FERMI LINAC O VERVIEW The 1.5 GeV 50 Hz Upgrade July 2013 C. Serpico

O UTLINE  FERMI Linac Layout  Operating Paramenters until RUN15  Energy and Rep. Rate Upgrade BTW Accelerating Structures Phase modulation technique  May 2013: the RF conditioning Vacuum bi-directional couplers faults  Hz  Linac Operating Condition  Conclusions Some comments… Summary What is the next? C. Serpico 2/27

L INAC L AYOUT 1/4 Linac Section # of Str. TypeInjector2S0A-S0B Linac14CERN Linac23CERN Linac32BTW Linac45BTW C. Serpico 3/27

L INAC L AYOUT 2/4 S0A – S0B accelerating structures Geometric Parameters ValueUnits Number of Cells 93 Cells' Length 33.33mm Total Length mm Min. Iris Radius 8.02mm Mean Iris Radius 9.73mm Acc. Str. RF Parameters ValueUnitsTypeMixed Operating Mode 2/3 pi Operating Frequency MHz Q factor R067.10Mohm/m Tau0.60Neper Filling Time 0.898µs C. Serpico 4/27

L INAC L AYOUT 3/4 CERN accelerating structure Geometric Parameters ValueUnits Number of Cells 134 Cells' Length 33.33mm Total Length mm Min. Iris Radius 8.02mm Mean Iris Radius 9.73mm Acc. Str. RF Parameters ValueUnits TypeConst. Grad. Operating Mode2/3 pi Operating Frequency MHz Q factor R065.00Mohm/m Tau0.84Neper Filling Time 1.255µs C. Serpico 5/27

L INAC L AYOUT 4/4 BTW accelerating structures Geometric Parameters ValueUnits Number of Cells 162 Cells' Length 37.50mm Total Length mm Iris Radius 5.00mm Acc. Str. RF Parameters ValueUnitsType Const. Imp. Operating Mode 3/4 pi Operating Frequency MHz Q factor R070.00Mohm/m Tau0.61Neper Filling Time 0.753µs SLED RF Parameters ValueUnits Operating Frequency MHz Q factor Beta10 Filling Time C. Serpico 6/27

K1GUN4.8 K2L0_147.0 K2L0_247.0 K3L1_155.2 K3L1_255.2 K4L1_X-20 K5L1_355.2 K5L1_455.2 K6L2_157.5 K6L2_257.5 K7L2_352.0 K8L3_ K9L3_ K10L4_ K11L4_ K12L4_ K13L4_ K14L4_ O PERATING P ARAMENTERS UNTIL RUN15 The Linac has been operated at 10 Hz with an overall energy gain of about 1270 MeV. C – Sections: ~ 55 MeV/section S – Sections: ~ 120 MeV/section Energy Budget on 06/2012 RF PlantKlystron Output Power [MW]Pulse Width [µs] K2  K7 ~322.0 K8  K14 ~223.0 C. Serpico 7/27

In May 4 weeks were foreseen for testing the power plants operation at 50 Hz repetition rate and the RF conditioning up to a power level corresponding to 1.5 GeV beam energy. E NERGY AND R EP. R ATE U PGRADE C. Serpico 8/27

E NERGY AND R EP. R ATE U PGRADE To get 1.5 GeV beam energy at the end of the Linac the 7 BTW S-type accelerating structures have to be pushed up to an energy gain of about 155 MeV/section (corresponding to a gradient of 26 MV/m).  Linac3: two sections  Linac4: presently five sections  From old Elettra injector.  Both Linac 3 and Linac 4 RF power distribution systems are equipped with SLED systems. C. Serpico 9/27

BTW A CCELERATING S TRUCTURES S-type structures are Backward Traveling Wave (BTW) structures comprised of 162 nose cone cavities coupled magnetically. Type Const. gradient, mag. coupled Mode 3 π /4 Frequency MHz Length6.15m Q11000 R sh 71-73MOhm/m Filling time 0.747µs  Past operation of the BTW accelerating structures showed that such structures suffered heavy breakdown phenomena when pushed to high gradient (no phase modulation implemented).  Single cell simulation shows that surface electric field on the magnetic coupling slot becomes of the same magnitude and even higher than the surface electric peak field on the nose. C. Serpico 10/27

T HE P HASE M ODULATION T ECHNIQUE SLED operation with Phase Modulation:  Amplitude of klystron output is kept constant during the RF Pulse  95º Phase Shift 0.77 µs before the end of the pulse  Phase changed gradually (slope = 149º/µs)  Phase kept constant during the last 200 ns of the RF pulse Note: Phase offset and Phase Slope Parameters adjustable by LLRF Sled Phase Modulation - 3 us t (us) Amp (a.u.) Measured Cavity Input Field 3µs pulse E pk-cav /E pk-kly = 1.83 (field ratio) P pk-cav /P pk-kly = 3.34 (power ratio) C. Serpico 11/27

Even if the RF pulse shape is not flat, for a 3 µs overall pulse width the energy gain loss with respect to the normal operation is just 3.3 % T HE P HASE M ODULATION T ECHNIQUE RF width [µs] E pk-cav /E pk-kly [field ratio] Gain Factor Normal Operation Phase Modulation Normal Operation Phase Modulation C. Serpico 12/27

M AY 2013: T HE RF C ONDITIONING From K2 to K7: (S0A-S0B & CERN sections)  Same Klystron Output Power as before (~32 MW)  Same Pulse Length as before  Repetition rate from 10 Hz to 50 Hz From K8 to K14: (equipped with SLED systems)  Klystron Output Power increased up to 32 MW (from the past 22 MW operation)  Pulse Length up to 3.0 µs  Input power to the structure increased from 62 MW up to 91 MW peak.  Repetition rate from 10 Hz to 50 Hz RF conditioning started on 6 th of May and it took about 4 full days to get to the target operating parameters. RF conditioning started on 9 th of May. Kly Output Power [MW] Pulse Width [µs] Rep Rate [Hz] Strucutre Input Power [MW] Up to RUN 15~ RUN 16~ C. Serpico 13/27

 All 3 directional couplers were installed between the output of the SLED system and the input coupler of the accelerating structures.  Vacuum leaks were detected only on the 50dB pick-up ceramic windows.  Cracks on ceramic windows were experienced on directional couplers produced by 2 different firms. During RF conditioning we experienced 3 major faults (on two different plants) on bidirectional couplers as a consequence of breakdown events in accelerating structures. M AY 2013: T HE RF C ONDITIONING Vacuum Bi-directional Couplers Faults C. Serpico 14/27

Bethe hole coupler type:  ~60 dB coupling on Forward Port  ~50 dB coupling on Backward Port  ~23 dB directivity M AY 2013: T HE RF C ONDITIONING Vacuum Bi-directional Couplers Changing the RF design of the coupler is needed! Bethe hole coupler with lower coupling on the Reflected Port (~60 dB) (the available length is 159 mm, mandatory!) C. Serpico 15/27

1.5 G E 50 H Z On 31 st of May an energy of GeV (on crest) at 50 Hz repetition rate was reached. C. Serpico 16/27

L INAC O PERATING C ONDITION  According to the FERMI Commissioning Plan, the commissioning of FEL2 line with a 1.5 GeV beam energy was foreseen starting from 10 th of June.  On 31 st of May, 1.5 GeV beam energy (on crest) was reached with the Linac operating at 50 Hz. While operating Linac with the above parameters, the breakdown rate was high to guarantee the desired uptime for the FEL2 line commissioning. To guarantee a reliable operation of the FERMI Linac during FEL2 commissioning in June, on 3 rd of June it was agreed to bring the Linac back to 10 Hz repetition rate while still pushing the energy gain at 1.46 GeV. Commissioning of FEL2 during June C. Serpico 17/27

L INAC O PERATING C ONDITION A comparison between the 50 Hz and the 10 Hz operation Faults rate and faults rate distribution before and just after switching back to 10 Hz on a 24 hours basis. Faults rate and faults 50Hz, ~1.5 GeV Faults rate and faults 10Hz, ~ 1.5 GeV C. Serpico 18/27

L INAC O PERATING C ONDITION A comparison between the 50 Hz and the 10 Hz operation Vacuum trends before and just after switching back to 10 HzK14 50 Hz Operation 10 Hz Operation 3.0e -9 mbar 1e -9 1e -8 1e -7 1e Hz Operation 10 Hz Operation K8 1.5e -9 mbar 1e -9 1e -8 1e -7 Most of the vacuum spikes have been detected by ion pumps closer to the structures input: breakdown events take place in structures input couplers and structures cells. C. Serpico 19/27

C ONCLUSIONS

S OME COMMENTS …  BTW accelerating structures were rated for working up to 35 MV/m accelerating gradient at 10 Hz repetition rate.  While operating as Elettra injector, such structures suffered heavy breakdown phenomena if pushed to gradient of MV/m. At that time no Phase Modulation technique was implemented.  In September 2011 Phase Modulation techinque was implemented. Some tests were done at 10 Hz on one plant: an overall energy gain of 165 MeV (~27.5 MV/m) was reached without any major issue. BTW accelerating structures …making a long story short … C. Serpico 21/27

S OME COMMENTS …  In May 2013, four weeks of RF conditioning were foreseen to reach a gradient of MV/m on each BTW structure while operating for the first time at 50 Hz repetition rate.  In May 2013 Just after reaching 1.5 GeV (with a gradient of 25 MV/m on each BTW structure) at 50 Hz repetition rate we had the third directional coupler fault.  We decided to reduce the gradient to about 23 MV/m on BTW structures and switch back to 10 Hz repetition rate. BTW accelerating structures …making a long story short … Is Hz a real limit for our BTW structures? C. Serpico 22/27

S OME COMMENTS … Breakdown diagnostic On all the plants Bethe hole bi-directional couplers are installed.  At the output of the klystrons On all the plants the backward power signal is used to shut- off the RF in case of breakdown events to protect the klystron itself.  At the input of the accelerating structures On RF plants equipped with SLED system, the forward power signal is just used for tuning the SLED cavity.  On RF plants equipped with SLED system, bi-directional couplers installed at the output of the klystron are not able to detect any breakdown event originated from the RF structure.  RF breakdown are detected using signals coming from ion pumps installed in the system. C. Serpico 23/27

S UMMARY Increase of operational margin at 1.5 GeV possible with the installation of the 2 accelerating structures already foreseen in the machine layout  1.5 GeV Linac energy (on crest) has been reached at 50 Hz repetition rate.  Stable operation of the Linac with about 1.46 GeV (on crest) beam energy at 10 Hz repetition rate. 1.Phase Modulation Improve pulse shape flatness 2.New Bi-directional Couplers Bethe hole couplers? Mechanical length of 159 mm is mandatory 3.Improve breakdown diagnostic Upgrades Achievements C. Serpico 24/27

W HAT IS THE NEXT … The Machine Advisory Committee (MAC meeting, June 2013) strongly suggested to consider the possibility of installing a SLED system on CERN-type structures. SLED on CERN-type accelerating structures? Acc. Str. RF Parameters ValueUnits Total Length~4.5m Q factor R065.00Mohm/m Tau0.84Neper Filling Time 1.255µs SLED RF Parameters ValueUnits Q factor ~ Beta6 Filling Time 1.5µs P kly RF Pulse Energy Gain Gradient [MW][µs][MeV][MV/m] NO SLED operation ~13 SLED operation ~17 C. Serpico 25/27  About 120 MeV more with a SLED system installed on 7 CERN type accelerating structures.  About 700 k€ SLED operation ~17

W HAT IS THE NEXT … All 6 meter long, BTW accelerating sections are 20 years old structures. A reliable solution to replace them is under investigation.  Each structure could be replaced by one accelerating module endowed with three 2-meter long accelerating structures.  Keeping the same power plant, new accelerating modules could guarantee a slightly higher energy gain.  With a 80 MW peak power klystron (commercially available) a 40 MV/m accelerating gradient could be achieved. A preliminary idea …high gradient 2-m long structures… C. Serpico 26/27

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