Reine Versteegen (CERN Fellow) and Michaela Schaumann (PhD Student) are closely working with John Jowett in the heavy-ion programme at CERN in getting protons and lead-ions colliding at the LHC. These asymmetric collisions pose technical challenges for the experiments related to numerous effects that appear due to the different charges and energies of the two beams. We met Reine and Michaela and discussed with them about their motivation and the challenges they faced before the proton-lead run.
A.M What has been your previous background and what motivated you to go in the field of heavy ions?
Michaela: I studied particle physics in Aachen where I also pursued my MSc on the LHC studying beam-beam interactions. Following the completion of my MSc thesis I decided to continue with a PhD on lead-lead interactions as part of the heavy-ion programme at the LHC. I discussed this opportunity with John Jowett, the leader of the heavy-ion programme and we both thought that extending my previous research would be a fantastic opportunity.
Reine: My first degree was on nuclear physics and engineering in the Grenoble Institute of Technology, where I had the chance to prepare a master in Accelerator Physics and Technologies (JUAS, Archamps). During my PhD I studied the optimization of the final focus for the International Linear Collider (ILC). After the completion of my PhD thesis I decided to apply for a fellow position at CERN. The ILC was at the study stage and I wanted to work on something more applied and get more familiar with accelerator operation. Moreover I wanted to study in depth the operation of a circular collider as I already had experience with linear colliders. I am glad that I was finally selected by the committee to join the heavy ion group and to work on the LHC, the biggest collider ever built.
Fig.1 Left to right, John Jowett, Rossano Giacchino, Reine, Ghislain Roy, Michaela, Django Manglunki, Mike Lamont Michaela Schaumann.
A.M. Why did you decide to go in the field of accelerator physics? What do you think is special in this field?
Reine: I believe that accelerator physics is a very rich area in physics. We are always trying to push the limits of the machines taking into account the requirements of the scientific community, in order to give them the chance to get the best data for testing new theories. The goal is exciting and the phenomena we get to study in order to achieve that goal are very diverse.
Michaela: I agree with Reine that this is the most exciting part of working in the field of accelerator physics. It seems that as an accelerator physicist you don’t have to spend your whole time sitting in front of a computer but you can actually see something and get more excitement from your results.
Fig.2 Michaela Schaumann measuring the evolution of the beam emittances during the first Stable Beams for physics on 20 January 2013
A.M What do you spend most of your time on here? What is your research topic?
Michaela: I am mainly working on improving the lead-lead luminosity which is also needed for the ALICE upgrade plans. In order to increase the luminosities at the LHC we need to cope with many difficulties which arise because of the interactions between the two beams and the design of the machine. I am carefully studying these phenomena and try to come up with new solutions in order to overcome the present obstacles and get the desired result.
The easiest way to increase the luminosity is to increase the number of bunches in the beam. This is done by modifying the injection scheme that is used at CERN. As this process is linear we could eventually achieve the total luminosity that we want at the LHC. We could also think of having more particles per bunch but then we get other limitations related to the increase in the density of the bunch. During this year I spent most of my time in analysing data from the last ion run and I was trying to find new interesting effects. I also looked at the results from well-studied effects that occur in lead-lead collisions like the intra-beam scattering.
Finally, I also worked in ways to get around the aperture restrictions that we encounter in ALICE in order to increase the luminosity. Because of these restrictions we can’t increase the number of bunches per beam neither the space between bunches and I am trying to find solutions to these problems in order to reach the higher luminosities which are needed for the study of QGP. ALICE performs collisions in the vertical plane which means that by applying a new crossing scheme perhaps a tilted one that would include the horizontal plane you might be able to get around these limitations and increase the luminosity. I am also working on a different approach which is based on having flat beams instead of the current round beams of lead. These are the two project that I am working on and I am trying to figure out which approach will be more useful in the future.
Reine: I am mainly working on the proton-lead asymmetric collisions. The main focus of my research is to study and possibly develop an analytical approach to describe the beam-beam electromagnetic interaction in the case of separated revolution frequencies (RF) which we get at the LHC during the injection and the ramp of asymmetric beams. I am also working on optimizing the setup of the machine for the forthcoming proton-lead and for that reason I analysed data from previous test runs of proton-lead beams.
Fig.3 Reine Versteegen and Mariusz Sapinski study data from the beam-gas ionization monitor during the first Stable Beams for physics on 20 January 2013
The problem of different RF in the case of asymmetric beams appeared at RHIC when they collided deuterium with gold. As RHIC is designed to collide asymmetric beams it is made of two separated rings in which different magnetic fields can be applied. In that way they could choose whether they would use the same magnetic rigidities or the same frequencies in both rings. First they had the same magnetic rigidity because it was the easiest thing in operating the system. However, this running mode caused big problems of instability and beam losses. Therefore they decided to step back and apply same RF frequencies in the two rings which eventually worked for them. However, at LHC we can’t apply different magnetic fields in the two rings as we have the so-called ‘2 in 1’ configuration for the magnets. This means that we have the same magnetic rigidity in the two beams. However given the small frequency difference, and based on simulations and the previous tests we don’t expect to get the same dramatic effects that they encountered at RHIC.
Fig.4 Radiofrequency cavities in the LHC tunnel had to be retuned to accelerate protons and lead ions (Image: CERN)
In order to demonstrate the feasibility of p-Pb collision at LHC and to tackle with the effect of having two different RF frequencies in the two rings, some tests were designed. They were meant to help us to explore the upper limit of proton intensity that would be tolerable for the lead beam stability. We had the first part of the test last year (2011). During that run few bunches of lead and few bunches of protons were injected, then we increased the number of proton bunches. Beams remained at injection energy (450Z GeV), they were not ramped to maximum energy (3.5Z TeV in 2011). This first test was successful. The plan was then to do a second test in which we would increase the proton energy from the injection level to top energy of 4Z TeV. This test was scheduled for last September (2012), just before the recent proton-lead pilot run but due to some technical problems we didn’t have the chance to do it.
Fig.5 Philippe Baudrenghien and Delphine Jacquet prepare the special manipulations of the LHC RF system required to accelerate proton and lead beams simultaneously.
A.M. Reine, I would like to ask you about the recent proton-lead run in LHC and what exactly have you achieved?
Reine: During the proton-lead run we had 13 bunches of lead against 13 bunches of protons that were injected at the energy of 450Z GeV and ramped to energy of 4Z TeV. During this pilot run we put the beams into collision without squeezing them. The pilot run had great importance both for the QGP studies which are of interest for ALICE but also for the accelerator department. It was the first time that we injected, ramped and had these asymmetric collisions in the machine, so we had the chance to test many new procedures in the LHC operation. We also had the oppportunity to learn more about the beams’ behaviour by getting a lot of data about the emittance, beam growth, the intensity evolution and the beam lifetime.
A.M Michaela do you only work on lead-lead collisions or do you look at proton-lead?
Michaela: I mostly work with lead-lead. Of course I participate in all the runs and data analysis from the proton-lead test run. Unfortunately due to the long-shutdown of 2013 I won’t have any new data from real collisions for my thesis. This is why I am planning to work with the data from the lead-proton collisions as they might be handy for my research. It is always better to have some real data rather than only simulations.
A.M. How well do your simulations fit with the data?
Michaela: The agreement is quite good although I am still getting some deviations which can’t be explained. This might be a problem of the simulation I am using and the assumptions that I am making; for example I am currently assuming that our beam profiles are Gaussian which is probably not the case. Wrongly assuming this Gaussianity affects the evolution of the bunch-length in the simulation. I am now trying different beam profiles in order to get better fits with the data from the physics runs.
Once I fix this I will be able to explore the role of other parameters and see what happens when the intensity increases or if we have smaller bunch-length or different emittances. From the beam parameters I can also study the evolution of the luminosity and see how we can get higher luminosities which is the ultimate goal!
A.M. Which are the main aims of the proton-lead collisions that will take place in January?
Reine: Proton-lead collisions were requested to benchmark Pb-Pb data. Of course the main goal of the run is to provide the experiments enough data for that. From the accelerator physics point of view, this way of operating the machine will be very new, and we hope to be able to collect data to study the effects I mentioned earlier.
Michaela: As John often mentions the proton-lead run is actually the first upgrade of the LHC since it was never mentioned in the design report. I think this will be a special moment not only for particle physics but also for accelerator physics since we will be trying a lot of new things.
A.M. Which are the main differences when you accelerate heavy ions compared to protons?
Reine. I would say the order of beam losses and the mechanisms that cause them. In the case of heavy ions we have a larger intra-beam scattering which causes higher losses in its package of heavy ions. You also have faster emittance increase which leads to shorter beam lifetimes. This also means that you have to refill more often. In the case of Pb-Pb collisions, secondary beams are also emitted from the interaction points due to parasitic reactions. This causes protection issues since they do not follow the same path as the main beam.
Michaela: For heavy ions collisions we have to be very fast. If the bunches are accumulated then we stay at low energies which means that there will be stronger effects of intrabeam scattering and higher losses. We need to avoid this situation as it significantly reduces the luminosity which after all means less data for physics analysis. The longer you need to inject and ramp your beams the worst they will get. The same phenomenon also appears in the case of proton beams but it is not as limiting as in the case of heavy ions.