Stony Brook Update: A bit more on Negative Ions

1. Stony Brook Update:A bit more on Negative Ions T.K. Hemmick for the Tent Crew

2. HV Preamp Amp HV Scope GEM HV HV Preamp Amp GEM GEM 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

3. HV Preamp Amp 5 mm HV Scope GEM HV HV Preamp Amp GEM GEM TOF Spectrometer: • Increased gap to mesh. • Allows “TOF” measurementto separate (somewhat)the prompt electrons fromthe late ions. electrons ions Vmesh=900

4. True Path ShouldInclude Diffusion 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.

5. Scans at various pressures • Data corrected for: • Noise • Lamp Drift • Ion tail • 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…

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

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 mm. 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. 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 mm). • 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 mm. • Lower Limit to 1/e lengths ~40 mm. • 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.