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发表于 2005-8-28 08:50:18
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http://fans.equn.com/dingcong/lhc/LHC/challenges.html
Technical challenges
技术挑战
Important parameters
重要参数
In the LHC the energy available in the collisions between the protons will reach 14 TeV range, that is about 10 times that of past and existing accelerators. But "energy" alone is not enough. To guarantee an "effective physics programme", i.e. to make possible all the discoveries that the LHC is planning to achieve, there is another very important parameter to consider: the "luminosity".
在LHC中，质子碰撞所产生的能量可以达到14特电子伏特，使以往的加速器的10倍。但是，只有能量是不够的。如果要想保证一个有效的实验，比如说完成LHC所计划的探索，还需要考虑一个很重要的参数：亮度。
The "luminosity" of a collider is a quantity proportional to the number of collisions per second. Whereas in past and present colliders the luminosity culminates around L = 1032cm-2 s-1, in the LHC it will have to reach L = 1034cm-2 s-1. This will be achieved by filling each of the two rings with 2835 bunches of 1011 particles each.
碰撞的亮度是与每秒钟碰撞次数成正比的。然而，在以往的加速器中的碰撞亮度最多达到L = 1032cm-2 s-1，在LHC亮度可以达到L = 1034cm-2 s-1。在每个环中注入2835群含有1011个粒子就可以达到这一点。
These unprecedented energy and luminosity values impose new and stringent requirements on the machine design and performances.
这种前所未有的能量和亮度给设计与运行带来了严酷的考验。
Some disturbing effects
干扰效应
While touring their four hundred million revolutions around the machine, a number of effects contribute to dilute the beams and degrade the luminosity. Let's see the main ones and how CERN scientists are trying to cure them.
当粒子正在环行4亿圈时，有几种效应会使粒子束变弱，亮度变暗。让我们来看一下几个主要问题，以及CERN的科学家如何试着解决它们。
Beam-beam effect
粒子束效应
When two bunches cross in the center of a physics detector only a tiny fraction of the particles collide head-on to produce the wanted events. All the others are deflected by the strong electromagnetic field of the opposing bunch. These deflections, which are stronger for denser bunches, accumulate turn after turn and may eventually lead to particle loss. This beam-beam effect was studied in previous colliders, where experience showed that one cannot increase the bunch density beyond a certain beam-beam limit to preserve a sufficiently long beam lifetime. In order to reach the desired luminosity the LHC has to operate as close as possible to this limit.
当两群粒子在一个屋里探测器中间相遇时，只有一小部分的粒子会迎头相撞，产生我们想要的结果。其他粒子会被反向的群的电磁力推偏离。这种偏离，对密集的粒子群效应更大。如果在每一圈的偏离都累积起来，很可能会导致意外的粒子损失。这种粒子束效应已经在以前的加速器中被研究过。实验表明，我们如果想使粒子束保存实验所需的更长的时间，粒子束的密度就不能超过一个极限。为了达到我们想要的亮度，LHC需要尽可能地靠近这个极限。
Collective instabilities
集体不稳定性
While travelling down the 27 km long LHC beam pipe at a speed close to the speed of light, each of the 2835 proton bunches leaves behind an electromagnetic wake-field which perturbs the succeeding bunches and can lead to beam loss. These collective instabilities can be severe in the LHC because of the large beam current needed to provide high luminosity. Their effect is minimized by a careful control of the electromagnetic properties of the elements surrounding the beam, and by sophisticated feedback systems.
当在27千米的LHC管道中以接近光速运行时，每一个质子群都会留下一个影响下一群粒子的电磁场。这可能会导致粒子束的丧失。这种集体不稳定性在LHC中会变得更严重，因为为了达到更高的亮度，我们需要更大的粒子束。仔细地控制在粒子束周围的元素的电磁属性与利用改进了的反馈系统可以将这种效应减到最低。
Chaotic motion
混沌运动
In addition to the beam-beam interaction already mentioned, tiny spurious non linear components of the guiding and focusing magnetic-fields of the machine can make the motion slightly chaotic, so that after a large number of turns the particles may be lost.
除了已经提到的粒子束效应以外，引导以及聚焦的电磁场如果稍微与线性预定有偏离的话，会导致粒子束有混沌效应。这样的话，在许多次环行后，粒子可能会丢失。
In the LHC the destabilizing effects of magnetic imperfections is more pronounced at injection energy, because the imperfections are larger and a because the beams occupy a larger fraction of the coil cross section.
在LHC中，粒子注入能量中，不完美的电磁场产生的不稳定效应更为明显，因为缺陷的程度更大，粒子束占据环绕横截面的一个更大的部分。
There are two ways to cure this:
- We must evaluate the Dynamic Aperture (the fraction of the coil cross section within which particles remain stable for the required time) and make sure that it exceeds the dimension of the injected beam with a sufficient safety margin
- For the time being, no theory can predict with sufficient accuracy the long term behavior of particles in non linear fields. Instead we use fast computers to track hundreds of particles step by step through the thousands LHC magnets for up to a million turns. Results are used to define tolerances for the quality of the magnets at the design stage and during production.
解决它有两种途径：
1.我们必须估计动态孔（在要求时间内粒子束能保持稳定的环绕横截面部分）的大小，保证它比注入的粒子束横截面更大，并留下一道足够安全的边沿。
2.直到现在，还没有一个理论能充分精确地预言在非线性场中粒子束的长期行为。所以，我们用计算机模拟追踪数以百计的粒子一步步穿过LHC的磁铁上百万圈。模拟结果被用来确定在设计以及制造中电磁铁质量的可以容忍的偏差。
It's here that you can help! Join the SixTrack challenge downloading CERN@home and help CERN scientists to fight the effect of chaotic motion!
您能够在这里帮助我们！参加LHC@home，运行SixTrack程序，帮助科学家们对付混沌运动的影响！
Quenches
熄灭
Despite all precautions the beam lifetime will not be infinite, in other words a fraction of the particles will diffuse towards the beam pipe wall and be lost. In this event the particle energy is converted into heat in the surrounding material and this can "quench" the magnet out of its cold, superconducting state. A quench in any of the 5,000 LHC superconducting magnets will disrupt machine operation for several hours! To avoid this a collimation system will catch the unstable particles before they can reach the beam pipe wall, so as to confine losses in well shielded regions far from any superconducting element. To design effective controls systems, safety engineers are using extremely advanced computer programmes to perform coupled mechanical-magnetic-thermal analyses of stresses induced by a quench.
不管怎么细心，粒子束的寿命不会是无限的。也就是说，一部分粒子会散射到管壁并且丢失。在这种情况下，粒子的能量会在周围的金属材料中转化成热，又可能会导致需要低温的超导电磁铁“熄灭”。在LHC中5000个超导磁铁的任一个的熄灭，都会使机器的运行中断几个小时。为了避免这样的事情，一个瞄准系统会在不稳定的粒子碰到管壁之前捕获它们，从而限制在远离超导电磁铁的隔离区域内的损失。为了设计更有效的控制系统，安全工程师们正在用先进的电脑程序去研究耦合的磁铁受一次“熄灭”产生的热量的影响的分析。 |
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发表于 2005-8-27 22:28:22