High Power S-Band RF Load for SLAC Linac. A. Krasnykh, F-J Decker, et al. SLAC 2014

Transcription

1 High Power S-Band RF Load for SLAC Linac A. Krasnykh, F-J Decker, et al. SLAC 2014

2 The LCLS Injector showed the phase jitters (PJ) at L0A, L0B, and L1S stations on beginning of injector setup. Initial PJ measurement setup employed both RF signals from Input and Output WG coupler of each accelerating section. Initial thoughts was that the PJ problem linked with the 5045 klystrons (see movie L1S, July 2008) Introduction I was asked to jump into the PJ investigations and two main week components were selected. They are RF Loads and Thyratrons. *) PJ was acceptable for the Injector commission but it was close to the upper spec value.

3 Original and Present RF Setups Originally 20 MW peak klystrons were employed. All vacuum RF components ( waveguide bends, power dividers, RF loads, etc.) are matched with a power level from available RF sources. 7 RF loads per each station: 4 High and 3 Low power loads

4 Comparison of RF Power Levels Klystron Output Power, MW Power at SLED Output, MW Power in Waveguide Feeders after First Power Divider Power at the Input Port of Accel. Section, MW Power at the Output of Accel. Section (Power at Load), MW Original RF Setup 24 n/a Present RF Setup *) *) From our Blue Book bible

5 High Power RF Loads in LCLS Injector ~50MW In ~14 MW out 5

6 Result of RF Load Klystron Test Lab A dozen of original Kanthal RF loads were tested. We found not one good RF load for P>3 MW peak. It was confirmed that the original loads can absorb RF peak power no more than 3 MW peak without breakdown. 6

7 Dilemmas # Actions Result Comments 1 To find good RF loads from existing spares 2 To find a national vendor(s) and to buy RF loads 3 To find an international vendor(s) and to purchase 4 To find alternative technical solutions for the RF linac setup 5 To find a company capable of design and fabrication Unsuccessful Unsuccessful a) NIHON KOSHUHA Co., Ltd, Japan b) BINP, N-sk, Russia There are couple risky proposals a) Muons Inc. b) INTA Technologies c) SLAC (a HY approach) 14 Loads had been tested, practically all linac RF loads were inspected, load lifetime is unknown? vacuum dry spec a) 5 loads tested, 2 were accepted, $$ b) vacuum dry spec Alternatives were not tested experimentally a) & b) Under DoE SBIR grants

8 Result of Linac Inspection of the RF Loads For example, waveforms of reflection Sector #21 All waveforms have an unstable (harsh) periods. The harsh period is delayed. The delay is equal to a round trip of RF power from the KLY to the RF Load and back, i.e. ~1.8 usec.

9 Result To Find National Vendor(s) to Buy the RF Loads Existing SLAC vacuum dry RF Load A vacuum dry specification was a main stopper for the National Vendors. Their technical solutions are similar to the SLAC H2O RF Load concept. Such proposals are not acceptable for the SLAC linac.

10 High Power Loads from Japan and Russia Five SiC-type loads were tested: accepted two and failed three loads. HFFS simulation shows that the SiC surface gradient is high 30 MW peak 35 surface of the SiC pins) This load does not meet the SLAC requirements to be the dry load It is a resonant load (sensitive to temperature, incident power level, etc.) TE020 mode

11 Possible Temporary Technical Solution Double RF window approach with a vacuum interlock Item #4 Item #7 Each RF window had been tested at 60 MW, 180 pps, 3.5 usec before to be installed in 5045 assembly Double overhead power (i.e. four RF windows are employed)

12 Possible Temporary Technical Solution #2 (cont.) ~ 1usec W, MeV Pin, MW Pout, MW P_ref@ KLY, MW L0A L0B Stable RF Power termination Good vacuum (no RF window) 5045 is less 0.3 MW (is still rather high) 5045 tube practically OFF (i.e. no KLY beam) Multi bunch mode is feasible

13 Anatoly -- Help Yourself with the RF Load Problem! Permanent solution is needed for the SLAC RF Loads. 1,000 loads are needed (i.e. a cheap cost load concept) for whole linac *) Lifetime load should be forever under a continuously mode of the linac operation It was proposed to work with Muons Inc. and INTA Technologies to design and fabricate the RF load for the SLAC linac under the DoE SBIR grants. Alike Anatoly worker fabricates dirty absorber parts for his RF loads Muons Inc. & SLAC An absorber is based on the lossy dielectric. TE01 mode in round WG is a working mode. M. Neubauer is the Muons PI (design and fabrication). Anatoly is responsible for the SLAC sub-grant INTA Tech. & SLAC An absorber is based on the low conductive layer. TE10 mode in rectangular WG is a working mode. Anatoly is a designer and tester. Mr. LeClair is the PI for the load technology and its fabrication.

14 Concept of the Muons RF Load Cross section view of the lossy ceramic cylinder in the compression assembly *). A prototype configuration of the complete dry vacuum load *). The Muons RF Load design employs a mode converter. A design of the mode converter is the SLAC responsibility. *) courtesy Muons Inc. Accelerating Section and RF Load 14

15 Mode Convertors for the Muons RF Load TE10-TE11-TE01 The S-Band wrap around mode converter had been scaled from the Sami Tantawi X-Band converter Under Muons-SLAC SBIR DoE Grant a new compact mode converter for Muons RF Load had been designed with V. Dolgashev 15

16 Design of Dielectric Cylinders for the Muons RF Load Lossy material in the TE01 round waveguide will be mechanically confined in compression (without brazing), in order to eliminate tensile stresses in the lossy material. A heat gun is used to evaluate an RF loss sensitivity vs. temperature A number of lossy porcelain ceramics were fabricated by Muons with varying amounts of SiC powder incorporated. Dielectric constant and loss tangent had been measured at SLAC. The case where 52% by dry volume of the SiC powder was used produced a lossy ceramic rings with an average dielectric constant of i, yielding a loss tangent 16 of 0.05 as measured by the Agilent probe method.

17 Where Are We with the Muons RF Load Design? Two prototypes of the mode convertors were proposed at SLAC for the Muons RF Load Design The prototypes were manufactured by Muons Inc. S-parameters of prototypes were measured/studied at SLAC. A compact version of the mode converter was recommended for the Muons RF load. A matrix of the porcelain with the SiC powder doping was sintered by Muons Inc. for the RF lossy dielectric material. The lossy ceramic cylinders in the compression assembly is currently in the fabrication stage by Muons Inc. This development is sponsored by the Phase II DoE SBIR Grant. Activity at SLAC is conducted under Muons-SLAC CRADA with a total SLAC budget of $450k paid by Muons Inc. Remaining to do: Fabricate cylinders, test them individually and after welding them in array to test at mw-kw levels, vacuum and thermal cycle tests of the pre assembled unit. Fabricate the high power version of the TE10-TE01 mode converter, low level RF test, vacuum leak test, bake up cycle tests. Build the load assembly and perform low-middle-high power tests

18 Development of New Load Based on High Resistive Absorber (SLAC design) This high power RF load concept is a conservative approach of further development of the original design. Formation of RF Absorbing Layer Original New A Kanthal wire (matrix of components) and a flame spray technology are used to produce the RF absorption layer. Similar to the Kanthal matrix (but atomized powder instead of the wire) and a plasma spray technology are used to produce an the RF absorption layer Kinetic energy of melted drops, their temperature, and size are dissimilar. Oxidation process of components is also different. Interaction particles with a base substrate is different too. Etc.

19 Development of New Load Based on High Resistive Absorber (cont.) Original New Tapering concept No tapering concept In 2012 we (at SLAC) were working with INTA Technologies that was granted a Phase I SBIR to develop the new high power RF load. The load design was proposed at SLAC. INTA was a responsible for the technology and manufacturing methodologies including welding of Skarpass RF flanges, which are made from dissimilar materials.

20 Development of New Load Based on High Resistive Absorber (cont.) During the Phase I INTA fabricated a pilot version of the RF Load that had been conditioned and tested up to 30 MW peak power at SLAC. 30 MW peak, 1 kw average, 30 Hz, cooling by two fans We (SLAC & INTA Technologies) did not get a Phase II SBIR support. INTA Technologies stopped to work on this project in 2013.

21 Development of New Load Based on High Resistive Absorber (cont.) Main received results: The plasma deposition of ferritic FeCrAl alloy onto modified rectangular waveguide with kerfs on H-planes is a promising concept. The technology achieved a 5.5 db/m attenuation rate at 30 MW peak power and 1 usec pulse width with no evidence of breakdown activity in the vacuum envelope on the coated surfaces. All technological aspects of RF load fabrications had been practically solved. SLAC Relies Drawings SA was set for a fabrication in this year. Remaining for a development of the load prototype : Demonstrate a reliable RF absorption at 5 kw average RF power Demonstrate an acceptable lifetime at 20 MW peak and 5 kw average Conduct optimization of plasma coating to get an uniform RF absorption along the load length Cost optimization

22 Where Are We Now with the SLAC RF Load Design? APS Materials delivered coated aluminum extruded angles for eight load prototypes. Plasma spray quality for three pilot loads was not accepted after their visual inspection. Company will rework. A preparation of rejected pieces for a shipment back to APS is in progress. The accepted parts are in the fabrication process.

23 Where Are We Now with the SLAC RF Load Design? (cont.) VNA cold test of the WG parts which were accepted and welded Next steps are: Welding flanges, leak check, bake up cycles, mw and kw RF power levels after temperature cycles, assembly of cooling system, and in final, to perform an RF test at bldg044, etc.

24 Pilot S-Band Dry Vacuum Load for SLAC Linac Liquid cold plates: Cu pipes enclosed into Al alloy housings CF 8 l/s vacuum port with the Al-SS interface Estimated cost is less than $5k per each Extruded Al alloy angles with kerfs, a surface of which are plasma coated by the FeCrAlY matrix Skarpass RF Flange made by the Al-SS explosion bond technology Al extrusions was from APEL Extrusion, Inc. Plasma coating performed by APS Martials, Inc. Dissimilar Al-SS plates purchased from Spur Industries Liquid cold plates fabricated by Wakefield-Vette, Inc. SLAC labor

25 How RF Load Activity Does Relate to Improvement of the LCLS Phase Stability? **) A qualified speculation: RF Power propagates from klystron to the RF load through an accelerating waveguide. The present RF load can not absorb the residual RF power and creates a local vacuum burst during each pulse. A result of the vacuum burst propagates into the beam line. A baseline pressure will be higher than for a case of a proper RF power termination. The collision of the bunch during its acceleration with the residual gas is higher accordingly. A result of the collision has two folds: (1) the non recoverable emittance rise and (2) the pulseto-pulse jitter due to statistical uncertainty of a collision. Both effects are not needed for LCLS. **) PJ is acceptable but we are looking the improvements