Title: Final Design Review for The SNS BLM System R' Witkover, D' Gassner
1Final Design Review forThe SNS BLM System R.
Witkover, D. Gassner
2Choice of Detector
- Ion Chamber
- Good uniformity, linearity, stability
- Resistant to radiation
- Scintillator/Photomultiplier
- Wide dynamic range, fast response
- Requires frequent calibration
- Susceptible to radiation Damage
- Pin Diode
- Good uniformity, linearity, stability
- Not well suited to pulsed beam
- 107 Max pulse rate ? 1000 cnts during macropulse
- Ion Chamber is best choice
3FNAL Ion Chamber
- Designed by Shafer for Tevatron 1982
- Sealed glass, Argon filled
- Packaging improved for RHIC to reduce noise
4FNAL Detector Production Problem
- Poor uniformity with preferred polarity
(electrons to center electrode) so RHIC used
opposite - BUT Slower ion transit time and earlier
saturation
5Problem Solved
- Notching of outer electrode eliminated crimp at
support lower gradient
6Original BLM Detector Design For SNS
- Advantages
- Low Cost ( 450 assembled)
- Long history of Design/Manufacture (1982 -)
- Proven record in Accelerators (Tevatron, RHIC)
- Disadvantages
- Recent problems with sole source vendor (2001)
- Poor unit-to-unit response for preferred bias
polarity in later units - Slow Ion Collection time (600 µsec at 3000 V)
- Poor Collection Efficiency (saturation) at high
loss rate (35-40 for 1 local loss at 3000V)
7Improving Ion Transit Time
- Ion speed vion µ0 E(P0/P) µ0
(V/d )(P0/P) - Where µ0 Ion Mobility, V Voltage, P/P0
Pressure (Atm) -
- Transit Time t d/ vion d2/µ0 V
(P0/P) - For plane geometry d Electrode gap
- For cylindrical geometry d2 (a2 b2)/2 ln
(a/b) - Where b Inner radius and a Outer radius
- The new design uses the same outer diameter of
1.5 inches but an inner diameter of 1 vs 1/4-inch
and active length of 6.7 vs 4 inches to get the
same sensitivity.
8Efficiency and Ion Transit Time vs Inner Radius
9Calculation of Collection Efficiency
- Difficult calculationsolutions available for
certain regimes - Static
- Duration long compared to ion transit time
- Electron-Ion pairs continually created drift in
opposite directions - Pulsed
- Duration short compared to ion transit times
- Electrons rapidly collected leaving only ion
cloud - Solutions shown due to Boag and others
10Collection Efficiency for Continuous Loss
Specific Charge density Equivalent Gap
I sig Chamber signal current a Outer
radius b Inner radius a Townsend
recombination coefficent k1 Electron
mobility k2 Ion mobility e Electronic charge
Efficiency
11Collection Efficiency for Continuous Loss
12Collection Efficiency for a Pulsed Beam
Specific Charge density
Collection Efficiency
13Collection Efficiency for a Pulsed Beam
14New BLM Detector Design
- Advantages
- Better HV design
- Should improve unit reproducibility
- Should allow higher bias Voltages
- Better Collection Efficiency for high loss rate
- Much faster Ion Collection time
- Low-skill design- can be made by multiple
vendors - Disadvantages
- Development time and funding not in original
scope - Unforeseen production problems may arise
- Increased cost
- Long-term stability unknown
15SNS Prototype Ion Chamber
16Simulated Linac/HEBT Response
17New Design Prototypes
- 3 prototypes built
- 1 Unable to hold 3 kV with Argon filled with N2
- Installed in Linac for tests
- Tests done at linac at 200 MeV, 2-4 kW local
loss - 2 Redesigned ceramic insulators, Signal
electrode. Higher Voltage feedthru - Held 3.7 kV with Argon
- Installed in Booster to AGS (BTA) Line Tests done
at 1.2 GeV 5-10 W local loss - 3 Shown to vendors as model
18Beam Test of FNAL SNS Prototype BLM
19Detector Saturation Tests
20Pulsed Beam Tests in BNL BTA Line
21X-ray Sensitivity of RHIC BLM
X-rays produced by RHIC 200 MHz RF cavities
Vertical Scale is in kV And Pico-Amps
22Future X-ray Tests
- Future tests required
- 1/8, ¼, 3/8, ½ thickness lead shields
- Estimates from LANL and SNS suggest 10s of R/hr
X-rays at 30 cm - BNL test Signal 1000 pA 51 R/hr during
pulse.
23Status of Detector
- New detector design shown to 3 vendors
- LND (Long Island, NY)
- ADC (Ithaca, NY)
- Far West (San Diego, CA)
- LND built prototype undergoing testing
- ADC .preparing estimate
- Far West. preparing estimate
- Preliminary estimate at BNL Material and labor
costs 800, vs 450 for FNAL design. Industrial
production cost will probably be higher.