Fund Project:Project supported by the National Natural Science Foundation of China (Grant No. 11655003), the Innovation Project of IHEP, China (Grant Nos. 542017IHEPZZBS11820, 542018IHEPZZBS12427), the CAS Center for Excellence in Particle Physics (CCEPP), China, and IHEP Innovation, China (Grant No. Y4545170Y2)
Received Date:08 December 2020
Accepted Date:25 January 2021
Available Online:26 June 2021
Published Online:05 July 2021
Abstract:The accurate calibration of the beam energy of the circular electron-positron collider (CEPC) is performed to accurately measure the mass width of Higgs particle and the mass of W/Z boson, thus providing the basic experimental basis for the accurate test of the standard model. Based on this, the error control of beam energy is required to be at a level of 10–5. Compton backscattering method is suitable for high precision calibration of beam energy in the Hundred GeV high energy electron collider. In this work, the CEPC beam energy is predicted to reach a theoretical accuracy of about 3 MeV by using the accurate measurement of the scattered photon energy after microwave electron Compton backscattering. Firstly, TM01 mode microwave transmission in circular waveguide is selected according to the design requirements, and the electromagnetic field distribution and Poynting vector under this condition are solved. According to the photon distribution and transmission in the waveguide, the design idea is proposed to simplify the complexity of calculation, and the parameters conforming to the design requirements are solved by combining the simultaneous equations of the high purity germanium detector sensitivity and the background of synchrotron radiation. Using the optimal set of waveguide inner diameter, microwave wavelength and electron incident angle data, the derivative of the differential scattering cross section with respect to energy and the collision brightness are obtained when the microwave power is 100 W. The scattered photon density of 15 MeV energy is further obtained, and the signal-to-noise ratio is analyzed according to the photon density of synchrotron radiation under this energy. The feasibility of the scheme is demonstrated theoretically and the technical difficulties and problems to be further studied are discussed. Keywords:Compton backscattering/ circular electron-positron collider/ beam energy calibration/ microwave
由坡印廷矢量的结算结果可以分析能流沿坐标轴的变化情况, 坡印廷矢量的z, ρ分量仅与z, ρ坐标变化有关, 且坡印廷矢量z, ρ分量的常数项数量级基本一致, 因此在分析能流的变化情况时, 可以仅考虑含坐标z, ρ的函数变化情况. 由坡印廷矢量的结算结果可以得到Sz, Sρ沿z, ρ坐标在波导空间中的变化情况如图2(c)和图2(d)所示. 分析(12)式前式可知: 坡印廷矢量的z分量Sz在平行于z轴方向上, 其大小沿z轴呈sin2(βz)函数周期分布, 沿z轴方向的周期情况如图2(b); 同一截面内, Sz大小与角度φ无关, 仅与ρ有关, 随着ρ由0增大至半径a, Sz的大小先由0增大至极值点后减小至非零值, 沿ρ方向的变化情况如图2(a). 分析(12)式后式可知: 坡印廷矢量的ρ分量Sρ其大小沿z轴呈sin(βz)cos(βz)函数周期分布, 周期为与Sz相同, 沿z轴方向的周期情况如图2(b); 同一截面内, Sρ的大小与角度φ无关, 仅与ρ有关, 随着ρ由0增大至半径a, Sρ的大小先增大再减小至0, 沿ρ方向的变化情况如图2(a). 图 2 波导中坡印廷矢量变化情况 (a)各分量沿ρ方向变化情况; (b) 各分量沿z方向变化情况; (c)坡印廷矢量z分量在空间中的变化情况; (d) 坡印廷矢量ρ分量在空间中的变化情况 Figure2. Poynting vector variation in the waveguide: (a) The variations of each Poynting vector’s components along the ρ axis; (b) the variations of each Poynting vector’s components along the z axis; (c)variations of the z component of Poynting vector in space; (d) variations of the ρ component of Poynting vector in space.
由图4可知, 满足以上两个限制条件的同时该方程组有解, 若使用标准圆形波导管, 则可以从中得到9组数据, 对应的波导半径a、微波波长λ等参数如表2所列. 图 4 单模传输波长-内径解 Figure4. Solution of wavelength and inner diameter for single mode transmission.
a/m
λ/m
vg
cosψ/cosθ
Tz/m
Tt/S
6.35 × 10–3
1.39 × 10–2
5.45 × 10–1c
5.45 × 10–1
1.28 × 10–2
7.80 × 10–11
5.5 × 10–3
1.30 × 10–2
4.46 × 10–1c
4.46 × 10–1
1.46 × 10–2
1.09 × 10–10
4.76 × 10–3
1.18 × 10–2
3.13 × 10–1c
3.13 × 10–1
1.89 × 10–2
2.01 × 10–10
4.17 × 10–3
1.07 × 10–2
1.88 × 10–1c
1.88 × 10–1
2.85 × 10–2
5.05 × 10–10
3.57 × 10–3
9.32 × 10–2
3.54 × 10–1c
3.54 × 10–1
1.32 × 10–2
1.24 × 10–8
3.18 × 10–3
8.27 × 10–2
8.12 × 10–1c
8.12 × 10–1
5.09 × 10–2
2.09 × 10–9
2.78 × 10–3
7.11 × 10–2
2.11 × 10–1c
2.11 × 10–1
1.69 × 10–2
2.67 × 10–10
2.39 × 10–3
5.84 × 10–2
3.51 × 10–1c
3.51 × 10–1
8.32 × 10–3
7.91 × 10–11
2.18 × 10–3
5.16 × 10–3
4.26 × 10–1c
4.26 × 10–1
6.06 × 10–3
4.74 × 10–11
表2单模传输时微波-电子系统各参数值 Table2.Parameters of microwave-electronic system in single mode transmission.
表中Tz为坡印廷矢量轴向周期长度, Tt为能流传过坡印廷矢量轴向最小周期长度所用的时间. CEPC在Higgs模式运行时, 每束电子束团间隔时间为680 ns, 为保证系统的时间控制精度, Tt应接近百纳米量级, 故本方案采用第五组数据: 波导半径a = 3.5685 × 10–3 m, 微波波长λ = 9.319 mm, 电子入射夹角的余弦值cosθ = 0.035405. 对撞角θ约为88°, 电子束轴向跨度Δz = 4.4 mm × cosθ ≈ 0.156 mm, 坡印廷矢量轴向周期长度Tz = 13.1601 mm, Δz/Tz ≈ 0.59%. 由于电子所在能流面上所含ρ方向传播的光子占比很小, 故可以认为与电子束发生散射的微波光子均沿z传播. 23.3.散射光子数密度计算 -->