Experimental Demonstration of Relativistic Electron Cooling

We report on an experimental demonstration of electron cooling of high-energy antiprotons circulating in a storage ring. In our experiments, electron cooling, a well-established method at low energies ($<500\mathrm{MeV}/\mathrm{\text{nucleon}}$), was carried out in a new region of beam parameters, requiring a multi-MeV dc electron beam and an unusual beam transport line. In this Letter, we present the results of the longitudinal cooling force measurements and compare them with theoretical predictions.

Figure 1

Schematic layout of the Recycler electron cooling system and accelerator cross section (inset).

Figure 2

The calculated longitudinal LF cooling force as a function of the antiproton energy deviation.

Figure 3

Measured (solid curve) and Gaussian model (dashed curve) equilibrium distribution functions for a beam of $5\times {10}^{10}$ antiprotons with the transverse emittance of $2\mu \mathrm{m}\mathrm{rad}$ ($n$, 95%) cooled by a 500-mA electron beam. The rms momentum spread is $0.17\mathrm{MeV}/c$. Nonvanishing tails are due to amplifier noise.

Figure 4

The measured (diamonds) and the fitted (dashed line) rms momentum spread as a function of time. The electron beam current (solid line) was turned off at 180 seconds and on again at 810 seconds.

Figure 5

The evolution of the antiproton beam momentum distribution function as the antiprotons are being dragged by the electron beam to a new equilibrium after the energy jump. Left curve, the initial distribution; center, 5 minutes later; and right, 17 minutes after the energy jump. The number of antiprotons was $4\times {10}^{10}$; the transverse emittance was $1.6\mu \mathrm{m}\mathrm{rad}$ ($n$, 95%).

Figure 6

The momentum distribution function (arbitrary linear scale) of $3\times {10}^{12}$ bunched antiproton beam cooled by a 500-mA electron beam. The antiproton beam transverse emittance was $7\mu \mathrm{m}\mathrm{rad}$ ($n$, 95%).