Linac4 BCC Meeting 43 - H - stripping test - 23 rd April 2013

Linac4 BCC Meeting 43 - H- stripping test - 23rd April 2013

H0/H- current monitor

F. Zocca, F. Roncarolo, B. Cheymol, A. Ravni, S. Burger, G.J. Focker, J. Tan

BE/BI

F.Zocca - Linac4 BCC Meeting 43 - H- stripping test - 23rd April 2013

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)


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)

H0/H- current monitor concept

Functional specifications

Robust and simple (lifetime ≈ 20 years, no maintenance) Radiation dose of 0.1-1.0 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


Monitor geometry preliminary proposal

90 mm

56 mm

3 mm 5 mm

R = 25 mm

3 mm

3 mm

3 mm 3 mm

3 mm

50 m

m

37 mm

42 mm

90 mm

56 mm

R = 25 mm

40 mm 45 mm

10 mm10 mm10 mm

ring (- 1000 V)

ring (- 1000 V)monitor plates

dump

Top view

Signal platesPolarization frames

Missing on the front frame only


Courtesy of C.Bracco

Beam envelopes

SiC Dump 110 mm + 30 mm Pol. Frames

H0 H-

Separation H0/H+ = 2.8 mm Distance dump edge/ H0 beam = 0.5 mm

Separation H0/H- = 10.8 mm


Plates material: titanium

MaterialConductivity

(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*

SignalQ (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 YES

Copper 5.96 × 10 7 98 K 1356 K 1.0 % - 1.22 - 0.60 NO

Tungsten 1.89 × 10 7 229 K 3683 K 6.4 % - 0.68 - 0.33 NO

Fully acceptable

Acceptable (not ideal)

Not 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

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

* taking into account losses due to electron backscattering and secondary emission


Charge signal estimate

Q (e/H-) = -2*(1-h) + 2*SEYP + 2*SEYBS + YD

Q (e/H0) = - (1-h) + SEYP + 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 ≈ 100-400 keV)

Proton energy = 160 MeV electron energy = 87 keV

Material h SEYP SEYBS YD Q (e/H-) Q (e/H0)

Titanium 0.23 0.038 0.0114 0.025 - 1.42 - 0.70


Polarization rings: E-field effect

frame (- 1000 V)

frame (- 1000 V)

View from the top

monitor plates

Simulations including surrounding beam pipe

E-field map on the monitor platesEffect due to the missing lateral frame

CST Particle Studio tracking simulation

Electron energy range = 10-30 eVIsotropic angular distributionStationary conditionNo space charge


E-field + B-field effect (BSW4 magnet 0.4T)Uniform vertical B-field of 0.4 TStationary condition, no space charge

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

10 mm


Expected signals

Q (e/H-) Average Pulse Current MIN

Average Pulse Current NOM

Average Pulse Current MAX

H-

SEY + BS + YD - 1.42 113 nA 0.56 mA 11 mA

BS + YD - 1.52 121 nA 0.6 mA 12 mA

Full deposition -2 160 nA 0.8 mA 16 mA

H0SEY + BS + YD - 0.70 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

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 × 1011 for H0 and H- alike

Desired dynamic range = 5 × 107 - 5 × 1012 particles (for H- and H0 alike)


Status

Material choice Design principle Theoretical study of sensitivity – range of expected signals

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

Almost finalized:

On-going developments:



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


The presence of the bending magnet L4T.MBH.0250…

… allows us for a more complete test of the laser-wire scanner

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

Test of the stripping unit (≈ 20 cm)


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

Documents

Linac4 BCC Meeting 43 - H - stripping test - 23 rd April 2013