how to control the local beam density at the ntof target

L.Gatignon, 14/02/2008 Local beam density control 1

How to control the local beam densityat the nTOF target

Lau Gatignon / AB-ATB-SBA

Thanks to Rende Steerenberg, Olav Berrig, Paolo Cennini

and many others for input and help


How to control the local beam densityat the nTOF target

1) Beam optics / spot

3) Bunch width 4) Intensity limitation

2) Parasitic vs dedicated cycle


An increased beam spot will decreasethe local energy density in the target

Increase the beta function in the beam optics

Note that the 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 -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


IF

ID

IHZ IVT

The focusing and steering options are the following:

For matchingto TT2 line


The original optics, up to 2004, with QKE58


7 x 3 mm2 RMS


Old FTN optics after re-matching w/o QKE58

kqde=-0.10914, kqfo=0.15441


7 x 3 mm2 RMS


Increase the spot by factor 2 / plane ( x4)

kqde=-0.050722, kqfo=0.0590044


12 x 6 mm2 RMS


Increase the spot by factor 4 / plane ( x16)

kqde=-0.00666 ; kqfo = 0.01388


19 x 11 mm2 RMS


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 planeThis 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


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!


There is heavy demand on supercycles

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

Some (random) example in 2007:

nTOF


nTOF can be served with:

• dedicated cycles (up to 7 1012 ppp)• parasitic cycles (up to 4 1012 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)

EastnTOF


The bunch width can be lengthened,but is this useful?

Normally the bunch length is shortened by “Bunch Rotation”:

VRF phasejump by 180o

and wait few ms

VRF phasejump by 180o

after somedelay

Result: bunch length () from 50-60 nsec to ~25 nsec

momentum spread from 1.6 to ~3.2 permille

Courtesy Rende Steerenberg


Dedicated nTOF cycle Parasitic ( on any EAST) cycle

Bunch rotation (or not???)

Bunch rotation (or not???)

Rotate back(if needed)

Bunch rotation

Bunch rotation can (or not) be applied both in

dedicated and parasitic cycles


Before bunch rotation:

4 ~48 nsec,

hence

1 ~12 nsec


After bunch rotation:

4 ~22 nsec,

hence

1 ~6 nsec


Doubling the time resolution of the proton beam extends this problemto almost 10x lower values.

15 n

sec

reso

lutio

n

The bunch width degrades the TOF resolution andhence also the energy resolution

Is this worth it?


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


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

how to control the local beam density at the ntof target

Documents

local beam densityat

tt2 optics

dedicated ntof cycles

beam impact point

dedicated cycles

bunch length

pppparasitic cycles

limitedparasitic ntof