1. F. Zocca, F. Roncarolo, B. Cheymol, A. Ravni,
H0/H- current monitor
F. Zocca, F. Roncarolo, B. Cheymol, A. Ravni,
S. Burger, G.J. Focker, J. Tan
BE/BI
Linac4 BCC Meeting 43 - H- stripping test - 23rd April 2013

2. Outline H0/H- current monitor overview: Concept & specifications
Geometry & beam dynamics
Material
Sensitivity & expected signals
Status & on-going developments
Importance of the “half-chicane section” test for instrumentation
Other advantages in case of test implementation in L4T line (after bending)
F.Zocca - Linac4 BCC Meeting 43 - H- stripping test - 23rd April 2013

3. H0/H- current monitor concept
B.Goddard et al., 2010
H0/ H- current monitor needed in front of the dump
- to allow efficient setting up of the injection
- to monitor the efficiency of the stripping foil (detect degradation and failure)
The H0/H- current monitors are supposed to be plates intercepting the H0 and H- ions and acting as a Faraday cup for the stripped electrons (stripping & collection)
F.Zocca - Linac4 BCC Meeting 43 - H- stripping test - 23rd April 2013

4. Functional specifications
Robust and simple (lifetime ≈ 20 years, no maintenance)
Radiation dose of MGy per year
Vacuum level 10-8 mbar with beam
Withstand the BSW4 pulsed magnetic field of 0.4T and at the same time do not perturb the field by more than ≈ 1 %
Transverse dimensions (including support structure) not exceeding dump dimensions
Sensitive areas maximized to cover as much as beam halo as possible
Withstand the heat load in normal operation condition and a full Linac4 pulse load (2.5×1013 H- ions), in case of failure of the stripping foil, on a one-off basis, several times per year
Dynamic range: 5×107 – 5×1012 ions (for H- and H0 alike)
Absolute accuracy ± 20 %, relative accuracy ± 10 %
Time resolution: integral over the full injection time (few ms – 100 ms) – however higher resolution is welcome
F.Zocca - Linac4 BCC Meeting 43 - H- stripping test - 23rd April 2013

5. Monitor geometry preliminary proposal
Top view
Missing on the front frame only
Signal plates
Polarization frames
F.Zocca - Linac4 BCC Meeting 43 - H- stripping test - 23rd April 2013

6. SiC Dump 110 mm + 30 mm Pol. Frames
Beam envelopes
SiC Dump 110 mm + 30 mm Pol. Frames
Courtesy of C.Bracco
Separation H0/H- = 10.8 mm
Separation H0/H+ = 2.8 mm
Distance dump edge/ H0 beam = 0.5 mm
F.Zocca - Linac4 BCC Meeting 43 - H- stripping test - 23rd April 2013

7. Plates material: titanium
Requirements: good enough conductivity (for signal read-out) but compatible with BSW4 field quality, low thermal load, low neutron yield (low activation), high signal level
Fully acceptable
Among low-Z conductive materials, titanium is the only one with acceptable impact on the BS4 field quality thanks to the relatively “low” conductivity
Acceptable (not ideal)
Not acceptable
Material
Conductivity (1/Wm)
(for signal read-out)
Thermal load (DT) for a full Linac4 pulse
Melting point
Neutron yield
(w.r.t. n° of protons)
Signal
Q (e/H-) with NO external fields*
Q (e/H0) with NO external fields*
Compatibility with BS4 field
Graphite
6.1 × 10 4
67 K
3773 K
0.41 %
- 1.83
- 0.90
YES
Aluminum
3.77 × 10 7
50 K
933 K
0.57 %
- 1.63
- 0.80 NO
Titanium
2.34 × 10 6
80 K
1933 K
0.99 %
- 1.42
- 0.70
Copper
5.96 × 10 7
98 K
1356 K
1.0 %
- 1.22
- 0.60
Tungsten
1.89 × 10 7
229 K
3683 K
6.4 %
- 0.68
- 0.33
* taking into account losses due to electron backscattering and secondary emission
F.Zocca - Linac4 BCC Meeting 43 - H- stripping test - 23rd April 2013

8. Charge signal estimate
Proton energy = 160 MeV  electron energy = 87 keV
Q (e/H-) = -2*(1-h) + 2*SEYP + 2*SEYBS + YD
h = fraction of backscattered electrons (e- energy range ≈ 1-87 keV)
SEYP = Secondary Emission Yield of the Proton (e- energy range ≈ 1-20eV)
(SEY of H- entering the plate = SEY of proton exiting)
SEYBS = Secondary Emission Yield of one BackScattered electron
YD = fraction of “delta-rays” electrons emitted by the plate owing to
collisions with the proton beam (e- energy range ≈ keV)
Q (e/H0) = - (1-h) + SEYP + SEYBS + YD
Material h
SEYP
SEYBS YD
Q (e/H-)
Q (e/H0)
Titanium
0.23
0.038
0.0114
0.025
- 1.42
- 0.70
F.Zocca - Linac4 BCC Meeting 43 - H- stripping test - 23rd April 2013

9. Polarization rings: E-field effect
View from the top
CST Particle Studio
tracking simulation
Electron energy range = eV
Isotropic angular distribution
Stationary condition
No space charge
frame ( V)
monitor plates
frame ( V)
E-field map on the monitor plates
Effect due to the missing lateral frame
Simulations including surrounding beam pipe
F.Zocca - Linac4 BCC Meeting 43 - H- stripping test - 23rd April 2013

10. E-field + B-field effect (BSW4 magnet 0.4T)
Uniform vertical B-field of 0.4 T
Stationary condition, no space charge
10 mm
Secondary emission electrons (10 eV - 30eV)
Curvature radius for 30eV electrons ≈ 30 mm
Backscattered electrons (60 keV – 90 keV)
Curvature radius for 90keV electrons ≈ 2.6 mm
“Delta-rays” electrons (125 keV – 375 keV)
Curvature radius for 375 keV electrons ≈ 6 mm
F.Zocca - Linac4 BCC Meeting 43 - H- stripping test - 23rd April 2013

11. Expected signals Stripping foil of ≈ 200 mg/cm2 :
H- stripped to H0 ≈ 1 % ,
H- stripped to H+ ≈ 10-6 level BUT assume that 1% of H- from the LINAC4 beam will miss the foil and impact the dump
 nominal number of particles hitting the monitor per injection = 2.5 × for H0 and H- alike
Desired dynamic range = 5 × × 1012 particles (for H- and H0 alike)
Q (e/H-)
Average Pulse Current MIN
Average Pulse Current NOM
Average Pulse Current MAX H-
SEY + BS + YD 
113 nA
0.56 mA
11 mA
BS + YD 
121 nA
0.6 mA
12 mA
Full deposition  -2
160 nA
0.8 mA
16 mA H0
SEY + BS + YD 
56 nA
0.28 mA
5.6 mA
BS + YD - 0.75
60 nA
0.3 mA
6 mA
Full deposition  -1
80 nA
0.4 mA
8 mA
F.Zocca - Linac4 BCC Meeting 43 - H- stripping test - 23rd April 2013

12. Status Almost finalized: Material choice Design principle
Theoretical study of sensitivity – range of expected signals
On-going developments:
Final dimensions (w.r.t. dump size & beam dynamics simulations)
Mechanical support
Final integration in the system (design office)
Simulation of the impact on the BSW4 field (by magnet group)
Cables type and quantity
Read-out electronics
F.Zocca - Linac4 BCC Meeting 43 - H- stripping test - 23rd April 2013

13. What is important to test
Operational check including mechanical integration issues, thermic and mechanical stress (beam heat load and magnetic field)
Impact on the BSW4 field quality – beam positions & envelopes
Sensitivity to H- and H0 ions: effect of the E-field and of the B-field on the secondary electron suppression (and eventually on the backscattering effect suppression)
Read-out electronics: noise level and amplitude of picked-up induced signals on the plates  decision between charge integration or current read-out mode
Control signals for start/stop read-out according to injection start/stop
Calibration procedure of the H- and H0 currents: with unstripped H- beam of known intensity, profiting from BLMs for H0…?
Interlock reaction (protecting the injection system)
Long-term stability of the measured signals
F.Zocca - Linac4 BCC Meeting 43 - H- stripping test - 23rd April 2013

14. The presence of the bending magnet L4T.MBH.0250…
… allows us for a more complete test of the laser-wire scanner
Test of the stripping unit (≈ 20 cm)
40 cm long slot allocated between the steerers L4T.MCHV.0115 and L4T.MCHV.0135 for installing a laser station + profile monitor via stripped electron counting
F.Zocca - Linac4 BCC Meeting 43 - H- stripping test - 23rd April 2013

15. Conclusions
“Half-chicane section” test is of extreme importance for instrumentation commissioning
Despite the relatively simple working principle, many issues need to be checked for the H0/H- current monitor, regarding
- mechanical integration
- measurement sensitivity & read-out electronics
- interlock system
Beam instrumentation developments (laser wire emittance meter) would much profit from the presence of the bending section  we strongly support the option of the “half-chicane section” test in the L4T line
F.Zocca - Linac4 BCC Meeting 43 - H- stripping test - 23rd April 2013