The Vacuum Case for KATRIN

Transcription

1 The Vacuum Case for KATRIN Institute of Nuclear Physics, Forschungszentrum Karlsruhe,Germany, for the KATRIN Collaboration The vacuum requirements of the KATRIN experiment have their seeds in the physical goal to reach a sensitivity on the neutrino mass in the low sub-ev region. In short this means that on the one hand the tritium-beta-decay electrons have to be guided adiabatically from the so called Windowless Gaseous Tritium Source (WGTS) into the pre- and main-spectrometer. Simultaneously the flow of tritium molecules has to be reduced by at least ten orders of magnitude to avoid a contamination of the spectrometer region. On the other hand the background rate originated in the spectrometer region should be an order of magnitude lower than for the existing spectrometers in Mainz and Troitsk. From the comparison of measurements and simulations for the Mainz setup strong indications exist that with respect to the background the interaction between trapped charged particles in the spectrometer and residual gas molecules plays an important role. There are two different and complementary ways to lower the background rate from the spectrometer remove the trapped charged particles from the spectrometer volume and reduce the final pressure inside the spectrometer as much as possible. Based on the experience from Mainz and Troitsk, a final pressure below mbar is aimed for. In its first part this talk will give an outline of the WGTS and the differentially pumped transport system including key requirements and basic vacuum concepts. The second part deals with the requirements for the spectrometer region. Preliminary results of measurements with a vacuum test chamber will be presented.

2 The Vacuum Case for KATRIN Source and transport system: Physical requirements Reference setup Calculated tritium flow rates and suppression factors Spectrometer system: Physical requirements Main spectrometer drafts Measurements with a vacuum test chamber The Pre-spectrometer for KATRIN The vacuum requirements for the KATRIN experiment have their seeds in the aim to reach sensitivity on the neutrino mass of less than 0.2 ev/c 2!

3 Source and Transport System Physical requirements on the Source and Transport Systems: Same physical conditions for the effective tritium source Undisturbed and adiabatic transport of electrons into the prespectrometer Suppression of tritium flow rate between outlet of source tube and entrance of pre-spectrometer to values well below mbar l/s

4 Source and Transport System to pre-spectrometer V2-R V1-R WGTS CMS-R DPS2-R DPS1-R DPS1-F DPS2-F CPS1-F CPS2-F Tube V1-F V2-F V3-F V4-F Rear System WGTS Transport System Inner Loop Outer Loop WGTS: Windowless Gaseous Tritium Source DPS: Differential Pumping System CPS: Cryogenic Pumping System CMS: Calibration and Monitoring System Pumping system has to be tritium compatible: metal sealing, no organic materials

5 Source and Transport System to pre-spectrometer V2-R V1-R WGTS CMS-R DPS2-R DPS1-R DPS1-F DPS2-F CPS1-F CPS2-F Tube V1-F V2-F V3-F V4-F Rear System WGTS Transport System Inner Loop Outer Loop Column density of tritium molecules inside the WGTS tube: 5*10 17 molec./cm 2 Maximal allowed tritium flow rate into CPS1-F: 1Ci in 60 days Maximal allowed flow rate of tritium into the Pre-Spectrometer: mbar l/s

6 Source and Transport System: Reference Setup WGTS TRANSPORT SYSTEM DPS1-F DPS2-F CPS1-F CPS2-F V4-F Pre-spectrometer K 80 K 4.2 K RT Magnetic guiding of electrons by superconducting solenoids: WGTS and DPS1: B = 3.6 T diameter of tube: 90 mm DPS2: B = 5.6 T diameter of tube: 75 mm CPS1 and CPS2: B = 5.6 T diameter of tube: 75 mm Split coil magnet: B = 5.0 T Valves V1, V2, V3, V4: Diameter 200 mm, room temperature Minimal magnetic field at transported flux tube: 0.5 T V1-F V2-F V3-F Split coil magnet

7 WGTS Tube (1/3) Side View WGTS tube Setup: symmetric setup, 10 super conducting magnets length 10 m, diameter 90 mm pressure stability better 1%? temperature stability better ± 0.2 K at 30 K operating temperature 4.2 K < T < 150 K, allow bake out with K WGTS tube and DPS1 electrical insulated? allow ± 100 V potential variation

8 WGTS Tube (2/3) Side View Calculated tritium flow out of the WGTS tube (assumption of long pipes): Mode of gas flow determined by the product p d 17 2 Column density of 5 10 molecules/cm leads to -1 p d» mbar cm (at 300K) -2-1 Knudsen flow: 10 mbar cm < p d < 6 10 mbar cm Pipe conductance: L = 135 d l p Temperature and mass correction: L ~ 4 3 d l T T 30K 300K M M air tritium d p d p

9 WGTS Tube (3/3) Side View Calculated tritium flow out of the WGTS tube: q = L p - p ) ( inj end -3 With p inj = 2p = 4 10 mbar (at 30K) and neglecting p end follows q = 0.14mbar l/s (at 30K) in one direction Knudsen formula is a combination of a laminar and a molecular fraction Correction on temperature separately q = mbar l/s (at 30K) in one direction

10 DPS1-F (1/3) DPS1-F setup: Side View Needed to enable closed tritium loop (> 99.9% has to be recovered) Tube diameter 90mm, length app. 1m each, in super conducting solenoids (3.6T) Same electrical potential as WGTS tube, 4.2 K 150 K Each stage consists of one pumping port, served by two turbo molecular pumps Maximal allowed magnetic field at TMP: 5 mt Assumed conductance of pumping port: 2000 l/s, assumed pumping speed per TMP: 1850 l/s Resulting effective pumping speed per TMP unit: 960 l/s

11 DPS1-F (2/3) Side View Tritium flow and suppression factor of DPS1-F: Flow suppression factor K m of the differential pumping stage m: Sm,eff + Lmn Km = L mn q d -3 Molecular gas flow mode: p 1 d = = mbar cm (at 300K) 2S 3 d l Conductivity for connection pipe: L 12 = 12.1 = 61.3 (30K, T2 ) l s 1,eff

12 DPS1-F (3/3) Side View Tritium flow and suppression factor of DPS1-F: Conductivity of second tube, valve and first tube of transport section: L 2V L V3 l L 23 = = 32 ( L 2 V at 30K, L V3 at 80K) L + L s 2V V3 Suppression factors: K 1 = 32; K 2 = 61 Flow out of WGTS (tube+dps1-f): with = 1. 2 = WGTS K = 1 K2 a as correction factor for beaming effect. q q a mbar s l

13 DPS2-F (1/2) Top view DPS2-F setup: Reduce tritium flow into cryogenic pumping section to less than 1Ci in 60 days Diameter of tube 75mm, length 1m each in super conducting solenoids (5.6T), sections tilted by 20 Set to ground potential, working temperature: 80 K Each stage consists of one pumping port, served by one TMP, effective pumping speed per TMP unit: 960 l/s (assumed) Maximal allowed magnetic field at TMP: 5mT

14 DPS2-F (2/2) Top view Tritium flow and suppression factors of DPS2-F: 3 d tr l Conductivity of one transport system tube: L tr = 12.1 = 57 (at 80K, T2 ) l s Suppression factor for one stage: K = Total suppression factor of DPS2-F: K tr = First MC-simulations confirm calculations: K tr ~ qwgts -10 mbar l Tritium flow rate into CPS1-F: q tr = = K tr s Corresponds to less than 0.04 Ci tritium in 60 days

15 CPS1-F and CPS2-F (1/2) Top view CPS1 and CPS2 setup: Trap the remaining flow of tritium from DPS, maximal allowed flow into Pre- -11 Spectrometer 1 10 mbar l / s Diameter of tube 75mm, length 1m each in super conducting solenoids (5.6T), sections tilted by 20,wall temperature 4.2K, covered with e.g. argon frost Tritium load into CPS will be stored until the end of a tritium run (typically 60 days), removed by warming-up and purging with He gas Two separated sections to avoid migration of T 2 during warming-up Split coil magnet (5T) to house quench condensed sources

16 CPS1-F and CPS2-F (2/2) Top view Considerations on tritium flow rate and suppression factors: Accurate calculations not possible a) Sticking coefficient for T 2 at 4.2K unknown b) possibility of migration of T 2 due to its radioactivity (recoil energy) Active area of CPS: cm 2 (5 1m length, 75mm diameter) Ratio of active trap area to stored tritium is bigger than in Troitsk (argon frost) and Mainz (graphite) during standard operation Suppression factor of Troitsk setup (2 to 3 units): about 10 4 Mainz setup (2 units): 20 mci tritium evaporated, almost totally trapped Assumed effect of CPS varies between 10 4 and 10 8

17 Sources and Transport System Conclusion Tritium flow rate into DPS1:» 0.14 mbar l s (at 30 K) Tritium flow rate into DPS2:» mbar l s Tritium flow rate into CPS1:» mbar l s Maximal tritium flow rate into Pre-Spectrometer:?? «10-17 mbar l s?? Open Questions: Calculation of gas flow mode between laminar and molecular flow (WGTS) Behavior of tritium in cryogenic pumping sections with argon frost (graphite, etc.) coverage -- migration problem Suppression factor of cryogenic pumping system

18 Spectrometer System (1/3) Physical requirements on the spectrometer system: Pre-spectrometer: reject majority of decay electrons before entering the main spectrometer Main spectrometer: high resolution filter (~ 1:20000, high pass filter) for ß-electrons Electron motion has to be adiabatic Background from spectrometer system should be 1 mhz

19 Spectrometer System (2/3) Main sources for background and their counter measures : Cosmic rays: veto counter, screening electrode (J.B.) Intrinsic radioactivity: material selection, screening electrode (J.B.) Tritium contamination: differential and cryogenic pumping system Interaction between trapped charged particles and residual gas molecules

20 Spectrometer System (3/3) Counter measures: Avoid trapping conditions, spectrometer on retarding potential, shaping of spectrometer/screening electrode Remove trapped particles, e.g. with dipole electrode Reduce number of residual gas molecules? XHV conditions Aim: 10 times lower pressure than in Mainz: p final well below mbar

21 Options for the Main Spectrometer A B C A: NEG stripes in DN1000 pumping ports, estimated S eff = 80m 3 /s for Hydrogen At least ports needed (p end = mbar, outgassing rate mbar l/s cm 2 ) B: Reference Setup: Fix NEG stripes inside the main spectrometer, e.g. 4 layers on 200m 2 => ~ 60km getter stripes => $$$ C: Coating of inner surfaces with getter material, probably different surface preparation needed, a lot of open questions

22 Main-Spectrometer Sketch Possible main spectrometer setup: shaped along the calculated magnetic flux tube Pre-spectrometer: inner surface ~ 30 m 2, volume ~ 8.2 m 3 Main spectrometer: inner surface ~ 850 m 2, volume ~ 1500 m 3

23 Test Cylinder Material identical to the pre-spectrometer (stainless steel ) Diameter 50 cm, length 1.48 m, volume 0.3 m 3, inner surface 3 m 2 Final pressure, outgassing rates and residual gas composition measurements

24 Three stages of surface preparation: Test Cylinder Measurements Mechanically polished, cleaned with acetone and deionized water, baked out (300 C) Electro-polished, cleaned with acetone and deionized water, baked out (250 C) Cooled down to -20 C Delta-HELICOFLEX gaskets did not fulfill our requirements!

25 Test Cylinder Measurements (preliminary results) 1e-06 9e-07 8e e-07 pressure [mbar] 6e-07 5e-07 4e-07 3e-07 2e-07 1e time [s] p start = mbar outgassing rate: mbar l / s cm 2 p start = mbar outgassing rate: mbar l / s cm 2 p start = mbar outgassing rate: mbar l / s cm 2

26 Further Measurements Planned with Test Cylinder Continue tests of sealing: HELICOFLEX gaskets (without Delta!), gaskets delivered by different company, silver coating instead of copper Measurements (final pressure, outgassing rate, residual gas composition) with better thermal insulation to get more homogeneous temperature distribution Tests with NEG (SAES st707, getter stripes) in different length and geometries: aim for a final pressure below mbar Test of coating of inner surfaces with getter material: Is coating of inner surfaces useful, is it practical? What material should be used and how should it be fixed? Is tritium a problem for the getter material? How can the getter material be removed in case of problems?

27 Pre-Spectrometer Pre-filter to reject low energy decay electrons before they enter the main spectrometer But first: verify features of the main spectrometer design: Vacuum characteristics (final pressure, outgassing rate, sealing system) Spectrometer on retarding potential Electro-magnetic design (background, screening electrode, magnetic fields ) Almost all components of KATRIN will be tested with the pre-spectrometer!

28 Pre-Spectrometer drawing

29 Pre-Spectrometer First picture from the manufacturing at SDMS (France)

30 Open Questions - Spectrometer System Pre-treatment of vessel material Manufacturing of main spectrometer vessel (location) Transport of the main spectrometer vessel Cleaning and surface treatment Coating of inner surfaces with getter material? Manufacturing and mounting of screening electrodes Open Questions Source and Transport system Calculation of gas flow mode between laminar and molecular flow (WGTS) Behavior of tritium in cryogenic pumping sections with argon frost (graphite, etc.) coverage -- migration problem Suppression factor of cryogenic pumping system