Selected experiences of 6 years Rossendorf SRF Gun André Arnold on behalf of the ELBE Crew and the DESY-HZDR-HZB-JLab-MBI collaboration 1st TTC topical meeting on CW SRF June, 2013, Ithaca, NY, USA

Slide 2 Introduction 1.CW operation Mech. properties Intrinsic quality factor HOM coupler 2.Cathode Preparation Multipacting Dark current 3.New Cavity Summary Outline

Slide 3 Three CW operation modes high peak current operation for CW-IR-FELs with 13 MHz, 80 pC high bunch charge (1 nC), low rep-rate (<1 MHz) for pulsed secondary particle beam production (neutrons, positrons for ToF) low emittance (1 mm mrad), medium charge (100 pC) with short pulses for THz-radiation and x-rays by inverse Compton backscattering Introduction – SRF gun designed for ELBE HOM Coupler 3 TESLA Cells B peak = 110 mT E peak = 50 MV/m U = 33 J V acc = 9.4 MV G= Ω R/Q= Ω Q 0 = P diss = 26 W Choke Filter Cathode Electron Beam 10 MeV, 1 mA, 10 kW 77 pC - 1 nC Laser 1 W, 262 nm Half Cell Main Coupler

Slide 4 From in-house clean room cavity string assembly to SRF gun module completion e - -beam Laser beam Solenoid SRF gun in the acc. tunnel Faraday cup Screen 2 & slit masks 180° magnet Cherenkov radiator Length = 4m developed & manufactured by HZB connected to all peripherals and diagnostics Screen 1 Introduction – SRF gun in 2007

Slide 5 k peak = 0.69 Hz/(MV/m) 2 Pressure sensitivity 1. CW Operation – mech. Properties TESLA 9 cell: ~10 Hz/mbar Lorentz detuning using NWA in CW 24Hz (membrane pumps) 125Hz (1 st mech. resonance) 10Hz (LHe compressors) TESLA 9 cell: ~0.25 Hz/(MV/m) 2 Microphonics using LLRF controller time signals Reason is the weak half-cell back plane → additional stiffeners welded at new cavities

Slide 6 E acc E peak on AxisE kin CW6.5 MV/m17.5 MV/m3.3 MeV Pulsed RF8 MV/m22 MV/m4.0 MeV Formulas: Good News No Q degradation during the first 4 years of operation! Small improvement after HPP (but canceled by thermal cycle) Bad News measured Q 0 is 10 times lower than in vertical test Maximum achievable field 1/3 of the design value 50 MV/m) Cavity performance limited by FE & He consumption (>30 W) performance loss 1 ½ years ago, due to cathode exchanges ? Q vs. E measurement is an important instrument to identify cavity contamination! 1. CW Operation – In Situ Q 0 vs. E 2K Summary: Q 0 degradation possibly due to cathode exchanges

Slide 7 1. CW Operation – HOM hook coupler Ti:sapphire HOM feed through RF cable with coppered stainless steel inner conductor (same as DESY) No significant temp. increase in CW (∆  1K) (no significant HOM 13 MHz, 30 pC) Interesting possibility of hook couplers: R/Q calculation from energy loss per bunch, if mode damping caused by HOM couplers Measurement with spectrum analyzer (zero span) E pk =15 MV/m ∆  1K beampipe HOM1 HOM2 rhodium iron temp. sensor

Slide 8 2. Cathode – Preparation and Operation CathodeOperation DaysChargeQE in gun #090508Mo30< 1 C0.05% #070708Mo60< 1 C0.1% #310309Mo109< 1 C1.1% #040809Mo182< 1 C0.6% #230709Mo56< 1 C0.03% #250310Mo42735 C1.0% #090611Mo65< 1 C1.2% #300311Mo762 C1.0 % #170412MoFrom C~ 0.6 % Cathodes evaporated with Cs and Te (succes- sively or simultaneously) until QE is saturated Immediately after preparation QE drops fast to about 1% and remains const. also in the gun Up today: 9 Cs 2 Te cathodes used in the gun Most of them died because of vacuum problems fresh QE 8.5%, in gun 0.6% total beam time ~600 h extracted charge 260 C max beam current 400µA

Slide 9 2. Cathode – Multipacting MP was expected since the early days of the cavity design! And indeed it appeared at low field (<1 MV/m) for every Cs 2 Te cathode Characterized by high current (>1 mA, rectified) at the cathode and electron flash at view screens Biasing of the electrically isolated cathode up to -7 kV usually works (voltage is different for every cathode and position!) Anti multipacting grooves to suppress resonant conditions and coating with TiN to reduce secondary electron yield doesn’t work for Cs 2 Te coated cathodes  because of too high SEV due to Cs pollution? German BMBF project granted to further investigate MP and find solutions

Slide 10 Dark current = accelerated electrons produced by FE with wrong properties in space & time Comparison of dark current with low-bunch-charge photo beam: 1.Slit mask emittance measurement: dark current has similar transverse beam properties 2.180° bending magnet: large fraction has nearly same energy and energy spread (  emission near or from the cathode) 2. Cathode – Dark Current Properties -1.6 position (mm) Angle (mrad) -2.0 phase space dark current  n,rms =2.4 phase space photo beam 1pC  n,rms = position (mm) Angle (mrad) keV dark current (120 nA) 100 keV 30 pC photo beam DC 6 kV 16.2 MV/m screen intensity (a.u.)

Slide 11 3.whole energy spectra with and w/o cathode (intensity normalized to total current cathode curve) High energy part belongs to cathode itself Fractions with lower energy belongs to the cathode hole in the half cell or to other high-field regions 2. Cathode – Dark Current Origin 4.Total dark current for different cathodes only cathodes with Cs 2 Te layer have dark current 20 % dark current from cathode, 80% from cavity ~20%

Slide 12 New cavity can operate at 16 MV/m. Here we expect lower field enhancement factor β, that results in the same dark current as for the old cavity but now at 16 MV/m (blue curve) But extrapolation of FN fit for 20 % dark current emitted from cathode (ϕ = 3.5 eV for Cs 2 Te,) results in 40 µA cathode dark current [J. Teichert, FLS2012] 2. Cathode – Dark Current Extrapolation Further investigations within German Gun-Cluster collaboration and ARD  By far too much for CW accelerators  Need for cathodes with low dark current proper handling to prevent dust particles and surface damage proper materials for plugs with smooth surface photo layer properties - roughness, homogeneity, thickness - work function - crystal size, boundary and structure - post-preparation treatment (protect layer, heating, …) - pre-conditioning But how?

Slide 13 RRR 300 Nb cavity large grain Nb cavity Main aim: approach the design value of E pk =50 MV/m: Fabrication of two new cavities in collaboration with JLab (fabrication, treatment, test by P. Kneisel and co-workers) Slightly modification compared to old design to: Lower Lorentz force detuning, microphonics and pressure sensitivity Improve cleaning and simplify clean room assembly 3. New SRF Gun – The stony path to a new cavity additional half-cell stiffening (light green) larger cathode boring modified choke-cell pick-up flange

Slide 14 corresponding to a beam energy of 8 MeV existing SRF gun 3. New SRF Gun – The stony path to a new cavity My suggestion for future projects - Keep it simple and straightforward (KISS)

Slide 15 Pressure sensitivity and Lorentz force detuning higher than for TESLA cavities High microphonics but residual phase noise (closed RF loop) still sufficient  in all three cases stiffeners for the weak half cell needed No Q degradation of Cavity during first 4 years but then Q-drop due to cathodes?  NC cathodes and its exchange are a potential risk for SRF gun cavities No heating of Ti:sapphire HOM feed through in CW operation at E pk = 15 MV/m Long lifetime of NC photo cathodes in SRF gun (>1 yr, total charge 260 QE  1% ) Multipacting appears for Cs 2 Te coated cathodes only, suppression with DC Bias Cs 2 Te cathodes produces high dark current with similar properties as the photo beam,  for higher surface fields 40 µA are expected, which is a problem for CW accelerators RRR300 upgrade cavity (+vessel) tested up to 43 MV/m, cold mass assembly upcoming Summary

Slide 16 FEL spectraFEL detuning curve E kin at gun exit3.3 MeV Micro pulse repetition rate13 MHz Macro pulse repetition rate / length1.25 Hz / 2 ms Beam energy at FEL27.9 MeV Bunch charge / beam current20 pC / 260 µA Photo cathodeCs 2 Te RMS bunch length1.6 ps Normal. RMS emittance1 mm mrad ELBE infrared FEL (20 – 250 µm) April 11, 2013 stabilty beforefirst lasingoptimized Summary - First FEL Operation with the SRF gun NIM paper submitted

Slide 17 ELBE Crew in front of museum of clocks in Glashütte, Germany Thanks to our collaborators (HZB for diagnostics, MBI for the laser and DESY for preparation and testing of the 1 st cavity) and thank you for your attention! Acknowledgement We acknowledge the support of the European Community-Research Infrastructure Activity under the FP6 programme (CARE, contract number RII3-CT ) and the FP7 programme since 2009 (EuCARD, contract number ) as well as the support of the German Federal Ministry of Education and Research grant 05 ES4BR1/8.

Slide 18 Quelle: A. Arnold, et al., NIM A 577, 440 (2007) LHe supply 2.Ti spokes for cavity alignment and thermal decoupling 3.LN tank for cryo shielding 4.Mu metal shielding 5.Cathode with LN heat sink 6.LN tank for cathode and tuning system 7.LHe vessel with sc 3.5 cell gun cavity 8.RF coupler (10 kW) 9.Dual tuner isolation vacuum Introduction – 3D Cross-section of the Module

Slide 19 CW Operation – Cavity Dual Tuner stepping motor and gear box screw drive ½-cellTESLA cells ± 78 kHz± 225 kHz 1.2 nm/step1.6 nm/step 0.3 Hz/step0.7 Hz/step measured tuner values

Slide 20 Dark current energy spectrum with cathode #170412Mo Max. axis peak Energy peak peak/ Main peak 14.8 MV/m2.3 MeV140 keV24.6 %17 nA 15.5 MV/m2.53 MeV140 keV20.8 %17 nA 16.2 MV/m2.64 MeV146 keV16.3 %15 nA 16.7 MV/m2.87 MeV145 keV12.9 %20 nA Measured with dogleg dipole + YAG screen, normalized according to the total dark current measured from Faraday cup. DC 6kV Different energy  different position? Simulation needed. Dark current measurement and analysis

Slide 21 Fowler Nordheim formula for field emission current: Fowler Nordheim plot for RF fields: (J.W. Wang and G.A. Loew, SLAC-PUB-7684 October 1997) E: surface field amplitude in V/m, ɸ: work function in eV, β: field enhancement factor. ф Nb = 4.3 eV β ≈ 400 F-N plots for the SRF gun caivty Sharp emitter DARK CURRENT – Fowler Nordheim analysis