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Activities at LANL in the Field of High Power Superconducting Proton Linac Structures. T. Tajima and F.L. Krawczyk Los Alamos Neutron Science Center (LANSCE) LA-UR-05-3224. Outline. Accelerator Production of Tritium (APT) Project 100-170 MW (1.0-1.7 GeV, 100 mA) Proton Linac

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activities at lanl in the field of high power superconducting proton linac structures

Activities at LANL in the Field of High Power Superconducting Proton Linac Structures

T. Tajima and F.L. Krawczyk

Los Alamos Neutron Science Center (LANSCE)

LA-UR-05-3224

HPSL Workshop, Naperville, May 22-24, 2005

outline
Outline
  • Accelerator Production of Tritium (APT) Project
    • 100-170 MW (1.0-1.7 GeV, 100 mA) Proton Linac
    • Measurements of b=0.64, 5-cell 700 MHz elliptical cavities
  • Advanced Accelerator Applications (AAA) Project (Transmutation of Nuclear Waste)
    • 8 MW (600 MeV, 13.3 mA) Proton Linac
    • Measurements of b=0.29 (ANL) and 0.175 (LANL) spoke cavities
  • Q0 disease with low-b cavities

HPSL Workshop, Naperville, May 22-24, 2005

accelerator production of tritium apt project
Accelerator Production of Tritium (APT) Project
  • Purpose: Replenish Tritium Inventory for Nuclear Weapons (3 kg/year)
  • Method: Using spallation neutrons produced by a proton accelerator, split Lithium into Tritium and Helium.

HPSL Workshop, Naperville, May 22-24, 2005

700 mhz srf section specs
700 MHz SRF Section Specs

Numbers in parentheses are for 1.7 GeV case

HPSL Workshop, Naperville, May 22-24, 2005

apt b 0 64 700 mhz 5 cell cavity
APT b = 0.64, 700 MHz, 5-Cell Cavity

Cavity length : 116 cm

Cell diameter : 40 cm

Beam aperture : 13 cm

Ep/Eacc= 3.38

Hp/Eacc= 69.6 Oe/(MV/m)

HPSL Workshop, Naperville, May 22-24, 2005

facilities for srf activities at lansce
Facilities for SRF Activities at LANSCE

2 Inserts for 0.96 m diameter, 3 m deep cryostat

240 m2 Class-100 Clean room

Ultra-pure water system with 5700 liter storage tank

43 m

High-pressure rinse in a clean room.

Cryostat with movable radiation shield

30 m

Control, tuning

Pumps to pump down the cryostat to < 2 K

HPSL Workshop, Naperville, May 22-24, 2005

results of 6 prototype cavities at 2 k defects field emission and mp limited performance
Results of 6 Prototype Cavities at 2 K. Defects, field emission and MP(?) limited performance.

Half the cavities were measured by Jlab as well (J. Mammosser).

b=0.64

835 Oe

40 Epk

APT Goal

Defect at the middle cell equator

Calculated MP band by W. Hartung

HPSL Workshop, Naperville, May 22-24, 2005

slide8
Fitting of an Rs-T curve with the theoretical formula gives you Rres.In this case, Rres was determined to be 6.4 nW.Cavity: Sylvia

HPSL Workshop, Naperville, May 22-24, 2005

comparison with tesla cavities
Comparison with TESLA Cavities
  • Difference of Ep/Eacc and Hp/Eacc.
  • Difference of b. APT(0.64) vs. TESLA (1).
    • It might be more susceptible to multipacting (MP) due to the squeezed shape

APT cavity shape is not well optimized!

HPSL Workshop, Naperville, May 22-24, 2005

comparison with tesla cavities1
Comparison with TESLA Cavities

APT results (Eacc) were converted to TESLA equivalent numbers for comparison.

Best TESLA 9-Cell Cavity

2 K

TESLA 500

TESLA 800

APT

Eacc*: Hp/Eacc=42.6 was used.

HPSL Workshop, Naperville, May 22-24, 2005

issues that need to be addressed
Issues that need to be addressed
  • The APT Goal was achieved, but we need to achieve higher gradients for future machines to further reduce costs.
  • Q0 drops at medium and high gradient due to Multipacting (?) and Field emission.
  • Measures to take
    • MP free design of the cavity with MP codes.
    • Thermometry and other diagnostics for detecting heating spots to localize and identify the problem.
    • Improve contamination control during assembly to reduce field emission due to migrated particles.

HPSL Workshop, Naperville, May 22-24, 2005

advanced accelerator applications aaa project
Advanced Accelerator Applications (AAA) Project
  • Purpose: Transmutation of Nuclear Waste, i.e., reduce life of nuclear waste by about 2 orders of magnitude to <500 years.
  • Method: Accelerator-Driven System.

HPSL Workshop, Naperville, May 22-24, 2005

spoke cavity development for low energy sections of proton linacs
Spoke Cavity Development for Low-Energy Sections of Proton Linacs
  • Studies have been carried out at ANL since late ’80s (Jean Delayen, Ken Shepard)
  • Some advantages over elliptical cavities
    • Compact (~1/2 diameter of elliptical cavities for the same frequency)
    • Due to the lower frequency it can be operated at 4 K
    • Mechanically stable
    • Larger velocity acceptance

HPSL Workshop, Naperville, May 22-24, 2005

tests on anl b 0 29 340 mhz 2 gap spoke cavity
Tests on ANL b = 0.29, 340 MHz, 2-gap Spoke Cavity

Cell diameter : 44.2 cm

Cavity length : 30 cm

Beam aperture : 3.2 cm

Ep/Eacc : 3.18

Hp/Eacc : 85 Oe/MV/m

Previous tests at ANL showed field emission and limited at Eacc~5 MV/m

BCP ~100 micron and 1000-psi (~70 bar) HPR in a class-100 clean room were performed at LANL

HPSL Workshop, Naperville, May 22-24, 2005

vertical test results of anl b 0 29 340 mhz 2 gap spoke cavity
Vertical Test Results of ANL b=0.29, 340 MHz, 2-Gap Spoke Cavity

2 K

4K

AAA Goal

Ep 40 MV/m

Hp 1063 Oe

HPSL Workshop, Naperville, May 22-24, 2005

aaa b 0 175 350 mhz 2 gap spoke cavity
AAA b=0.175, 350-MHz, 2-Gap, Spoke Cavity

Two cavities were fabricated at ZANON, Italy.

Cell diameter : 39.2 cm

Cavity length : 20 cm

Beam aperture : 5 cm

Ep/Eacc : 2.82

Hp/Eacc : 69 Oe/MV/m

HPSL Workshop, Naperville, May 22-24, 2005

vertical test results of lanl b 0 175 350 mhz spoke cavities
Vertical Test Results of LANL b=0.175, 350-MHz, Spoke Cavities

With Nb blank flanges on large radial ports

Thoroughly cleaned with high-pressure water rinse after BCP.

The second EZ02 test was done after disassembly and high-pressure rinse.

4 K

4 K

Eacc, max = 13.5 MV/m

Ep, max= 38.0 MV/m

Hp, max = 994 Oe

Details in PAC03 Proceedings.

HPSL Workshop, Naperville, May 22-24, 2005

slide18

Very high gradients are not useful for low-b structures due to longitudinal envelope instability [T.P. Wangler and K.R.Crandall, Spoke Cavity Workshop, Los Alamos, Oct. 7-8, 2002.]

  • Our longitudinal beam-dynamics design approach has been to keep s0<90° and minimize the focusing period.
  • The cryomodules form piecewise constant FODO lattices where each period contains one cavity and one solenoid.
  • For 350-MHz proton linac in b range of 0.2 to 0.5 (20 to 150 MeV) we could use cavity gradients up to about 8 MV/m without longitudinal beam-dynamics problems.

HPSL Workshop, Naperville, May 22-24, 2005

some questions that need to be answered
Some questions that need to be answered.
  • What is the experimentally (practically) achievable Hp at 4 K? It has been ~1800 Oe at 2 K with TESLA cavities. (Due to the decrease of Hc with lower temperature, i.e., ~14 % from 4 K to 2 K, and the better thermal conductivity of liquid He at T<2.17 K (Lambda point), it is expected that the achievable field is higher at 2 K. )
  • Are there any difference between elliptical and spoke cavities in terms of the field limitations? It has been shown for spoke cavities that the maximum fields are almost the same at 4 K and 2 K, whereas some elliptical cavities have shown much better results at 2 K than at 4K.

HPSL Workshop, Naperville, May 22-24, 2005

3 q 0 disease with low frequency cavities tests with 350 mhz spoke cavities
3. Q0 disease with low-frequency cavities (tests with 350 MHz spoke cavities)
  • Q0 disease is a degradation of Q0 due to niobium hydrides (Tc~1.2 K?) precipitated on the surface from bulk during slow cooldown by holding the cavity at medium temperatures (70-150 K, worst at ~100 K). It had been said before our tests that low frequency cavities (<500 MHz) do not show it.
  • A systematic Q disease tests have been carried out with our 350 MHz spoke cavities
    • Spoke cavities made of RRR~250 Nb.
    • Original H contest is unknown.
    • No high temperature heat treatment. (<150 °C)
    • Standard BCP (HF:HNO3:H3PO4=1:1:2) ~150 mm at <15°C

HPSL Workshop, Naperville, May 22-24, 2005

q 0 e acc curves were taken after every 12 hours of holding at 100 k
Q0 – Eacc Curves were taken after every 12 hours of holding at 100 K

HPSL Workshop, Naperville, May 22-24, 2005

q 0 dependence on the holding time at 100 k
Q0 Dependence on the Holding Time at 100 K

HPSL Workshop, Naperville, May 22-24, 2005

additional surface resistance due to q disease
Additional Surface Resistance Due to Q disease

HPSL Workshop, Naperville, May 22-24, 2005

summary of the q disease tests
Summary of the Q disease tests
  • With our 350-MHz spoke cavities, little Q disease occurs after 150-mm BCP, up to ~24 hours of holding time at ~100 K.
  • Holding the cavity for longer time causes Q disease.
  • The additional surface resistance due to the Q disease increases linearly after the increase starts.
  • Q disease disappears if the cavity is warmed up to 180 K.

HPSL Workshop, Naperville, May 22-24, 2005

summary
Summary
  • Six b=0.64 700-MHz 5-cell cavities were prototyped and all of them exceeded APT specs, but needs better design in terms of optimum Hp/Eaccand MP, and further particulate contamination control to get higher gradients
  • Two b=0.175 350-MHz single-spoke spoke cavities were prototyped and both of them showed similar results and exceeded AAA specs.
  • It was found that Q0 disease occurs with our 350-MHz spoke cavities, but it does not occur if cooled down within 24 hours.

HPSL Workshop, Naperville, May 22-24, 2005

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