26 January 07 Trond Ramsvik TS / MME DC Spark Test System for CLIC

26 January 07 Outline Experimental Setup Breakdown characteristics of pure metals –E sat, Energy, Power Mechanical surface treatments –Molybdenum: EDM ↔ Rolled –Copper / GlidCop / CuZr: EDM ↔ Milled Heat treatments –Results from annealing with e-beam and oven Field Stability Mass Spectroscopy Studies Knowledge obtained from DC spark tests Further plans

26 January 07 Experimental Setup Sphere / Plane geometry HV supply 0 to + 12 kV UHV Sample Tip A-meter Field Emission Measurements HV supply 0 to + 12 kV UHV Sample Tip Q-meter Scope C Switch Breakdown Measurements Switch

26 January 07 Typical Conditioning Curves - Scaled

26 January 07 Electrode Material E sat Energy Material Displacement [MV/m][mJ] Graphite83  366  10< 13 % OFE Copper151   82< 24 % Aluminium155   26< 37 % Tungsten318   167< 37 % Molybdenum431 ±  201< 10 % 1 Chromium468   168< 32 % Titanium776   212< 50 % Saturated Breakdown Fields and Energies 1 Valid for non-heated Molybdenum Perry Wilson:

26 January 07 Saturated Breakdown Fields and Energies HDS 11 Ti CTF3, HDS 11 Ti: At ~ 75 ns, energy: ~ mJ DC sparks, Ti: Energy: ~ mJ

26 January 07 Power Flow (Scaled) RF HDS Ti P max = ns RF circular Mo: P max = ns DC: 1 mm in diameter => ~0.79 mm 2 RF circular Cu: P max = ns RF: circumference x width = 16 mm 2 P = P DC x(16/0.79) Lower power flow available in the discharge compared to RF

26 January 07 Chromium d gap = 16.7 μ m → 14.4 μ m 14 % decrease E sat = (491  11) MV/m Intensive breakdown conditioning of chromium shows: equal and higher breakdown fields than Molybdenum less erosion than Titanium.

26 January 07 Breakdown Conditions In addition to the type of electrode materials, the breakdown characteristics in vacuum for a given field depend on several other important parameters: Electrode Geometry and Gap Distance: Electrode Surface “Finishing” Treatment: standard metallurgical polishing techniques mechanical chemical electrochemical heat treatment Conditioning Processes: removal of contamination and surface smoothing field emission repeated breakdown events Residual gas pressure:

26 January 07 Molybdenum : EDM ↔ Cold Rolled Mo Rolled / Chem. cleaned Mo EDM

26 January 07 CuZr : EDM ↔ Milling

26 January 07 Glidcop : EDM ↔ Milling

26 January 07 Comparison: Cu – CuZr - GlidCop Material UNS C Chemical Composition Density Melt. Point Conductivity Temper state Tensile Strength Fatigue Strength (ultras. tests) Supplier S † L †† Electr.Therm. at 10 9 cycles Mg/m 3 K μ  -1 cm -1 (IACS%) Wm -1 ∙K -1 MPa Cu-OFE (C10100) Cu > 99.99% O 2 < 5 ppm (101%) 391 cold worked 50% Luvata Oy GlidCop ® Al-15 (C15715) Cu = % Al 2 O 3 = 0.15 % (90%) 365 hot extruded, (no cold working) SCM Metal Products Inc. CuZr (C15000) Cu = % Zr = % (93%) 367 aged and cold worked 40% Hitachi Cable Corp. † Solidus †† Liquidus

26 January 07 Comparison: Cu – CuZr - GlidCop E sat = (142  2) MV/m E sat = (121  2) MV/m E sat = (115  3) MV/m

26 January 07 Possible implications for CLIC  the breakdown characteristics are similar for all three Cu materials  the choice of mechanical surface finishing techniques are important to shorten the breakdown conditioning time.  EDM: ~50 and ~200 breakdown events for CuZr and GlidCop, respectively.  Milling: Immediate conditioning for both Cu materials  more extreme differences between EDM treated and rolled Mo electrodes  a final decision of cavity materials should be based on other parameters such as from the on-going fatigue measurements.

26 January 07 Mo - heated with e-beam Conditioning almost immediately to ~450 MV/m ~ 4 hours in air between heating and mounting in spark system

26 January 07 Mo – heated in oven Immediate conditioning not observed Faster conditioning No clear improvement in the conditioning speed with increasing temperatures recrystallized

26 January 07 Field Stability of Conditioned Mo o C for 2 h

26 January 07 Field Stability of Conditioned Mo 35 breakdown events 47 runs ~2900 ~1300 ~5600

26 January 07 Mass Spectroscopy Goal: To provide quantitative information about gas releases during breakdown events. Provide necessary input parameters for future pressure distribution calculations within the PETS system and the accelerating structure Pedro Costa-Pinto

26 January 07 Gas Releases - Mo Example: Release of Hydrogen Gas Pumping Speed: ~0.3 Litre/sec

26 January 07 Gas Releases - Mo Releases of H 2 and CO gas dominate Release of gas due to breakdown events Correlation Pressure Rise Number of Molecules Number of Molecules per unit pressure rise Releases of H 2 and CO gas dominate

26 January 07 Gas releases - Mo Low Density High Density Less energy needed to release H 2

26 January 07 Gas releases – heat treated Mo Current limiting resistor removed -> E gap = 1/2∙C dis ∙ U 2 Low Density An increase in the energy over the gap causes more gas releases

26 January 07 Gas Experiments – Air/Mo Laboratory Air

26 January 07 X-ray Photo Emission Spectroscopy Reference Mo Breakdown conditioning in UHV removes Oxides Breakdown conditioning in O 2 ambience causes a net formation of oxide film -> influences E sat

26 January 07 Summary - Gas Experiments Metal E sat at p  mbar (UHV) Relative decrease in E sat by increasing the gas pressure to mbar AirCOH2H2 Ar MV/m%% Cu164 ± 3000 not measured not measured W313 ± not measured not measured Mo438 ± Cr491 ± ??0 not measured not measured not measured Ti697 ± ?? not measured 0 not measured 0

26 January 07 Knowledge obtained from DC spark tests  the ranking of breakdown fields in RF and DC experiments is similar for high breakdown rates  the saturated breakdown fields vary with up to one order of magnitude among the studies electrode materials  the effort in finding the optimum material must continue  alloys?  the breakdown conditioning speed can be drastically improved by correct choice of pre-treatments  surface finishing technigues (milling, EDM,...)  heat treatments (e-beam ↔ oven)  breakdown rate experiments seem to give similar results in RF and DC  Should be given more importance in future studies  for molybdenum and tungsten the vacuum quality influences the ultimate breakdown fields

26 January 07 Future Plans Finish the construction of the new spark system: –Two systems running in parallel -> facilitate higher throughput of materials and preparation techniques – Improvements in the experimental setup XYZ movements E-beam heating → ~ 1000 o C –“In-situ” treatments several samples variation of energy over gap more convenient Upgrade of maximum voltage to ~30 kV Dedicate the “old” spark system to breakdown rate experiments –New setup to increase the repetition rate Goal ~ 0.5 Hz -> 500’000 runs corresponds to ~12 days Antoine Descoeudres

26 January 07 Contributors Sergio Calatroni Ahmed Cherif Antoine Descoeudres Gonzalo Arnau Izquierdo Samuli Heikkinen Holger Neupert Alessandra Reginelli Mauro Taborelli Ivo Wevers Walter Wuensch CLIC Study Team

26 January 07 Automatic Spark Conditioning Spark Scan Histogram Molybdenum (Mo) – Tip and Sample

26 January 07 Depth Profile - Mo Net Missing Volume: ,5  m  m 3 /spark ~3 0.8 J/spark

26 January 07 Comparison: OFE Cu – CuZr - GlidCop Electrode Material: Enhancement factor ( β ): E sat [MV/m] E local [GV/m] (average): √(2 σ /e 0 ) [GV/m] (average) [2]: Cu-OFE(46  5)57 [1](151  39)170 [1] [1] CuZr(86  23) † (120  26) GlidCop ® (32  3) †† (114  7) [1] M. Taborelli, M. Kildemo, S. Calatroni, Phys. Rev. ST-AB, 7, (2004) [2] A. Hassanein, Z. Insepov, J. Norem, A. Moretti, Z. Qian, A. Bross, Y. Torun, R. Rimmer, D. Li, M. Zisman, D. N. Seidman, and K. E. Yoon, Phys. Rev. ST-AB, 9, (2006) † Milled †† Electro Discharge Machined

26 January 07 Mass Spectroscopy FIG. 2. Calibration to determine the relation between the recorded current from the RGA and the corresponding CO pressure. (A) CO pressure as function of RGA ion current. The open blue squares represent the experimental values. (B) Evolution of the pressure during the first few seconds of a pumpdown of CO. The start pressure was ~5∙10-7 mbar. The measured data is represented by blue open circles. The red line shows the resulting linear fit through the measured points in both figure A and B.

26 January 07 Mass Spectroscopy

26 January 07 Field Stability of Conditioned Mo 500x 5000x