HEBT Design Considerations Jingyu Tang, Xiangqi Wang, Hao Hao, Jiajia Tian IHEP, USTC International Review Meeting on Accelerator Physics Design of C-ADS, September 19-20, 2011

Outlines Design goals and features of the HEBT HEBT for different phases Interface with target/reactor

Design goals and features of HEBT To transfer the beam from the linac exit to the target/reactor assembly or to the beam dump Beam preparation for irradiating on the target Treatment of back-streaming neutrons Proton beam window separating HEBT vacuum from the target Beam spot monitoring at the target Other interface issues

HEBT for different phases Beam characteristics from the linac in the different phases are quite different: beam energy and beam power Target/reactors or beam dumps are different: different designs and locations We need to study the requirements for injecting the beam into the target/reactors (not clear at the moment)  Discussions with the target system  For the checking point in 2013: no HEBT needed  For the checking point in 2015: only beam transport to beam dumps needed (50 MeV)

 For the checking points in 201X, 2022 and 2032: transport beam to both beam dumps and to T/R assemblies.  Beam dumps and T/R assemblies are off linac tunnel. Avoiding back-streaming neutrons and gamma (residual)  Once the linac is to be upgraded in energy, the old HEBT will be dismantled, old beam dumps and T/R assemblies will be no longer used. (compact linac design)

Interface with the target Supposed features  Achromatic vertical bending down  Uniform beam spot at the target  Collimation and stopping of back- streaming neutrons from the target  Proton beam window separating the vacuum from the target We need to discuss with the target system about the requirements of injecting beam into the target (e.g. 10 mA might be too high at Phases I,II

Achromatic vertical bending down Purposes  Vertical beam injection  Achromatism helps stabilization of beam spot at the target due to energy wobbling (depending on the beam uniformization method)  Filtering back-streaming neutrons  BPM at dispersive location for energy monitoring Method  Symmetric double-bending (achromatic)  Scaling with the beam energy

Uniformization of irradiation spot at the target Uniform beam spot is very useful to reduce the peak heat density in the target and in the proton beam window. Different methods to produce uniform beam spot are under study, e.g. by non-linear magnets, by raster magnets, by rotating magnetic field etc.  A very difficult situation here is the strong back-streaming neutron flux in the vertical HEBT section. Scanning magnets may not survive long there.

Proton beam window There are two beam windows between the HEBT and the target  The window separating the vacuum in HEBT from the target/reactor assembly (belonging to accelerator)  The window in the target vessel (belonging to target, perhaps without window or liquid surface window)  It might be possible to merge the two windows into one For the PBW in HEBT, three key issues  Heat deposit (more critical for lower energy at same current)  Radiation damage  Beam scattering effect (window close to the target)

A PBW based on aluminum alloy and cooling-water in sandwiched form is under study (for 0.6 GeV and 1.5 GeV, 10 mA)  Current density at PBW of 200 mm in diameter: mA/cm^2 ESS designed a multiple-pipe type PBW standing for 0.15 mA/cm^2 at GeV (total 5 MW, pulsed beam)

Back-streaming neutrons The back-streaming neutrons along the incoming proton path emitted from the target is extremely intense, due to the very high beam power and the large PBW (or large solid angle)  More than 10 times of the neutron flux at SNS (there sophisticated collimation system and shielding system are used, radiation-hard magnets are used) The neutrons may damage the devices in the HEBT or reduce their lifetimes, including magnets, beam diagnostics, vacuum devices, PBW etc.

Carefully designed collimation system will help reduce the neutron irradiation to the devices A neutron stop together with the last HEBT bending magnet helps remove the neutrons with small solid angles.