Profile Monitor SEM’s for the NuMI Beamline
Dharmaraj Indurthy, Sacha E. Kopp, (Tom Osiecki),
Zarko Pavlovich, Marek Proga, (Leif Ristroph)
University of Texas – Austin
www.hep.utexas.edu/~kopp/minos/sem/•

Foil Secondary Emission Monitors–––•

Preliminary Design––S. Kopp – U.T.-Austin
NBI2003

Data from other laboratories
Thermal modelling of foils/wires in theNuMI beam
Experience from our May 2003 prototype–‘Bayonet’-style insertion mechanism
Review of materials in & out of the vacuum
canTests of motion repeatability

Intro: Fermilab SEM’s•

Essential features of Fermilab
SEM’s:––––––––

W-Rh wires, Au plated (75 µm)
Ceramic circuit board with Pt-Agsolder pads for stringing wires
No clearing field applied
Frame is on all four sides of beam
Frame swings in-out like a door
SEM aging observed (signaldecreased by 37% by end ofKTeV run).
Each plane (X and Y) Causes
beam loss of order 6E-5 if have
1mm pitch
Wish to reduce device size along
beam direction

courtesy Gianni Tassotto
S. Kopp – U.T.-AustinNBI2003

Building on Past Experience …
While our requirements are different from SEM’s (“multiwires”) built at FNAL, the various
ingredients of the SEM we want to explore are not different from instrumentation
currently in use here and at other labs.
With time & budget constraints, we did not want to embark on an R&D effort. Thus, goingwith reasonably proven design choices was desirable.
Specifically, the proposed conceptual design has borrowed from:•

Active element – 5 µm Ti foils

CERN (G. Ferioli)•Motion Feedthrough (bellows)

LANL (D. Gilpatrick),
also MDC, Huntington catalogs•Feedback – Schaevitz LVDTFNAL (R. Reilly)•

Stepper Controls, Readback

FNAL (A. Legan)With some modification, the design presented here might be of general utility.

S. Kopp – U.T.-Austin
NBI2003

Candidate SEM Materials
Z

X0(cm)

λint(cm)

SEE(%)

Proposewire/foilThickness(µm)

BeamLoss(10-6)dBe

4

35.3

40.6?

foil25

12

SEE unknown; foils <0.001” difficult toprocure; biological hazard

C618.8

38.1

2-2.5

Wire33

2.7

Used at LANL, SLAC (wire scanner); veryfragile mechanically

Al

13

8.9

39.3

~7Foil5

2.5

SEE ages badly in beam (G. Ferioli)

Ti

22

3.6

27.5

3.5

Foil

5

3.6

Excellent longevity to 1020 dose (Ferioli)

Ni

28

1.46

~15a3-5?

Foil

10

13

Ages in beam [16]

Ag

47

0.87

~9b~6

Foil

5~10

Data from [11], but requires great carebecause oxidation will degrade signal.

W

74

0.35

9.64

Wire

75

60

SEE is for Au-plated [15]. Degrades in beam.
Experience of wire breakage if < 75µm?

Au79

0.30

8.8c~7Foil

1022Does not oxidize, but does adsorb CO [11];signal loss observed [13]ρ=8.9g/cc)
bScaled from λ (Cu) using λ-1∝A0.77
int
cValue for Pt (Z=78,ρ=21.5g/cc)
aValue for Cu (Z=29,S. Kopp – U.T.-Austin
NBI2003

Comments

dBeam loss calculated fromλint assuming

σbeam=1mm, 1mm pitch profile monitor, and0.2mm wide strips for foil detectors.

•

Wire heating grows with volume–For round wire:••––Gas cooling assumed nil
Blackbody radiation goes like surface area ~ r(Emissivity of bare Aluminum is poor ~ 0.1)

But note many materials have poor thermal
conduction (in W/cm-oC)
Don’t expect this to be dominant heat loss.

Suggests that surface to volume ratio is critical––•

5µm Ti foil

Conduction to the ends grows withcross-sectional area–•

Wide foil intercepts more beam – goes like width
dE/dx dumped in goes like thickness t

Blackbody cooling grows with surface area––•(see NuMI-B-929)

For flat foil•••Wider wire intercepts more beam -- goes like ~ rdE/dx dumped into wire grows – goes like ~ r

Foil/Wire Heating

Wire surface/volume ~ 2/r
Foil surface/volume ~ 1/t

Crude thermal model of center foil/wire–σ ~ 1mm beam at 4×1013/pulse every 1.9 sec– Assumed ε , kcond, Cp , dE/dx , ρ from CRC, PDG– Also tested if restrictive energy loss important (loss of δ rays out back of device – more imporatant for thin foils).
S. Kopp – U.T.-Austin
NBI2003

Foil vs. Wire?
•As a check of these assertions, tried ‘turningoff’ either blackbody radiation or thermal
conduction through foil/wire•NB: effect ofrestrictive energy
loss (δ rays) ignored– small at high Z

Ti foil•Looked at all materials, modelling withcorrect thermal and bulk properties•C and Al ideal,•Ti is not far behind.
S. Kopp – U.T.-Austin
NBI2003•NB: effect ofrestrictive energy
loss (δ rays) ignored– small at high Z

Beam-Induced Sag for Wire SEM’s•

•
•

•
•

Gravitational sag δy improves with greater stress (=T/A)
δy = gρAL2/T(T=tension, L=length, A=cross sect. area, ρ=density,
g=9.8m/s2)
Elongation from beam heating is linearly worsens
gravitational sag.
Yield stress is where wire breaks. Elastic limit typicallylower. For sake of discussion, assume can tension wire toyield stress.
Compare tension elongation to beam heating elongation.
Only Carbon is an attractive material for wire SEM

S. Kopp – U.T.-Austin
NBI2003accordion
springs

Foil Etching of Strips

Central beam
aperture
Foil edges for
Clamping/mounting

Halo foil

S. Kopp – U.T.-Austin
NBI2003

Stains/dirtAccordion Spring Tension

32 accordion folds•••••

Max elastic tension scales with foil width:
1mm width ⇒ achieve 0.7g
Max elongation at elastic tension limit does
not scale (?) with foil width––

may tension 32 folds by ~ 2.0mm
beam heating causes ~ 2.5µm

BEAM HEATING → ~1% TENSION LOSS
S. Kopp – U.T.-Austin
NBI2003

Tests performed of elasticity of accordion
springs (measure elongation vs appl tension)
NB: large systematic as foil “straightens out”
other (non-accordion) wrinkles
Observe near-elastic region and then regionof inelastic deformation of accordions (don’treturn to original length when tension
released).

Foil Cleaning

Photoresist to be cleaned•Sulfuric acid effective inremoving chem-etching
photo-resistive coating•Cleaning technique
improved (no burning!)•Found new aqueous-based
photo-resitive layer that iseasier to clean off.

Rinsing acid off in H2O bath

S. Kopp – U.T.-Austin
NBI2003

Dirty acid after cleaning

Foil Mounting••••

Epoxy to comb using Epo-Tek
H27D (cf UT-Austin condensed
matter physicists).
10-12 Torr vapor pressure
Cures at 200ºC, bakeable to 350ºC
Note handling affected a couple
strips (1mm pitch not maintained)

S. Kopp – U.T.-Austin
NBI2003

Rail(but shouldmoveoutside)

HV foil(but need accordions)

Bellows
Feedthrough(should be larger)“BeamOutHole”
Signal Foil
Sliding Paddle

S. Kopp – U.T.-Austin(but make from Ti!)
NBI2003

SignalCablesVacuumChamber LidSignal Connections

S. Kopp – U.T.-Austin
NBI2003

Assembled SEM Chamber

S. Kopp – U.T.-Austin
NBI2003

Motion ofFoil Paddle•••••Actuate paddle in/out of beam
Driven by DC stepper motor
Must repeat ‘in’ position within 50µm.
We achieve this via precise limit switch
Confirm ‘in’ position using LVDTLVDT•••

Schaevitz Sensors, Inc.
“High radiation” series~6mm full stroke, 1mV/µm outBellows•••Standard Bellows Corp.
20K cycle lifetime, 13cm stroke
6.3cm ID, effective area ~45cm2.

Limit Switch
Linear Stage

S. Kopp – U.T.-Austin
NBI2003•••

Crossed roller bearing, 20cm travel •Max 100kg axial load, 54 N-m torque ••Sold in PIC catalogue(end of travel)

Manufactured by Honeywell
Ceramic insulators
Used in Tevatron scraper systemEnd-on View•••••

Flange-to-flange distance is 23.5cm (less than required 26cm)
Cylindrical chamber fabricated from 8” OD pipe, 8” vacuum endcap
Upper lid is now 10” OD conflat (change from wire seal in prototype)
Cylindrical design sacrifices longitudinal space along beam for ease of manufacture.
Total mass to lift: <32kg.

S. Kopp – U.T.-Austin
NBI2003

Paddle Mounting to Manipulator•

Paddle to be bolted to the 5cm OD shaft––•

Cantilevered by ~22cm from the support
at the conflats at “connector box”––••

Deflection of tube is <25µm due to paddle
weightCan keep paddle weight <2kg including
clamps if make from Ti

Worried about vibration of paddle down in tunnel
Add roller bearing assembly inside vacuum
chamber lid––

•Cables transmitted up hollow shaft
Bolt slop used to help align paddle on jig

Two stiffly mounted rollers
Roller at top is spring-loaded to contact shaft

Now cantilever distance is <3cm when paddle is

S. Kopp – U.T.-Austin
drawn up toward lid (“in beam position”)
NBI2003

NB: some ventholes not shown

Repeatability Test•••••

LVDT measures position along
axial motion (cross-check withdial indicator)
Additional dial indicator
monitors lateral position of shaft
at fully-inserted or fullyretracted position.

S. Kopp – U.T.-Austin
NBI2003

Cycle motion up and down until motor
cuts off at the limit switch
Paddle weight simulated at end of shaft
Vacuum suction simulated by Pb brickover a pulleyMotion Test Results•••••••

We found better repeatability at upperswitch than the lower by factor 2.
We conjectured that it’s better to drive
the system to the upper switch, wheregravity + vacuum helps slow motor
down after switch engages.
Observed ~1µm accuracy, but longterm drift in system of order 10 µm.
Temperature in lab varies by ~1ºC
Motion manipulator is stainless steel(CTE~12×10-6/ºC), but stand isaluminum (CTE~25×10-6/ºC)
Differential expansion ofmaterials ⇒ shifts ~8µm/ºC
NuMI SEM will be allstainless and Ti (∆CTE~4×106/ºC), so effect there may besmaller, but at the ±10µmlevel things will move?

S. Kopp – U.T.-Austin
NBI2003temp
incr.
bumpedtable

Data shiftedby 350µm

temp
incr.

temp
decr.

NB: the plotted data for<55hrs have been offset by
350µm to allow them to fit
on the same plot. The
motion test table wasbumped by S.K.

Summary•

Foil SEM design borrows from demonstrated techniques employedelsewhere•

Design has solved salient requests made for NuMI beamline–––––Beam loss 7×10-6 (cf 1.2×10-4 current multiwire)
Longevity in 120 GeV beam up to ~1020 protons (cf 2×1018 for W, Au)
Accurate (1µm) insertion of foils without interruption of beam
Smaller size in beamline direction (23cm, cf 41cm current multiwire)
Integrates well into FNAL readout, controls•

Hopefully simplified design will allow completion for July 1, 2004•

Prototype detector yet to see beam; hope for exposure during
MiniBoone re-commissioning this/next week.

S. Kopp – U.T.-Austin
NBI2003