Secondary emission parameters

Choice of the model

Parameter

Description

switch_model

Optional, default 0. Secondary emission model:

0 or 'ECLOUD': the model used in the ECLOUD code.
1 or 'ACC_LOW': a more accurate treatment of low-energy impacts.
'ECLOUD_nunif': the ECLOUD model, with SEY parameters given per segment in the chamber file.
'ECLOUD_nunif_charging': the ECLOUD model with charging effects, with SEY and charging parameters given per segment in the chamber file.
'cos_low_ene': cosine dependence at low energy.
'flat_low_ene': flat dependence at low energy.
'furman_pivi': the Furman–Pivi model.
'from_file': interpolation of a user-supplied SEY curve (see sey_file).
'perfect_absorber': every impacting electron is absorbed; no secondaries are emitted.

ECLOUD model

These parameters are described in G. Iadarola’s doctoral thesis.

Parameter	Description
`Emax`	[eV] Impact energy at which the SEY is maximal.
`del_max`	Maximum SEY.
`s_param`	Optional; only for `switch_model = 'ECLOUD'`. Shape parameter of the true secondary SEY curve.
`R0`	Weight of the reflected (elastic) electron component.
`E0`	Optional, default set by the model. [eV] Shape parameter of the reflected SEY curve.
`E_th`	[eV] Maximum energy of true secondary electrons.
`sigmafit`	\(\sigma\) of the log-normal energy distribution of true secondaries.
`mufit`	\(\mu\) of the log-normal energy distribution of true secondaries.

Furman–Pivi model

The symbols refer to the Furman–Pivi paper (M. A. Furman and M. T. F. Pivi, Phys. Rev. ST Accel. Beams 5, 124404, 2002). See also CERN-ACC-NOTE-2019-0029.

Parameter	Description
`E_th`	[eV] Maximum energy of true secondary electrons.
`furman_pivi_surface`	Python dictionary with all Furman–Pivi model parameters. Its keys are listed below.

General keys

Key	Description
`'use_modified_sigmaE'`	bool. Modify \(\sigma_e\) in the same way as POSINST does.
`'use_ECLOUD_theta0_dependence'`	bool. Use the same incidence-angle dependence of the SEY as the ECLOUD model.
`'use_ECLOUD_energy'`	bool. Use the same secondary energy distribution as the ECLOUD model. Elastically reflected electrons then also use the ECLOUD emission angle.
`'conserve_energy'`	bool. Ensure that the total energy of the electrons emitted in one event does not exceed the energy of the impacting electron.
`'exclude_rediffused'`	bool. Set \(\delta_r = 0\).
`'choice'`	`'poisson'` or `'binomial'`: probability distribution used for \(P^\prime_{n,ts}\).
`'M_cut'`	int. Maximum number of macroparticles emitted in a single event.
`'p_n'`	NumPy array of `M_cut` floats: \(p_n\).
`'eps_n'`	NumPy array of `M_cut` floats: \(\epsilon_n\).

Backscattered electrons

Key	Description
`'p1EInf'`	float. \(P_{1,e}(\infty)\).
`'p1Ehat'`	float. \(\hat{P}_{1,e}\).
`'eEHat'`	float. \(\hat{E}_e\).
`'w'`	float. \(W\).
`'p'`	float. \(p\).
`'e1'`	float. \(e_1\).
`'e2'`	float. \(e_2\).
`'sigmaE'`	float. \(\sigma_e\).
`'theta_e_max'`	Optional; by default the angle is not limited. float, [rad]. Upper limit applied to the incidence angle when computing the angular dependence for backscattered electrons. Must satisfy \(0 < \theta_{e,\max} < \pi/2\).

Rediffused electrons

Key	Description
`'p1RInf'`	float. \(P_{1,r}(\infty)\).
`'eR'`	float. \(E_r\).
`'r'`	float. \(r\).
`'q'`	float. \(q\).
`'r1'`	float. \(r_1\).
`'r2'`	float. \(r_2\).
`'theta_r_max'`	Optional; by default the angle is not limited. float, [rad]. Upper limit applied to the incidence angle when computing the angular dependence for rediffused electrons. Must satisfy \(0 < \theta_{r,\max} < \pi/2\).

True secondaries

Key	Description
`'deltaTSHat'`	float. \(\hat{\delta}_{ts}\).
`'eHat0'`	float. \(\hat{E}_{ts}\).
`'s'`	float. \(s\).
`'t1'`	float. \(t_1\).
`'t2'`	float. \(t_2\).
`'t3'`	float. \(t_3\).
`'t4'`	float. \(t_4\).

Other parameters

Parameter	Description
`sey_file`	Required for `switch_model = 'from_file'`. Path to a file giving the reflected and true secondary yields as a function of energy. Examples are in `examples/sey_files/` in the repository.
`flag_costheta_delta_scale`	Optional, default `True`. Scale the true secondary SEY curve with the impact angle.
`flag_costheta_Emax_shift`	Optional, default `True`. Shift the true secondary SEY curve in energy with the impact angle.
`switch_no_increase_energy`	Optional, default `0`. `0`: off. `1`: no longer used. `2`: ensure that true secondaries do not have more energy than the impacting electron.
`thresh_low_energy`	[eV] Maximum impact energy for which the `switch_no_increase_energy` check is applied.
`scrub_en_th`	[eV] Minimum impact energy of scrubbing electrons, used to estimate the scrubbing current.
`secondary_angle_distribution`	Required. Emission angle distribution of new electrons, where \(\theta\) is the angle between the surface normal and the initial velocity: `'cosine_3D'`: \(dn/d\theta = \cos\theta\sin\theta\), which includes the surface element \(\sin\theta\) in spherical coordinates (reference). `'normal_emission'`: \(\theta = 0\). The legacy `'cosine_2D'` distribution (\(dn/d\theta = \cos\theta\)) is no longer supported from PyECLOUD v8.9.0 and raises an error. To reproduce the behaviour of PyECLOUD 6.6.0 or earlier, use a version older than v8.9.0.