1. G. Marcus, Y. Ding, J. Qiang 02/06/2017
CuRF to LCLS-II SXR: 180 pC, horn truncated, max K, tuning e-beam energy Latest 100 pC LCLS-II solution: Sudden increase in charge impact on FEL performance
G. Marcus, Y. Ding, J. Qiang
02/06/2017

2. CuRF to LCLS-II SXR
Previous studies explored the LCLS-II SXR performance when fed by an e- beam from the CuRF linac at 10 GeV
Undulator K was tuned to examine the performance at 10, 5, and 1.5 keV
High beam power coupled with large K (1.5 keV case) produced some relatively extreme performance (~ 14 mJ, ~ TW power after taper!)
This current study explores the performance for a fixed K (max ~ 5.5) while tuning the e-beam energy
E-beam energies of ~ 4 and 6 GeV produce photons at 250 and 550 eV, respectively
E-beams tracked in ELEGANT by Y. Ding
Part of ongoing attempt to define an envelope of maximum operating power and fluence along the beamline.

3. Cu linac to LCLS2 SXR, 4 or 6 GeV
LCLS beam with BC1 collimation, from 250pC to 180pC. Final current ~2kA.
BC2 energy setting of 5 GeV makes a better longitudinal phase space.
Compensation chicanes (CC) at DL2 area are on. CSR from CC is negligible at 2kA current level. It affects the final local chirp.

4. BC2 at 3 GeV, final 6 GeV. @ BC2 END @ undulator entrance.
@ undulator entrance, CC off.
@ BC2 END
@ undulator entrance.
The LTU wakes and CSR make the bunch tail energy loss, partially due to tail horn.

5. BC2 at 5 GeV, final 6 GeV. @ BC2 END @ undulator entrance.
With increasing BC2 to 5GeV, the L2 wakefield induced nonlinear effect is relatively smaller, resulting a more uniform current profile. The final phase space is more linear.

6. BC2 at 5GeV, final 4 GeV

7. E-beam slice properties @ ~ 4 GeV

8. E-beam slice BMAG @ ~ 4 GeV

9. FEL performance at Eγ = 250 eV (~4 GeV beam energy)
E ~ 5.8 mJ
DTC = 0.37

10. Location of the waist at Eγ = 250 eV
Location of waist:
z = m, Projection RMS
z = m, Projection FWHM
z = m, Max slice RMS
z = m, Max slice FWHM
z = m, Peak intensity

11. 250 eV divergence from forward propagation and farfield distribution
Divergence calculations from the projected farfield distribution
Projected RMS divergence (x) = e-05
Projected RMS divergence (y) = e-05
Projected FWHM divergence (x) = e-05
Projected FWHM divergence (y) = e-05
Slice through peak RMS divergence (x) = e-05
Slice through peak RMS divergence (y) = e-05
Slice through peak FWHM divergence (x) = e-05
Slice through peak FWHM divergence (y) = e-05
==================================================
Divergence calculations from field propagation
Projected RMS divergence (x) = e-05
Projected RMS divergence (y) = e-05
Projected FWHM divergence (x) = e-05
Projected FWHM divergence (y) = e-05
Slice through peak RMS divergence (x) = e-05
Slice through peak RMS divergence (y) = e-05
Slice through peak FWHM divergence (x) = e-05
Slice through peak FWHM divergence (y) = e-05

12. FEL performance at Eγ = 550 eV (~6 GeV beam energy)
E ~ 9.6 mJ
DTC = 0.48

13. Location of the waist at Eγ = 550 eV
Location of waist:
z = m, Projection RMS
z = m, Projection FWHM
z = m, Max slice RMS
z = m, Max slice FWHM
z = m, Peak intensity

14. 550 eV divergence from forward propagation and farfield distribution
Divergence calculations from the projected farfield distribution
Projected RMS divergence (x) = e-06
Projected RMS divergence (y) = e-06
Projected FWHM divergence (x) = e-05
Projected FWHM divergence (y) = e-05
Slice through peak RMS divergence (x) = e-06
Slice through peak RMS divergence (y) = e-06
Slice through peak FWHM divergence (x) = e-05
Slice through peak FWHM divergence (y) = e-05
==================================================
Divergence calculations from field propagation
Projected RMS divergence (x) = e-06
Projected RMS divergence (y) = e-06
Projected FWHM divergence (x) = e-05
Projected FWHM divergence (y) = e-05
Slice through peak RMS divergence (x) = e-06
Slice through peak RMS divergence (y) = e-06
Slice through peak FWHM divergence (x) = e-05
Slice through peak FWHM divergence (y) = e-05

15. CuRF to SXR table Q [pC] E [GeV] K Eγ [keV] E [mJ] ΔtFW [fs] ΔEγ [eV]
Ppk [GW]
z0 [m]
σp,(c) {x,y} [μm]
FWHMp,(c) {x,y} [μm]
w0 (x,y) [μm]
σθ,p,(c) {x,y} [μrad]
FWHMθ,p,(c) {x,y} [μrad]
DTC
180 10
5.5
1.5
14.1 42
4.2
336
-26
{63.2, 52.9},
({49.3, 39.6})
{81.8, 72.5},
({77.3, 69.3})
(65.6, 58.9)
{3.5,3.4},
({3.3,3.1})
{7.4,7.4},
({8.0,7.9})
0.49
2.8
5.0
4.4 38
9.1
116
-12
{30.3, 24.5},
({25.7, 20.9})
{48.9, 45.3},
({48.0, 44.4})
(40.8, 37.7)
{1.5, 1.4},
({1.4,1.3})
{3.0,3.1},
({3.2,3.2})
0.69
1.7
10.0
0.4
12.3 11
-14
{18.3, 16.8},
({17.8,16.3})
{40.9,37.6},
({40.9,37.5})
(34.7,31.8)
{0.9,0.9},
({0.8,0.8})
{1.8,1.7},
({1.8,1.7})
0.72 4
0.25
5.8 78 74
-60
{235,232},
({131,130})
{169,168},
({146,146})
(124, 124)
{14.6,14.5},
({12.0,11.9})
{24.8,24.9},
({26.6,26.0})
0.37 6
0.55
9.6 77
2.4
125
-36
{110,106},
({72.6,69.8})
{119,112},
({98.9,92.2})
(84.0,78.3)
({6.5,6.4})
{14.6,14.9},
({16.1,16.3})
0.48

16. Impact of doubling charge on FEL performance
What happens to the FEL performance if the charge at the cathode instantaneously doubles from 100 to 200 pC?
Q: Is it possible to either accidentally (through random fluctuations) or intentionally change the charge, once at a given set point, and drive the FEL beyond the BCS damage limit before feedback integration times would trip off?
As a first pass, we looked at this numerically by simply doubling the charge from 100 to 200 pC at the cathode without making any changes in the linac, compression, transport, or undulator.

17. 100 pC e-beam properties

18. 130 pC e-beam properties

19. 200 pC e-beam properties

20. Energy gain curve comparison (~ 260 eV)
~ 2.4 mJ
~ 2.4 mJ
~ 1.0 mJ

21. Conclusions and future work
We have generated two ‘curves’ for the performance of the LCLS-II SXR undulator when fed by an e-beam from the CuRF linac
Maximum beam energy and tuned K
Maximum K and tuned beam energy
Should help to define an envelope of maximum operating power in this scenario
Would be nice to compare single simulations presented here to analytic estimates that can explore a much larger parameter space
Found that doubling the charge on the cathode without any commensurate change along the linac, transport or undulator negatively impacts the FEL performance
Will explore this at a less severe change at the cathode