将其命名为PASER (Particle Acceleration by Stimulated Emission of Radiation),中文可翻译成:粒子激射器——这台装置可以利用和激光器相同的原理加速电子束。

来自海法(Haifa)以色列技术学院(Israel Institute of Technology)的Samer Banna说:到目前为止这台装置只能将电子的速度增加初始速度的0.15%,但是它可以用来制成微型的粒子加速器以及放在桌面上的X射线装置。 Samer Banna及其同事将在《物理学评论快报》上发表他们的工作。



由于是使用原子来加速电子,这使得这种粒子加速器和别的加速器完全不同。来自加利福尼亚斯坦福直线加速器中心的Eric Colby认为这与先前的任何东西都不一样。这种特殊的加速电子的方法使得粒子激射器比现在的加速器——其中粒子的加速是由强大的电磁场造成的——的效率要高很多。Colby对于粒子激射器的潜力非常乐观,他说:“虽然现在这种装置只是一个可爱的玩具,但是有足够的理由相信在加速技术方面取得巨大的进展是可能的。”

以色列技术学院的Levi Schächter相信这台粒子激射器同样可以作为一台X射线源。如果从这台装置中出来的高速电子运行的轨道被弯折,它们就会产生象激光一样的X射线,这可以用在医学和纳米科技中。但是Schächter并不愿意确切地猜测这项技术的未来到底会怎样。希伯来人常说:“做出预言是很困难的,特别是预言未来的时候。”

不可能用电子制造出和激光完全一样的装置,因为泡利不相容原理(Pauli Exclusion Principle)决定了不可能同时存在相同状态的电子。而对于其它的一些粒子,比如光子和引力子或是某些原子核,如果存在一种体系能发射这些粒子的话就有可能产生和激光器相同的激射器。比如在今年六月份,有一组研究人员报道,可以用半导体技术制造出能够产生具有高度一致性声子束的声子激射器(SASER)[1]。


[1] A. J. Kent et al. Phys. Rev. Lett 96, 215504; 2006