LHC & Bunch Crossing: Exploring the Why & How

[touqra]

I was reading about the LHC and came across the term "bunch crossing" in this sentence "The bunch crossing time will be 25ns and at full luminosity there will be approximately 22 proton-proton collisions per bunch crossing. "

It seems obvious that this means the protons are coming in bunches which will last for 25 ns. Why can't they make a continuous proton beam? Sorry if this question sounds stupid.

 

[ZapperZ]

There are several reasons, but I'll list just 1.

When charge particles are accelerated, the most common technology today is to use a "linac" structure. These consist of a series of cavities. However, these cavities are power by a high-power RF source from a Klystron. Essentially, what you have in each of the cavity is a standing wave oscillating E-field along the line of motion of the particles. So when you have a series of such cavities, and you only have one RF source for each linac structure, you then have E-field oscillating back and forth in each cavity.

So what happens here is that you need to inject the charge particle bunch at just the right phase when it enters the first cavity to get an acceleration. But you must also time it just right (by building just the right length between one cavity and the next), so that by the time it leaves the first cavity, enters a drift tube, and then enters the next cavity, the E-field phase will again start to build up in the right direction. This continues though the length of the linac. This is how most particle accelerator works.

So as you can see, if you have a continuous bunch, you will not get the same effect, because you will have bunches that will enter the linac at the wrong phase.

So why do you use oscillating RF as the source of the E-field for acceleration. Because we can get higher gradient at the right phase for smaller amount of power than using a static, continuous E-field. It is "cheaper" and less taxing on power consumption.

Zz.

 

[sir-pinski]

No question is ever stupid when it comes to learning about complex scientific concepts like the Large Hadron Collider (LHC) and bunch crossing. In fact, the term "bunch crossing" is a crucial aspect of how the LHC operates and is essential for achieving its scientific goals. Let's explore why and how the LHC uses bunch crossings.

First, it's important to understand that the LHC is the world's largest and most powerful particle accelerator. It is designed to accelerate protons to nearly the speed of light and then collide them together, creating a burst of energy that mimics conditions that existed just after the Big Bang. This allows scientists to study the fundamental building blocks of matter and the forces that govern them.

Now, why can't the LHC use a continuous proton beam instead of bunches? The answer lies in the sheer amount of energy and power needed to accelerate and control these particles. A continuous beam would require a constant and immense amount of energy, which is not feasible for the LHC. Additionally, controlling and focusing a continuous beam would be extremely difficult and could cause unwanted collisions and damage to the accelerator.

This is where bunch crossings come in. By dividing the proton beam into smaller bunches, the LHC can maintain a more manageable energy level and control the collisions more precisely. The 25ns (nanoseconds) bunch crossing time refers to the time interval between each bunch of protons passing through a specific point in the accelerator. This allows for a controlled and synchronized collision between two bunches of protons.

Furthermore, the number of proton-proton collisions per bunch crossing is also carefully controlled. This is because each collision produces a vast amount of data that needs to be analyzed by scientists. Too many collisions at once would overwhelm the detectors and make it difficult to extract meaningful information.

In conclusion, the use of bunch crossings is a crucial aspect of the LHC's operation. It allows for a more manageable and controlled acceleration and collision process, ultimately leading to the groundbreaking discoveries and advancements in our understanding of the universe. So, your question was not stupid at all, and I hope this explanation helps clarify the concept of bunch crossings in the context of the LHC.