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HESR for Pedestrians

The Institute for Nuclear Physics 4 (IKP-4) of the Research Center Jülich (FZJ) is in charge of 65 M€ investment for the High Energy Storage Ring (HESR), which is a German contribution to the international facility FAIR at Darmstadt. Accelerator assembly and commissioning will happen as soon as the necessary buildings and other infrastructures are completed by the FAIR GmbH. The dipole and quadrupole magnets were designed by IKP-4 together with experts from ZEA-1, and were directly contracted with Jülich specifications by FAIR GmbH. All other contracts have been and will be handled by the Jülich purchase department. IKP-4 leads the HESR consortium with members from FZJ, GSI and partners from Romania and Slovenia.

HESR Tasks

The HESR is a synchrotron into which electrically charged particles are injected with a magnetic rigidity of 12.76 Tm. These particles are subsequently accelerated or decelerated according to the requirements from various experiments. Beam size and quality can be controlled within certain range. It is known that for a bending dipole, charged particles with higher energy, i.e. travel faster, are bent less than charged particles with lower energy, i.e. travel slower. In an accelerator the situation is similar: The force bending the particles onto a defined beam path must match the particle velocity. Therefore, during acceleration the current through the magnets has to rise synchronously with the velocity to keep the particles inside the beam tube.

The fundamental research experiments that HESR is designed to serve for is to study the properties of the so called strong interaction (this is the force that keeps the constituents of the nuclei together against the electrostatic repulsion). Many details still need investigation. However, patience will be required. There is an example in the history of nuclear physics where it took about 50 years between a new discovery and an established application: Around 1930 it was discovered that the proton has a new property, which was called spin. Systematic investigations of the spin of the proton created the fundamental knowledge which led in the 1980s to the development of the first usable magnetic resonance imaging device which is nowadays a routine tool in medicine. Moreover, mass storage discs for computers with high information density could be developed (Giant Magneto Resistance), and there is still more emerging from this discovery.

Particle properties in the HESR
Magnetic Rigidity / TmProtons: Kin. Energy / GeVProtons: velocity / speed of lightProtons: relativistic mass factor
At injection12,763*)0,9714,2
At minimum energy51,770,8481,89
At maximum energy5014,10,99816,0

*) Injection energy is chosen to correspond to the production energy of anti-protons. Thus, the HESR is suited for the acceleration of anti-protons.

HESR consists of two straight sections with length 132 m and two arcs of 155 m length. The overall inner diameter of the beam pipe is 89 mm, neglecting details in the region of the experiments. The average bending radius of the arcs is nearly 50 m. One of the straight sections is reserved to house a spectrometer, whereas the other one might be equipped at a later time with an electron cooler to improve the beam quality. Start and end of the arc sections are also suited to house experiments.

Main components of HESR

HESRFloor plan of HESR

Magnets

The dipole magnets (north pole and south pole are above and below the beam pipe) produce a homogenous magnetic field which are bending the particle path onto (part of) a circle. HESR needs 44 of such magnets. Each one is 4.2 m long. Including girder its mass is app. 35 t. The magnetic field can be varied between 0.17 and 1.7 T. The current in the coils must be up to 3000 A to produce the required magnetic field. As during acceleration the magnetic field is varying, eddy currents will emerge within the magnet iron and will weaken the field which is needed for particle deflection (induction, Lenz’ rule). To minimize the weakening of the field, the iron core of the magnet is made of many lamellas which are isolated against each other. A water pressure of 6.5 bar is sufficient to press 62 l per minute through the cooling channels of the coils. At this flow the temperature increase of the cooling water at maximum load (100 kW) can be limited to 30°C.

Quadrupole magnets (north pole, south pole, north pole and south pole are arranged around the beam pipe) are being used to focus the particle beams. In HESR, app. 108 particles will circulate. Most of them, however, will not travel on the ideal path. Instead, most of them will travel with some distance to the ideal orbit and even might travel with a small angle to it. Quadrupole magnets have no field on the center line. It rises linearly with the distance to the center line. Thus, particles off the center see forces bending them back to the ideal path. This magnet acts like an optical lens. Each quadrupole magnet act in one plane (e.g. the horizontal one) focusing and in the other plane (in this example the vertical one) defocussing. If the direction of the current is reversed, the focusing and the defocusing effect are interchanged. In total, 84 quadrupole magnets will be installed in HESR. Each of them will be 60 cm long and have a weight of 5.2 t. It takes a power of 12 kW with a maximum current of 426 A.

For fine tuning of further beam properties other magnet types are needed: 66 sextupole magnets and 53 steerer magnets. Steerer magnets are small dipole magnets. Both magnet variants are produced with a length of 30 cm and will have a mass of app. 300 kg. In the arcs, they will be mounted up- and downstream of the quadrupole magnets. All sextupole magnets, steerer magnets and their power converters will be supplied by our Romanian partner.

Power converters

All magnets are grouped into “families” according to their function. Thus, the number of power converters shall be minimized. Furthermore, the number of power converter variants shall be kept at minimum. We want to keep the number of cables spanning from one arc to the other along the straight close to zero. Thus, all magnets in the northern half of the HESR will be supplied by power converters in the northern supply hall. The remaining magnets will be supplies from the southern supply hall. In total, 125 converters with power between 9 and 3500 kW will be installed.

RF station

Beam particles will be accelerated in HESR at exactly one place, the acceleration gap. There, the electric component of a high frequency wave is used to let a voltage of up to 2500 act on the charged particles when they cross the gap. With each crossing their velocity increases and subsequently magnet currents and frequency of the electromagnetic wave can be increased accordingly. The particles need about 2 µs for one revolution. The RF station is operating between 440 and 520 kHz. In terms of radio engineers, the frequency is within the medium wave regime. The RF work package also takes care of another specific detail: The HESR has to accumulate the required number of particles. In the case of anti-protons, we are expecting the injection of 108 particles every 10 seconds. Reasonable experiments need app. 1010 particles. For accumulation, the scheme will be the following: Using a dedicated wave form (barrier bucket) the circulating particles can be confined to one half of the ring, and the remaining half will be empty. Fresh particles will be injected into the empty half of the ring. It can be done within 1 µs. Subsequently, the barrier between the two halves will be removed, and the two buckets will merge during the next 10 seconds supported by beam cooling. This process will be repeated until 100 injections have occurred. The number of particles then will have reached its final value and the beam can be prepared for the experiment.

Injection

For injection into HESR strict rules will be applied: All necessary actions have to be completed within 1 µs (= 1000 ns). Step 1: The injection kicker magnet has 250 ns to deliver a stable magnetic field with the required strength. Step 2: Within 500 ns the new particles will flow into the HESR. Step 3: Within the remaining 250 ns the magnetic field must decay to a value which can be tolerated for the circulating beam. Some key data: The deflection angle of the injection kicker is 6.4 mrad (1.152°). The deflecting field will be generated by a current of nearly 5000 A. Even if designing the magnet with lowest possible inductance it is unavoidable that the current exceeds the required value and returns to it afterwards (over shoot). A similar situation is when the magnet current is switched off. A residual field at the injection kicker above a low limit would send the already stored particles to the chamber wall and the particles would be lost. The details of the pulsed operation of this kicker magnet depend on the geometry of any conductive material surrounding the current bearing parts. The system performance will be evaluated with a prototype. First measurements are scheduled for 2015.

Beam Diagnostics

For operating any accelerator including the HESR it is essential to measure the position at many places along the beam path without disturbing the beam too much. Of course, the intensity and the time structure of the beam are also of interest. Development and operating of such devices is covered by the beam diagnostics work package. Due to place restrictions it was necessary to integrate the beam position monitors into the sextupole magnets. Thus, the gap in the sextupole magnets is 140 mm instead of 100 mm in all other magnets. The performance of the first beam position monitors will be measured in 2015. Most other beam diagnostics components can be purchased. The read-out electronics will be supplied by our Slovenian partners.

Vacuum

When circulating particles in an accelerator hit other atoms their charge state will change and they will be lost. Consequently, setting up a suitable environment for charged particle beams always involves preparation of high quality vacuum. A large variety of tasks are bundled in the vacuum work package. All chambers which need evacuation are designed, manufactured and controlled. Number and kind of vacuum pumps are decided as well as number of valves and gauges. In addition, for all components girders have to be designed and manufactured allowing precision positioning on the 0.1 mm level for entrance and exit. Thus, any component has to be equipped with at least 3 fiducials. For being prepared for operation at a pressure lower than 10-9 mbar all beam tubes which later on will be accessible only with severe effort will be prepared with a getter layer (NEG) acting like a vacuum pump. This pumping ability can be activated by heating to app. 250°C. The corresponding increase in length of all components has to be taken into account already now to avoid deformations during a heating cycle. Also, smooth transport of components during assembly and in case of maintenance is within the responsibility of this work package which might be some challenge especially for the 4.2 m long and 35 t heavy dipole magnets.

Beam cooling (stochastic cooling, electron cooling)

Each cooling method aims to reduce the spread of particle properties as position, direction of flight, velocity. If these parameters have a large spread the beam would be considered hot, otherwise cold.

In the start version the HESR will be equipped only with stochastic cooling. Many times per revolution (with a frequency of 2-4 GHz) the position of the particle beam is recorded at the sensor or pick-up. A correction signal is derived and sent to the actor or kicker (not to be confused with the injection kicker magnet). When the measured particles pass the kicker the correction signal is applied and the corrected particles will travel along a path closer to the ideal orbit. As the particles and the correction signal both travel with nearly the same velocity (speed of light), the correction signal has to take a short cut to be available at the kicker system in time. Pick-up and kicker are made of the same hardware. It has been developed and manufactured in Jülich. At the NICA accelerator in Russia these devices proved their capabilities.

A possible prototype for an electron cooler at injection energy of the HESR is being operated successfully at COSY to cooled proton beam with momentum of 2.4 GeV, the highest proton energy so far reached by e-cooler.

Integration of experiments

Integration of the experiments into the accelerator is an important task which needs careful planning. Effects of the targets (objects that are hit by the particle beam) on the accelerator need to be minimized. The space for the detectors needs to be reserved. If dedicated magnets are to be used, decisions on compensating measures need to be taken. Frequently, the experiments require small beam spots at the target, which are only possible if considered already during the planning. State-of-the-art experiments need close cooperation between accelerator operators and experimentalists, and both areas are being merging. At the moment, there are two users planning experiments at HESR: One is the PANDA collaboration which is dedicated to experiments with protons and anti-protons in the energy region of the charm quark. The other user is the SPARC collaboration aiming at questions related to heavy ion physics.


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