Crystalline Beams - Present Experimental Status

Experiments related to Crystalline Beams are in the phase of rapid expansion. Until recently, only some aspects and neccessary technologies were examined. Theoretical advances , success with experiments involving neccessary cooling techniques and high interest for this topic caused several European goverments to invest in building machines particularly for the Crystalline Beam investigation. As most focused projects one can mention ASTRID in Denmark and CRYSTAL in Italy.

However, Crystalline Beam is NOT YET OBSERVED phenomenon. Best "approximations" we did observe are the ordered states of ions confined in Pauli traps:

consisting of "ring" and "caps" electrodes which can trap ions from the some beam by varying electric fields applied to them. These ions can be cooled down by a version of laser cooling up to a point at which they "crystalize" in some structure. Interesting result shown below represents cases of two, three, four and seven ions crystalized in the ordered structure by Pauli trap mechanism. The average ion separation is 20 micrometers (picture by H.Walther, Max Planck Institute, Germany).

Ions in the Pauli trap are not exactly Crystalline Beam as they are "static". Also, "micromotion" is still present in that case. To go to the next step experimentalists devised a very small accelerator ring consisting entirely of one quadrupole magnet:

In such a small ring it is much easier to observe structure similar to the actual Crystalline Beam expected in the larger machines under construction. Laser cooling was used in this system, too.

As a result of experiments with above mentioned quadrupole stroage ring, group from the Max Planck Institute in Germany have observed the following crystalline structures of Mg ions. Depending on the number of ions "forced" to "cohabitate" the ring, structures varied from the simple (low density) string to the helical structures (at higher densities). Order of particle separation is similar to a Pauli trap case. Micromotion is absent.

These two observations are as close to a crystalline beam as we got by now. There are other, more or less similar experiments done in other laboratories through the world. Couple of observations in the NAP-M ring in Novosibirsk and ESR ring in Darmstadt could be explained by crystalized beam. However, they are not completely conclusive.

There are many related topics which need investigation before we succesfully construct "working crystalline beam" machine. Questions about the limits of laser (and other) cooling mechanisms, space-charge influence on the emmitance and tune shifts caused by it, general particle stability questions (extreme stability is needed and not many experimental data is known about effects of such common accelerator design issues as impedance, rf buckets, nonlinear elements, etc., at such extreme conditions.

This was compilation of the data from Crystalline Beams and Related Issues book, edited by D.Maletic and A.G.Ruggiero. For more info consult the source.

Dusan Maletic