1LBS line alternative – L4 BCC meeting - 25/07/2013 – JB Lallement

An alternative to the LBS measurement line for Linac4Reconstructing the longitudinal emittance

JB Lallement

Linac4 BCC meeting – 25/07/2013

2LBS line alternative – L4 BCC meeting - 25/07/2013 – JB Lallement

Present solution : LBS upgrade

Solution presented in October 2012 (BCC -36).

Adaptation of the present LBS line to the Linac4 beam.

• 2 L4T bending magnets• 2 L4T quadrupoles• 1 SEM-Grid• 1 BCT• 1 dedicated beam dump

3LBS line alternative – L4 BCC meeting - 25/07/2013 – JB Lallement

Present solution : Drawbacks

Feasible but :• Expensive• Time consuming• A planning headache• Integration issues not solved yet

J. Humbert

4LBS line alternative – L4 BCC meeting - 25/07/2013 – JB Lallement

Alternative solutions

• Use of a spectrometer (LBS like) in a different location:• In the Linac4 tunnel• After BHZ.20• In the PSB underground

In all cases need dedicated line (switching magnet) and a beam dump, civil engineering etc...

• Use another method:• Reconstruct the longitudinal emittance with a classical BSM• Use a Laser stripping diagnostic (not ready to propose a workable solution)

5LBS line alternative – L4 BCC meeting - 25/07/2013 – JB Lallement

Why reconstructing the longitudinal emittance?

X’

X

Y’

Y

E

Φ

The beam is characterized by its 6 D emittance:Horizontal, Vertical and longitudinal emittances

E

ΦΔE

ΔΦ

What we are looking for !!!

6LBS line alternative – L4 BCC meeting - 25/07/2013 – JB Lallement

Reconstructing the longitudinal emittance

The longitudinal emittance can be determined like in the transverse case using the “ 3 grids method” or the “ 3 gradients method”.

X’

X

X’

X

X’

X

Quad

Quad

This technique will be used at 50,100, 160 MeV and in the LBE line.

7LBS line alternative – L4 BCC meeting - 25/07/2013 – JB Lallement

Reconstructing the longitudinal emittance

Let’s replace the quadrupole by a cavity and the grid by a BSM.We measure the bunch length at 352.2 MHz.

E

Φ

E

Φ

E

Φ

Cavity

Cavity

The space charge effect are taken into account in the reconstruction with the “Forward method”.

8LBS line alternative – L4 BCC meeting - 25/07/2013 – JB Lallement

Reconstructing the longitudinal emittance: Location

What do we need?• A BSM (bunch shape monitor).• A cavity : The “debuncher”.• Find a location and some settings to do the measurement.

• Some free space to locate the BSM.• Some reasonable distance from the cavity (level arm).• The beam should still be “small” (RMS phase < 50-60°).

BHZ.20

Debuncher

J. Humbert

9LBS line alternative – L4 BCC meeting - 25/07/2013 – JB Lallement

Reconstructing the longitudinal emittance: Location

Proposed layout

Present layout

Two changes induced in the sector 6 (plateau): Steerer and support position.-> No impact on the beam dynamics !

10LBS line alternative – L4 BCC meeting - 25/07/2013 – JB Lallement

Reconstructing the longitudinal emittance

Scanning the amplitude (red: OFF, green: 0.7 MV “nominal”, blue: 1.2 MV)

Scanning the phase (red: 90°, green: -90°, blue: 0°-180°)

We can vary both the amplitude and the phase of the cavity : “3 Phase-Ampl method”.Have to make sure that the cavity has a measurable effect on the beam phase.

Horizontal bend Vertical stepCavity BHZ.20

11LBS line alternative – L4 BCC meeting - 25/07/2013 – JB Lallement

Reconstructing the longitudinal emittance

-270 -240 -210 -180 -150 -120 -90 -60 -30 0 30 60 9020

25

30

35

40

45

50

55

RMS beam phase @ BSM vs cavity phase

Cavity phase (°)

RMS

beam

pha

se a

t BSM

(°)

-1.2 -1 -0.8 -0.6 -0.4 -0.2 010

15

20

25

30

35

40

RMS beam phase @ BSM vs cavity amplitude

Cavity effective voltage (MV)

RMS

beam

pha

se a

t BSM

(°)

Factor of 2 Max and Min phase !

12LBS line alternative – L4 BCC meeting - 25/07/2013 – JB Lallement

Reconstructing the longitudinal emittance: Cavity characteristic

Depending mainly on the input beam parameters and distance between the cavity and the diagnostic. The cavity effect on the bunch length is clearly measurable!

Expected resolution at the BSM< 15 keV for 1° (conservative).

For the LBS, 100 keV/mm

RMS beam phase at BSM vs cavity parameters

13LBS line alternative – L4 BCC meeting - 25/07/2013 – JB Lallement

Experience in other Labs: SNS and J-PARC

-0.10

-0.08

-0.06

-0.04

-0.02

0.00

0.02

0.04

0.06

0.08

0.10

-0.50 -0.40 -0.30 -0.20 -0.10 0.00 0.10 0.20 0.30 0.40 0.50

Phase, rad

Ener

gy D

evia

tion,

MeV

Tank 1 Entrance

Tank 1 Exit

BSM

“LINAC4 … Uniquely positioned to carry out many valuable physics studies.”, D. Jeon

A. Aleksandrov, SNS

A. Feschenko, D. Jeon, et al., INR, SNS

A. Feschenko, D. Jeon, et al., INR, SNS

M. Ikegami, J-PARC

14LBS line alternative – L4 BCC meeting - 25/07/2013 – JB Lallement

Our recent experience: 3 MeV test stand !

1000 1500 2000 2500 3000 3500 400010

20

30

40

50

60

70

Beam RMS phase vs Cavity amplitude

MeasurementsSimulationSim (double E spread)Reconstructed

Buncher 2 Amplitude (A.U.)

Beam

RM

S ph

ase

(°)

BU.1 fixed

BU.2 varrying

BU.3 OFF

BSM

MeasurementReconstruction

RFQ

15LBS line alternative – L4 BCC meeting - 25/07/2013 – JB Lallement

Our recent experience: 3 MeV test stand !

BU.1 fixed BU.2 varrying BU.3 OFF

BSM

MeasurementReconstruction

RFQ

Expected : 21 keVReconstructed : 22 keV

Reconstruction technique and diagnostic performance were validated !

16LBS line alternative – L4 BCC meeting - 25/07/2013 – JB Lallement

Input from diagnostic and RPBeam aperture increased to 100 mmQuadrupole shielding?Ok for mid 2015Cost approx. 170 kCHF(U. Raich, A. Feschenko)

Tungsten melting point = 3680 K400 µs, 40 mA, 1 Hz

(F. Zocca)

Approx. 1% of the beam intercepted by the 100 µm wire. Assuming 100% stripping -> around 30 W losses in BHZ.20 (400µs, 40 mA)

• Quite often during commissioning (we can reduce the pulse length).• Limited to few minutes every week during operation.• Effects could be mitigated by having a thicker chicane in L2 tunnel.

17LBS line alternative – L4 BCC meeting - 25/07/2013 – JB Lallement

Solution LBE-BSMLBE as foreseen (back on the left) and a BSM in the L4T sector 6 (keep the present LBS for ions ≠location?).Many issues suddenly solved (disappeared) and cost drastically reduced.

• Replacing [2 bends, 2 quads, 1 BCT, 1 SEMgrid, 1 beam dump, etc …] by [1 BSM].• Integration issue solved.• No civil engineering and transport (PSB wall, PS access etc…).• Having the LBE, we keep a beam dump at PSB injection.

J. Humbert

18LBS line alternative – L4 BCC meeting - 25/07/2013 – JB Lallement

Any question?

19LBS line alternative – L4 BCC meeting - 25/07/2013 – JB Lallement

Reconstructing the longitudinal emittance

Let’s replace the quadrupole with a cavity and the grid by a BSM.

Cavity

0 1 2 3 4 50

0.5

1

1.5

2

2.5

3

3.5

MeasurementExpectedReconstructed

Cavity setting

Beam

size

(A.U

)

Φ

E