Target Efficiency Simulation

A simulation program was developed g[9] to study the basic properties of a halo target. Single halo particles are generated and tracked through the HERA proton machine until they are absorbed in the target or hit an aperture limitation. The particle tracking in linear optics uses single turn transport matrices, coupling is introduced artificially by a skew quadrupole. Diffusion is taken into account by the former given parametrisation; scattering and energy loss of protons in the target are simulated, losses or interactions are calculated. The simulation contains a lot of parameters, not all of them are well defined:

Geometries:

the detailed information concerning the HERA ring geometries and its aperture limitation is rather complex and even not always well known. The simulation uses actually only one limit in each transversal direction; this seems to be a proper approach as long the collimator system defines the narrowest part of HERA.

Optics:

The simulation assumes linear optics; the severe question is, whether the region outside 4 is dominated by nonlinear effects. At HERA nonlinear impacts are expected, e.g. the dynamic aperture or stable resonances in the halo region. But the very high intrinsic proton lifetime is an indication that the machine is even for larger betatron amplitudes in good approximation linear.

Diffusion:

There exists just a poor knowledge and understanding about diffusion processes in the beam halo. Statistical physics with its basic transport equations together with some measured data provides the frame. The high proton lifetime and the interpretation of HERA-B target data indicates rather small drift velocities. A deeper understanding of beam halo dynamics is an important goal of actual and further target studies, and the simulation program is therefore a powerful tool.

Fluctuations:

The real proton machine shows a wide variety of fluctuations and disturbances, something what is until yet neglected in the simulation.

The target efficiency is either limited by diffusion or by multiple scattering. In the first case more target material and material with larger Z improves the efficiency. But the HERA-B target is mainly dominated by multiple scattering. Fig. 8 shows for this case the results of simulations with various target materials and different target positions for collimators located at 7 and 9 .

  
Figure 8: Target efficiency as function of the wire position for various target materials and two different collimator positions.

The following list tries to summarize the most important results of the simulations: