# Presentation: 004098

## Expected signal strength,detection schemes.

- A.Aleksandrov.

## Basic assumptions

- Cross-section is well known therefore stripping efficiency calculation is a matter of algebraic manipulation (tech. notes)
- Range of beam parameters:
- SCL energy: 186MeV-1000MeV
- Current:         20mA-38mA
- Pulse length: 50us-1000us
- Beam size:    1.5mm- 2mm-  2.5mm :nom.emm.
-                           2.0mm-2.8mm-3.5mm :2 x nom.emm.

**Laser parameters:**
- NdYAG pulsed; 1064nm; 12ns; 200mJ;
- Note: much more powerful laser are available: ex. 3J NdYAG
- Higher risk of window damage, more expensive optics, etc.

## Calculated stripping efficiency r [%]

|  |  |  |  |  |  |  |  |
| --- | --- | --- | --- | --- | --- | --- | --- |
|  |  | 1.5 x 2.5      mm | 2 x 2      mm | 2.5 x 1.5      mm | 2 x 3.5      mm | 2.8 x 2.8      mm | 3.5 x 2      mm |
| 200 mJ | 300MeV | 19% | 23% | 30% | 14% | 17% | 23% |
|  | 1GeV | 13% | 16% | 21% | 9% | 12% | 16% |
| ∞
 | - | 20% | 25% | 33% | 15% | 18% | 25% |

- Nominal emittance

- 2 x Nominal emittance

- It is safe to assume stripping efficiency of 15%

**12ns**

- Ibeam

-  % of Ibeam

## Signal strength and noise estimation

- Assuming 15% stripping efficiency, 20mA beam current, 20:1 transformer ratio, 50 Ohm load we have 12 ns pulse of 7.5mV amplitude. (laser wire at beam center)
-  S/N estimation  (M.Kesselman): ~35dB
- Optimized filter (gaussian, 40MHz BW)
- Thermal noise + amplifier noise (4 x 50ohm resistor)
- allows measurement down to ~ 3 sigma
- No beam noise (can be dominant factor)
- Differential measurements to mitigate beam noise effect:
- 2 BCMs hundreds meters apart + long cables  + beam energy variations ( commissioning) make it difficult

## Signal strength (number of electrons x 108) for 20mA beam

|  |  |  |  |  |  |  |  |
| --- | --- | --- | --- | --- | --- | --- | --- |
|  |  | 1.5 x 2.5      mm | 2 x 2      mm | 2.5 x 1.5      mm | 2 x 3.5      mm | 2.8 x 2.8      mm | 3.5 x 2      mm |
| 200 mJ | 300MeV | 2.8 | 3.5 | 4.5 | 2.1 | 2.5 | 3.5 |
|  | 1GeV | 1.9 | 2.4 | 3 | 1.4 | 1.7 | 2.3 |

- Nominal emittance

- 2 x Nominal emittance

- It is safe to assume number of released electrons of 2·108

- U = 2e8*1.6e-19/12e-9*50 = 133mV  on 50 Ohm load (laser at beam center)

- Laser beam

- Electron collector

- Bending magnet

## Detection of released electrons

- Advantages:
- large number of electrons
- charge integrating amplifier similar to BLM
- Energy of electrons is well defined
- Electron beam is well collimated
-  Drawbacks
- External magnets are required
- In vacuum collectors are required
- Might suffer  from beam loss background

- } easy to collect

- } good S/N

## Background charge

- Sources of background are
- Stripping on residual gas
- 38mA; 200MeV; 1e-9 torr nitrogen; 1ms:  3.3e6 electrons/m
- 10-3 background  in 10cm detector
- 20mA; 200MeV; 1e-9 torr nitrogen; 50us:  8.7e4 electrons/m
- < 10-4 background  in 10cm detector
- Beam losses
- 1W/m -> Ne = 2e6  for 10cm^2 collector area at 200MeV
- 		    Ne = 4e5  for 10cm^2 collector area at 1000MeV
- Can be major source of background (~1e-2)
- Time gating can help
-  Experience from other machines? (PSR, etc.)

## Magnet requirements

- Electrons released in the process of photo-detachment have well defined velocity along the beam. They can be easily deflected by magnetic field and collected. Bending radius for electrons in a uniform magnetic field is

- Reasonable choice for bending radius is

- where L is available longitudinal space between laser beam and electron detector, d is diameter of the vacuum chamber. For current warm section design L=12cm, d=7cm  therefore R=20cm is a reasonable choice for estimation of required magnetic field strength.

## Calculated values of the magnetic field for each warm section of the SCL 

| E[MeV] | 186 | 204 | 223 | 241 | 259 | 277 | 296 | 314 | 332 | 351 | 369 | 387 |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| B[Gs] | 70 | 74 | 77 | 80 | 84 | 87 | 90 | 94 | 97 | 100 | 103 | 105 |
| E[MeV] | 438 | 489 | 540 | 591 | 642 | 694 | 745 | 796 | 847 | 898 | 949 | 1000 |
| B[Gs] | 113 | 121 | 128 | 136 | 143 | 150 | 157 | 163 | 170 | 177 | 183 | 190 |

-  Required magnetic field spans from 70Gs at lowest energy to 190Gs at the linac exit. As beam energy can deviate from design values during commissioning/operation it is desirable to have magnets with variable field.
-  Optimal solution might be hybrid PM/EM design where 130Gs are generated by permanent magnet and EM coils generate 60Gs.
-   Note that electron detector been discussed is not an imaging device therefore high quality of the field is not required.
-   Deflection of the ion beam is negligibly small an can be corrected by baseline steering correctors therefore no compensating magnet is required.

## Signal strength (number of electrons x 108) for 20mA beammode locked laser 

- Signal strength (number of electrons x 108) for 20mA beammode locked laser

|  |  |  |  |  |  |  |  |
| --- | --- | --- | --- | --- | --- | --- | --- |
|  |  | 1.5 x 2.5      mm | 2 x 2      mm | 2.5 x 1.5      mm | 2 x 3.5      mm | 2.8 x 2.8      mm | 3.5 x 2      mm |
| 200 mJ | 300MeV | 150 | 200 | 240 | 110 | 130 | 170 |
|  | 1GeV | 70 | 90 | 110 | 50 | 60 | 80 |

- Nominal emittance

- 2 x Nominal emittance

- It is safe to assume number of released electrons of 1·1010

- U = 1e10*1.6e-19/12e-9*50 = 6.7V  on 50 Ohm load (laser at beam center)

- NdYAG

- beam

- Mode locked

## conclusions

- One can expect about 15% neutralization yield at beam center for realistic commissioning beam parameters
- Expected S/N ratio is about 35db but beam noise can be major limiting factor
- Electron collection option might extend dynamic range considerably while not preventing from using BCM
- Electron collection  allows use of mode-locked laser as an attractive option