User defined Ez chirp in SALAME algorithm

[AlexanderSinn]

Use

hipace.salame_Ez_target(zeta,zeta_initial,Ez_initial) = "Ez_initial"

For a flat Ez field at the SALAME beam (this is the default) or something more custom if needed

hipace.salame_Ez_target(zeta,zeta_initial,Ez_initial) = "
x=(zeta-zeta_initial+0.1)*100;
Ez_initial*(1-0.05*
if(x<-7, 0,
    if(x<-3, 3*sqrt(1-x^2/49),
        if(x<-1, (6*sqrt(10)/7 + (1.5-0.5*abs(x)) - (6*sqrt(10)/14)*sqrt(4-(abs(x)-1)^2) ),
            if(x<-0.75, 9-8*abs(x),
                if(x<-0.5, 3*abs(x)+0.75,
                    if(x<=0.5, 2.25,
                        if(x<=0.75, 3*abs(x)+0.75,
                            if(x<=1, 9-8*abs(x),
                                if(x<=3, (6*sqrt(10)/7 + (1.5-0.5*abs(x)) - (6*sqrt(10)/14)*sqrt(4-(abs(x)-1)^2) ),
                                    if(x<=7, 3*sqrt(1-x^2/49),
                                        0
                                    )
                                )
                            )
                        )
                    )
                )
            )
        )
    )
)/3)"

• Small enough (< few 100s of lines), otherwise it should probably be split into smaller PRs
• Tested (describe the tests in the PR description)
• Runs on GPU (basic: the code compiles and run well with the new module)
• Contains an automated test (checksum and/or comparison with theory)
• Documented: all elements (classes and their members, functions, namespaces, etc.) are documented
• Constified (All that can be const is const)
• Code is clean (no unwanted comments, )
• Style and code conventions are respected at the bottom of https://github.com/Hi-PACE/hipace
• Proper label and GitHub project, if applicable

[MaxThevenet]

Great PR, thanks!

[MaxThevenet]

Suggested change

	// update target with user funciton
	// update target with user function