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 MgF 2 window. Simple system: Two Modes: –GEMS same; collect charge on the Grid –Grid && Top-Top same Adjust dV of GEM Adjust dV of 1 st gap Collect charge on mid GEM Preamp HV GEM Amp Scope

3 TOF Spectrometer: Increased gap to mesh. Allows “TOF” measurement to separate (somewhat) the prompt electrons from the late ions. Preamp HV GEM 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” scaled for effective transmission coefficients: –V eff =V applied * (1 atm/P applied ) –Losses under-estimated for low pressure measurements. True Path Should Include 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… Data corrected for: Noise Lamp Drift Ion tail

6 Fully Corrected scans Upper limits to 1/e length can be calculated: –Take “full yield”, Y 0 as peak of 0.1 atm scan. –Blame loss at high field: Y = Y 0 e -5mm/L0 –Learn L 0 as a function of effective V. –Compare to known cross sections… 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. 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: –λ a = 200 μm at 7 eV –λ d = 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 Forward bias 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.