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Stony Brook Update:A bit more on Negative Ions

T.K. Hemmick for the Tent Crew

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Brief Reminder• Charge Transfer Tests• Flash Lamp pulsing through MgF2 window.• Simple system:

• Two Modes:– GEMS same;

collect charge on the Grid– Grid && Top-Top same

• Adjust dV of GEM• Adjust dV of 1st gap• Collect charge on mid GEM

Preamp

Preamp

HV

HV

HVHV

GEM

GEM

GEM

Amp

Amp

Scope

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TOF Spectrometer:

• Increased gap to mesh.

• Allows “TOF” measurementto separate (somewhat)the prompt electrons fromthe late ions.

Preamp

Preamp

HV

HV

HVHV

GEM

GEM

GEM

Amp

Amp

Scope

5 mm

Vmesh=900

ionselectrons

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Procedure

• Low pressure allows one to achieve large mean-free-path (MFP) without sparking.

• Large MFP with significant field allows one to measure transmission with higher electron-ion collision energies.

• High Energies are necessary to achieve absorption cross sections.

• Results can be “partly” scaledfor effective transmissioncoefficients:– Veff=Vapplied * (1 atm/Papplied)

– Losses under-estimated for lowpressure measurements.

True Path ShouldInclude Diffusion

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Scans at various pressures

• Before absorption, existence of gas leads to some loss (8% loss 1 atm vs 0.1 atm).

• Tails do not align all that well after scaling.• Should normalize for primary yield…

e peak 3 us shaping time

0123456789

10

0 2000 4000 6000 8000 10000 12000

Adjusted drift gap (V / 5 mm)

Pu

lse

hei

gh

t (V

) 0.1 atm

0.2 atm

0.4 atm

0.6 atm

0.8 atm

1 atm

Data corrected for:• Noise•Lamp Drift•Ion tail

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Fully Corrected scans

• Upper limits to 1/e length can be calculated:– Take “full yield”, Y0 as peak of 0.1 atm scan.– Blame loss at high field: Y = Y0e-5mm/L0

– Learn L0 as a function of effective V.– Compare to known cross sections…

e peak 3 us shaping time all corrections

0

1

2

3

4

5

6

7

8

9

10

0 2000 4000 6000 8000 10000 12000

Adjusted drift gap (V)

Pu

lse

he

igh

t (V

) 0.1 atm0.2 atm0.4 atm0.6 atm0.8 atm1 atm

Data corrected for:• Noise•Lamp Drift•Ion tail•Equal yield at ~2000 V/5mm

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Attenuation length – measured, Upper Limit

• Remember that because of decreased diffusion, the measurements are further from the truth for the lowest pressures.

• Fortunately, these measurements seem to be saturating in the 0.4 and 0.6 atm results.

• Can compare to Lower Limit from hitting the resonant cross section(s) exactly.

• Lowest point is yellow curve at ~300 m.

Attenuation Length

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

0 2000 4000 6000 8000 10000 12000

Adjusted drift gap (V)

L (

Mic

ron

s)

0.1 atm

0.2 atm

0.4 atm

0.6 atm

0.8 atm

1 atm

Rising QE

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Mean Free Path – theoretical Lower Limit

• Assume that the electron energy during a collision is exactly in resonance (worst case).

• λ = 1/nσ• “Worst case” scenario:

– λa = 200 μm at 7 eV

– λd = 40 μm at 15 eV

• Since these MFP’s are smaller than the measured lengths, it is not impossible that the losses are indeed due to ion transport.

0.01

0.1

1

10

100

0.01 0.1 1 10 100

Electron energy e (eV)

s (

10-1

6 cm

2 )

Ar - - - - CF4 ——

sm

sexc

s

ion

sion

sn 4sn 3

sm

sa

sn ind

sd

Fig. 1. Electron scattering cross-sections in Ar and CF4: elastic momentum transfer (σm), vibrational excitation (σν4, σν3, σνind ), electron attachment (σa), dissociation (σd), excitation (σexc), and ionization (σion).

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What would we do to learn final answer:

• Pure Theory:– Transport in gas through HBD collection field

and measure the result w/ and w/o the absorption cross sections running.

• Measurement:– Take transmission vs. field result and run this

through the HBD collection field.

• Common Denominator:– Need the HBD Collection Field (in reverse bias).– We’ve done 2D field simulations (Maxwell).– We must purchase $$$ code to do 3D.

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Forwardbias

• NOTE: Maxwell display is non-standard:– Field lines are not continuous.– Density of field lines has no meaning.– COLOR of the field lines specifies field strength.

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Reverse Bias

• Different than TKH’s imagination:– In reverse bias collection region is “tall” (> 150 m).– A non-zero region of the cathode area does not enter hole.

• These calculations must be re-done in 3D…

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Summary• At low fields, there is no loss.• At high fields, there is a loss region prior to the

gain region:• Limits on max loss in worst field:

– Upper Limit to 1/e lengths ~300 m.– Lower Limit to 1/e lengths ~40 m.

• Field of HBD goes from:– Low field, good transmission.– Medium field, high absorption.– High field, gain (home free!)

• Need to convolute more realistic field profiles with the absorption limits to get effective transmission.

• Results are probably not too bad as long as the regime of medium field is fairly short in length…seems likely to TKH’s imagination.

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Other News

• These will be the last measurements of the transmission for a while (even though further conclusions can be forth-coming based upon E-field calculations).

• We’re now getting ready for the rebuild:– Clean tent survey with brand new dust meter.– Clean up the tent’s bad spots.– Beginning survey of status of extra GEMs.

• Expect to make a cathode for scintillation measurements by end of week.

• Will start thinking about practical shades design.