An alternative to the LBS measurement line for Linac4 Reconstructing the longitudinal emittance
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Transcript of An alternative to the LBS measurement line for Linac4 Reconstructing the longitudinal emittance
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’
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E
Φ
The beam is characterized by its 6 D emittance:Horizontal, Vertical and longitudinal emittances
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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”.
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X
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Quad
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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
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RMS beam phase @ BSM vs cavity phase
Cavity phase (°)
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t BSM
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-1.2 -1 -0.8 -0.6 -0.4 -0.2 010
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RMS beam phase @ BSM vs cavity amplitude
Cavity effective voltage (MV)
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se a
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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
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-0.50 -0.40 -0.30 -0.20 -0.10 0.00 0.10 0.20 0.30 0.40 0.50
Phase, rad
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gy D
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tion,
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Tank 1 Entrance
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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 !
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Beam RMS phase vs Cavity amplitude
MeasurementsSimulationSim (double E spread)Reconstructed
Buncher 2 Amplitude (A.U.)
Beam
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ase
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BU.1 fixed
BU.2 varrying
BU.3 OFF
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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
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MeasurementExpectedReconstructed
Cavity setting
Beam
size
(A.U
)
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E