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1
Stony Brook Update:A bit more on Negative Ions
T.K. Hemmick for the Tent Crew
2
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
3
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
4
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
5
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
6
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
7
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
8
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).
9
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.
10
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.
11
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…
12
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.
13
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.