1. How to control the local beam density
at the nTOF target
Lau Gatignon / AB-ATB-SBA
Thanks to Rende Steerenberg, Olav Berrig, Paolo Cennini and many others for input and help
L.Gatignon, 14/02/2008
Local beam density control

2. Local beam density control
How to control the local beam density
at the nTOF target
1) Beam optics / spot
2) Parasitic vs dedicated cycle
3) Bunch width
4) Intensity limitation
L.Gatignon, 14/02/2008
Local beam density control

3. Local beam density control
An increased beam spot will decrease the local energy density in the target
Increase the beta function in the beam optics Note that the b function is proportional to size2 If the size is doubled in each plane, the density decreases ~ x4 at least if the beam spot is not too small compared to the shower size
It is ‘easy’ to increase the b-value by up to 16x/plane This reduces the density by roughly a factor 16!
Intermediate solutions can be found ‘easily’
Please note that TT2 optics has changed since 2004 (QKE58)
Many thanks to Olav Berrig for preparing and providing the optics files with the new matching TT2-FTN
L.Gatignon, 14/02/2008
Local beam density control

4. Local beam density control
The focusing and steering options are the following: IF ID
IHZ
IVT
For matching to TT2 line
L.Gatignon, 14/02/2008
Local beam density control

5. Local beam density control
The original optics, up to 2004, with QKE58
L.Gatignon, 14/02/2008
Local beam density control

6. Local beam density control
7 x 3 mm2 RMS
L.Gatignon, 14/02/2008
Local beam density control

7. Old FTN optics after re-matching w/o QKE58
kqde= , kqfo=
L.Gatignon, 14/02/2008
Local beam density control

8. Local beam density control
7 x 3 mm2 RMS
L.Gatignon, 14/02/2008
Local beam density control

9. Increase the spot by factor 2 / plane (b x4)
kqde= , kqfo=
L.Gatignon, 14/02/2008
Local beam density control

10. Local beam density control
12 x 6 mm2 RMS
L.Gatignon, 14/02/2008
Local beam density control

11. Increase the spot by factor 4 / plane (b x16)
kqde= ; kqfo =
L.Gatignon, 14/02/2008
Local beam density control

12. Local beam density control
19 x 11 mm2 RMS
L.Gatignon, 14/02/2008
Local beam density control

13. How to choose final optics?
→ There is a relatively free choice in increase of the beam spot by up to a factor ~4 in each plane This would typically reduce the deposited energy density by about a factor of ~16
The shape of the beam impact can be influenced as well e.g. increase the vertical spot and decrease the horizontal one
Simulations (FLUKA) can evaluate the impact of the increased beam spot on the neutronics and on the heating of the target
There is also the possibility to change the beam impact point from time to time (e.g. once or twice per shift). But is the beam is very large, room for maneuver is limited
L.Gatignon, 14/02/2008
Local beam density control

14. Parasitic nTOF cycles are attractive
The PS serves many users: LHC, SPS, EASTA, EASTB, EASTC, MD, AD, nTOF,…
The basic PS period is 1.2 seconds and each user requires one or two basic PS periods
The SPS cycle is much longer and therefore an overall supercycle is defined to serve e.g. LHC, NA, CNGS and MD A typical supercycle was 16.8 s in 2007, 48 s in 2008!
L.Gatignon, 14/02/2008
Local beam density control

15. There is heavy demand on supercycles
Some (random) example in 2007:
nTOF
Each EASTA, B, C serve the East experimental area and take two PS basic periods each. There are many EAST cycles per s.c.
A dedicated nTOF cycles takes 1 PS basic period
L.Gatignon, 14/02/2008
Local beam density control

16. Local beam density control
nTOF can be served with:
dedicated cycles (up to ppp)
parasitic cycles (up to ppp)
Dedicated nTOF cycle
Parasitic ( on any EAST) cycle
Parasitic cycles cannot be so intense they perturb the 10x smaller bunch for the EAST
But there are many EAST cycles available ‘for free’
As they are less intense, one may get the protons more favorably distributed over time (less strongly peaked)
East
nTOF
L.Gatignon, 14/02/2008
Local beam density control

17. Local beam density control
The bunch width can be lengthened,
but is this useful?
Normally the bunch length is shortened by “Bunch Rotation”:
Courtesy Rende Steerenberg
VRF phase jump by 180o
after some
delay
VRF phase jump by 180o
and wait few ms
Result: bunch length (4s) from nsec to ~25 nsec
momentum spread from 1.6 to ~3.2 permille
L.Gatignon, 14/02/2008
Local beam density control

18. Local beam density control
Bunch rotation can (or not) be applied both in dedicated and parasitic cycles
Bunch rotation
(or not???)
Bunch rotation
(or not???)
Rotate back (if needed)
Bunch rotation
Dedicated nTOF cycle
Parasitic ( on any EAST) cycle
L.Gatignon, 14/02/2008
Local beam density control

19. Local beam density control
Before bunch rotation:
4 s ~48 nsec,
hence 1s ~12 nsec
L.Gatignon, 14/02/2008
Local beam density control

20. Local beam density control
After bunch rotation:
4 s ~22 nsec,
hence 1s ~6 nsec
L.Gatignon, 14/02/2008
Local beam density control

21. Local beam density control
The bunch width degrades the TOF resolution and
hence also the energy resolution
15 nsec resolution
Is this worth it?
Doubling the time resolution of the proton beam extends this problem to almost 10x lower values.
L.Gatignon, 14/02/2008
Local beam density control

22. There are ways to limit the intensity from the PS
One may limit the maximum intensity per PS pulse This is done by sending the beam on the internal dump in case the intensity per shot exceeds a threshold (BCT at injection)
One may restrict the RMS current (by software) This would need monitoring of the current in a BCT (FESA modifs) and a modification to two rectifiers to allow fast switching of the beam settings to go onto the D3 dump. Plus the implementation of a software surveillance task. Is this important?
A hardware solution is in principle also possible Based on a direct BCT reading or image current in a wire in the nTOF control room
L.Gatignon, 14/02/2008
Local beam density control

23. Local beam density control
Conclusions
It is possible to increase the beam spot by up to 4x in each plane, thus reducing deposited energy density
It seems better for the target to take as much as possible of the total integrated intensity from parasitic cycles
It is possible to double the bunch width, but this has a negative impact on the energy resolution
There are easy ways to prohibit excessive fluxes per shot. If really necessary one may also restrict the RMS flux
L.Gatignon, 14/02/2008
Local beam density control