High Energy Electron Cooling D. Reistad The Svedberg Laboratory Uppsala University

Low energy electron cooling is a well established technique. CELSIUS cooler was used to cool protons and ions up to 500 MeV/nucleon Electron Cooling Principle CELSIUS Electron Cooler Principle of electron cooling Björn Gålnander, SFAIR-meeting,

where is the “effective” electron velocity is a Coulomb logarithm. What is special with high-energy electron cooling? It has become popular to use the so-called “Parkhomchuk formula” for the (“magnetized”) electron cooling force:

What is special with high-energy electron cooling? It has become popular to use the so-called “Parkhomchuk formula” for the (“magnetized”) electron cooling force: One can derive a “cooling time”: where a e is the electron beam radius,  is the (un-normalised) rms. emittance,  * is the beta-value at the cooling section, and  eff is the effective electron angle at first sight, the  5  5 -scaling looks quite discouraging…

note the importance of we have to make the effective angle very small, e.g. and would have given at 8 GeV in the HESR with

HESR 0.45 ­ 4.5 (8) MeV up to 1 A 2,000 gauss 5 mm mbar 24 m continuous longitudinal magnetic field 2,000 gauss

the magnetic field is too low for the electrons to appear as “disks”. Therefore, formula tells us that cooling becomes 125 times slower, compared to case when, however Coulomb logarithm becomes better by factor about 4. Still, the “unmagnetized cooling” is much weaker than “magnetized cooling”. In passing, we note that if then “magnetization” may not help (RHIC).

Most important difference between HESR and Recycler electron coolers is the magnetic field. However, we need to remove some of the advantage, because it is not useful to have the beam spot on the target too small. We need to keep the transverse emittance from becoming too small, even though this makes the cooling force weaker. But keeping the emittance from becoming too small also eliminates (most of) intra-beam scattering. The resulting becomes about the same.

HESR Electron Cooler Energy MeV Current 1 A Solenoid field T Straightness (rad rms)1·10 -5 Interaction length24 m Bending radius4 m Björn Gålnander, SFAIR-meeting, Magnet modules Based on adjustable pancakes to achieve straightness

0.8 mm 80 % 3.5 where CHOICE OF BEAM SIZE AT TARGET

Experience from existing electron coolers is that it is easy to not align perfectly and get broad transverse distributions. Experience is also that it is much more critical to align correctly to get small transverse beam than to get small Δp/p EMITTANCE STABILIZATION

Effect of misalignment between electron beam and ion beam in CELSIUS: electron-cooled 200 MeV/u Ar 18+. Curves represent theoretical profiles (if constant diffusion rate) measured with magnesium- jet beam profile monitor

EMITTANCE STABILIZATION Transverse and longitudinal cooler 8 GeV for different tilts

Calculated equilibrium transverse beam profiles of GeV electron-cooled antiprotons on target in units of the initial rms. beam size of 0.56 mm Calculated aspect of the beam on the target for GeV electron- cooled antiprotons on target EMITTANCE STABILIZATION

RESULTS 8.9 GeV/c, pbars momentum spread of 90 % of pbars emittance of 50 % of pbars calculated lifetime 6,000 s

RESULTS 8.9 GeV/c, pbars (no stochastic cooling) momentum spread of 90 % of pbars emittance of 50 % of pbars

CONCLUSIONS High energy electron cooling at HESR can maintain momentum spread at several in presence of internal hydrogen target. Magnetized cooling is necessary, and requires 0.2 T (or more). With hydrogen pellet target, it becomes necessary to control the beam emittance, else the emittance becomes too small. This can be achieved by tilting the electron beam. This removes some of the cooling force, but also intra-beam scattering. Electron cooling gives the antiproton beam a dense core, but also a low-energy tail. (This tail can be removed with stochastic cooling, if employed simultaneously with the electron cooling).