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Dynamic cavity desynchronisation in FELIX
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH
Nuclear Instruments and Methods in Physics Research A 341 (1994) ABS 26-ABS 27 North-Holland
Section A
Dynamic cavity desynchronisation in FELIX G.M.H. Knippels a, R.J. Bakker a, A.F.G. van der Meer D . Oepts a, P.W. van Amersfoort a, J.N. Hovenier b
D.A . Jaroszynski a,',
a,
" FOM-Instituut voor Plasmafysica `Rynhuizen, Postbus 1207, 3430 BE Nieuwegein, The Netherlands b Delft University of Technology, Faculty of Applied Physics, Postbus 5046, 2600 GA Delft, The Netherlands
In a free-electron laser operating with short electron bunches, like FELIX [1], the lethargic start-up can be counteracted by changing the cavity desynchronism. A large desynchronism results in a quick start-up but also in low saturated power and long micropulses . A small desynchronism results in high saturated power with short micropulses but with a slow start-up [1,2]. It is therefore tempting to vary the cavity desynchronism during the macropulse, starting with a large desynchronism to reach a high small-signal gain, and switch to a small desynchronism to obtain a high saturated power level [3]. The cavity desynchronism is varied by ramping the electron bunch repetition frequency rather than by adjusting the cavity mirrors mechanically . A cavity length detuning AL has the same effect as a frequency detuning given by 0f = fAL/L. In FELIX, the nominal cavity length, L, is 6 m and the nominal repetition frequency, f, is 1 GHz. A single master oscillator is used as source for the RF system, that provides the input power for the chain of accelerator components : triode electron gun, prebuncher, buncher, and two linear accelerators . Hence, a suitably chosen signal, fed into the FM input of the master oscillator, can be used to change the spacing between the train of micropulses . The exact response of the RF system is not known but we found that the optical signal reacts with a delay of typically 3.5 ws with respect to the input signal . In Fig. 1 we show three experimentally obtained optical macropulses, for a wavelength of 42 [,Lm . At a fixed repetition frequency of 1 GHz and a fixed cavity length, a build-up time to saturation close to the shortest possible value (roughly 3.5 ws, taking into account the beam loading transient of 1 .5 Lts at the start of the macropulse) is obtained at a cavity length detuning
I Permanent address: Heriott-Watt University, Physics Department, Riccarton, Edinburgh EH14 4 AS, UK.
AL = -27 wm . The net corresponding small-signal gain is of the order of 20%. The largest saturated power, however, is obtained at a detuning AL = -4 wm . In this case the laser operates just above threshold, with the consequence that the time at which saturation is reached fluctuates by typically 1-2 g,s from macropulse to macropulse . The optical macropulse with dynamic cavity desynchronisation has the anticipated feature of a high initial gain per pass and a high saturated power. The shot-to-shot reproducibility is now comparable to the situation at AL = -27 pm . The fluctuations in the saturated power level are reduced from 14% to 7% . The origin of the limit-cycle oscillation of the saturated power, as observed in Fig. 1, is a direct consequence of the short pulse nature of FELIX, and has been reported earlier [1]. This stable oscillation is known as a "limit cycle" and originates from the temporal evolution of the macropulse shape and its energy . The oscillation period can be controlled by the cavity
10 8
3 Y
a0
o.
6 4 2 0
0
1
2
3
4
5
6
7
8
9
10 1 1
12 13
time (ws ) Fig. 1. Optical macropulses measured at A = 42 pm, as obtained at a fixed cavity desynchronism of OL = -27 N m, at a fixed desynchronism of AL = -4 Wm and with dynamic cavity desynchronisation.
0168-9002/94/$07 .00 © 1994 - Elsevier Science B.V . All rights reserved SSDI0168-9002(93)E0734-A
G .M H. Knippels et al. I Nucl. Instr. and Meth . in Phys. Res. A 341 (1994) ABS 26-ABS 27
desynchronisation. The large oscillation period in the case of a dynamically altered cavity desynchronism, of the order of 5 ws, illustrates the improved stability of the micropulse shape during the saturated part of the macropulse . This work has been performed as part of the research programme of the "Stichting voor Fundamenteel Onderzoek der Materie" (FOM), and was
ABS 27
made possible by the financial support from the "Nederlandse Organisatie voor Wetenschappelijk Onderzoek" .
[1] D.A . Jaroszynski et al., Phys . Rev. Lett . 70 (1993) 3412 . [2] S. Benson et al ., Phys . Rev. Lett . 48 (1982) 235. [3] D.A . Jaroszynski et al ., Nucl . Instr. and Meth . A 296 (1990) 480.
EXTENDED SYNOPSES
Nuclear Instruments and Methods in Physics Research A 341 (1994) ABS 26-ABS 27 North-Holland
Section A
Dynamic cavity desynchronisation in FELIX G.M.H. Knippels a, R.J. Bakker a, A.F.G. van der Meer D . Oepts a, P.W. van Amersfoort a, J.N. Hovenier b
D.A . Jaroszynski a,',
a,
" FOM-Instituut voor Plasmafysica `Rynhuizen, Postbus 1207, 3430 BE Nieuwegein, The Netherlands b Delft University of Technology, Faculty of Applied Physics, Postbus 5046, 2600 GA Delft, The Netherlands
In a free-electron laser operating with short electron bunches, like FELIX [1], the lethargic start-up can be counteracted by changing the cavity desynchronism. A large desynchronism results in a quick start-up but also in low saturated power and long micropulses . A small desynchronism results in high saturated power with short micropulses but with a slow start-up [1,2]. It is therefore tempting to vary the cavity desynchronism during the macropulse, starting with a large desynchronism to reach a high small-signal gain, and switch to a small desynchronism to obtain a high saturated power level [3]. The cavity desynchronism is varied by ramping the electron bunch repetition frequency rather than by adjusting the cavity mirrors mechanically . A cavity length detuning AL has the same effect as a frequency detuning given by 0f = fAL/L. In FELIX, the nominal cavity length, L, is 6 m and the nominal repetition frequency, f, is 1 GHz. A single master oscillator is used as source for the RF system, that provides the input power for the chain of accelerator components : triode electron gun, prebuncher, buncher, and two linear accelerators . Hence, a suitably chosen signal, fed into the FM input of the master oscillator, can be used to change the spacing between the train of micropulses . The exact response of the RF system is not known but we found that the optical signal reacts with a delay of typically 3.5 ws with respect to the input signal . In Fig. 1 we show three experimentally obtained optical macropulses, for a wavelength of 42 [,Lm . At a fixed repetition frequency of 1 GHz and a fixed cavity length, a build-up time to saturation close to the shortest possible value (roughly 3.5 ws, taking into account the beam loading transient of 1 .5 Lts at the start of the macropulse) is obtained at a cavity length detuning
I Permanent address: Heriott-Watt University, Physics Department, Riccarton, Edinburgh EH14 4 AS, UK.
AL = -27 wm . The net corresponding small-signal gain is of the order of 20%. The largest saturated power, however, is obtained at a detuning AL = -4 wm . In this case the laser operates just above threshold, with the consequence that the time at which saturation is reached fluctuates by typically 1-2 g,s from macropulse to macropulse . The optical macropulse with dynamic cavity desynchronisation has the anticipated feature of a high initial gain per pass and a high saturated power. The shot-to-shot reproducibility is now comparable to the situation at AL = -27 pm . The fluctuations in the saturated power level are reduced from 14% to 7% . The origin of the limit-cycle oscillation of the saturated power, as observed in Fig. 1, is a direct consequence of the short pulse nature of FELIX, and has been reported earlier [1]. This stable oscillation is known as a "limit cycle" and originates from the temporal evolution of the macropulse shape and its energy . The oscillation period can be controlled by the cavity
10 8
3 Y
a0
o.
6 4 2 0
0
1
2
3
4
5
6
7
8
9
10 1 1
12 13
time (ws ) Fig. 1. Optical macropulses measured at A = 42 pm, as obtained at a fixed cavity desynchronism of OL = -27 N m, at a fixed desynchronism of AL = -4 Wm and with dynamic cavity desynchronisation.
0168-9002/94/$07 .00 © 1994 - Elsevier Science B.V . All rights reserved SSDI0168-9002(93)E0734-A
G .M H. Knippels et al. I Nucl. Instr. and Meth . in Phys. Res. A 341 (1994) ABS 26-ABS 27
desynchronisation. The large oscillation period in the case of a dynamically altered cavity desynchronism, of the order of 5 ws, illustrates the improved stability of the micropulse shape during the saturated part of the macropulse . This work has been performed as part of the research programme of the "Stichting voor Fundamenteel Onderzoek der Materie" (FOM), and was
ABS 27
made possible by the financial support from the "Nederlandse Organisatie voor Wetenschappelijk Onderzoek" .
[1] D.A . Jaroszynski et al., Phys . Rev. Lett . 70 (1993) 3412 . [2] S. Benson et al ., Phys . Rev. Lett . 48 (1982) 235. [3] D.A . Jaroszynski et al ., Nucl . Instr. and Meth . A 296 (1990) 480.
EXTENDED SYNOPSES