1. He Zhang MEIC R&D Meeting, 07/09/2015
Effect of the Electron Beam Momentum Spread on Friction Force and Cooling Rate for MEIC
He Zhang
MEIC R&D Meeting, 07/09/2015

2. Magnetized Friction Force
Parkhomchuk formula: 𝑭=−𝑽 4 𝑍 2 𝑒 4 𝑛 𝑒 𝐿 𝑝 𝑚 𝑉 2 + Δ 𝑒,𝑒𝑓𝑓
Ion beam 𝑉 2 = 𝑉 ⊥ 2 + 𝑉 ∥ 2
Electron beam Δ 𝑒,𝑒𝑓𝑓 2 = 𝑣 𝑒𝑓𝑓 2 + 𝑣 𝑒,∥ 2
𝐿 𝑝 has a weak dependence on 𝑣 𝑒,∥ , ignore it.
Consider a proton beam at 100 GeV. Normalized emittance 0.3 um. Momentum spread ~ 10 −4 . Cooler length 30m. 𝛽 ⊥ =10 m, 𝛼 ⊥ =0 at the center of the cooler.
𝛾=108.4, 𝛽=
𝜎= 𝛽 ⊥ ⋅ 𝜀 𝑛 𝛽𝛾 =0.166 mm, 𝜎 ′ = 𝜀 𝜎 = 𝜀 𝑛 𝛽𝛾𝜎 =1.66×1 0 −5 .
𝑉 ⊥,𝑙𝑎𝑏 = 𝜎 ′ 𝛽𝑐=4990 m/s, 𝑉 ⊥ =𝛾 𝑉 ⊥,𝑙𝑎𝑏 =5.41× m/s
Assuming momentum spread 5× 10 −4 , 𝑉 ∥ = 𝑉 ∥,𝑙𝑎𝑏 =1.5× m/s
𝑉 2 = 𝑉 ⊥ 2 + 𝑉 ∥ 2 =31× m2/s2, 𝑉=5.6× m/s
He Zhang

3. Magnetized Friction Force
Electron beam Δ 𝑒,𝑒𝑓𝑓 2 = 𝑣 𝑒𝑓𝑓 2 + 𝑣 𝑒,∥ 2
𝑭=−𝑽 4 𝑍 2 𝑒 4 𝑛 𝑒 𝐿 𝑝 𝑚 1 𝑉 Δ 𝑒,𝑒𝑓𝑓 2 𝑉 = 𝑭 𝟎 1 𝑉 2 (1− Δ 𝑒,𝑒𝑓𝑓 2 𝑉 2 ), for Δ 𝑒,𝑒𝑓𝑓 2 𝑉 2 ≪1.
Effective transverse velocity 𝑣 𝑒𝑓𝑓 =0 for an ideal case.
𝑓 1 = Δ 𝑒,𝑒𝑓𝑓 2 𝑉
𝑓 2 =1− Δ 𝑒,𝑒𝑓𝑓 2 𝑉 2
Δ 𝑒,𝑒𝑓𝑓 = 𝑣 𝑒,∥ = 𝑑𝑝 𝑝 ⋅𝑐
He Zhang

4. Cooling Rate
Consider a proton with emittance 𝜀, the dynamic invariant is
𝐼=𝛽 𝑥 𝛽 ′ 2 +2𝛼 𝑥 𝛽 𝑥 𝛽 ′ +𝛾 𝑥 𝛽 2 =2𝜀, 𝛼,𝛽,𝛾 are TWISS parameters at the cooler
Consider an easy case with 𝛼=0.
Assuming the friction force gives the proton a kick, Δ𝑥 𝛽 ′ , NO change on 𝑥 𝛽 .
Δ𝐼=𝛽 𝑥 𝛽 ′ +Δ 𝑥 𝛽 ′ 2 −𝛽 𝑥 𝛽 ′ 2 =2𝛽 𝑥 𝛽 ′ Δ 𝑥 𝛽 ′ +𝛽 Δ𝑥 𝛽 ′ 2
Cooling effect is weak: Δ 𝑥 𝛽 ′ ≪ 𝑥 𝛽 ′
Δ𝐼∝ Δx 𝛽 ′ ∝Δp∝𝐹𝑡∝𝐹
Above is only for a single proton. Consider the proton beam:
Cooling rate:
〈 Δ𝜀 𝜀 〉 1 𝑇 =〈 Δ𝐼 𝐼 〉 1 𝑇 ∝〈𝐹〉 1 𝑇
〈〉 means average on all particles. ?
He Zhang

5. Numerical Calculation (BETACOOL)
Proton beam at 100 GeV, 𝜀 𝑛 =0.4 um, dp/p = 4E-4. Gaussian bunch.
Electron beam: Gaussian bunch, 𝜎 ⊥ =0.2 mm, 𝜎 ∥ =2.1 cm, ne=1.4E10/bunch
Cooler: 2 sections, 30 m, B=2T, 𝛽=10 m, 𝛼=0
IBS rate: 𝑅 ⊥ = , 𝑅 ∥ = , 100% coupling
Dispersion = 0
Longitudinal
Transverse
He Zhang

6. Numerical Calculation (BETACOOL)
Insufficient cooling in transverse direction, and surfeit cooling in longitudinal direction.
Set dispersion function to transfer the cooling effect.
Dispersion = 0.7 m
Longitudinal
Transverse
He Zhang

7. He Zhang