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

HPSL Workshop, Naperville, May 22-24, 2005 1

Activities at LANL in the Field of High Power Superconducting

Proton Linac StructuresT. Tajima and F.L. Krawczyk

Los Alamos Neutron Science Center (LANSCE)LA-UR-05-3224


Outline1. Accelerator Production of Tritium (APT) Project

– 100-170 MW (1.0-1.7 GeV, 100 mA) Proton Linac– Measurements of =0.64, 5-cell 700 MHz elliptical

cavities

2. Advanced Accelerator Applications (AAA) Project (Transmutation of Nuclear Waste)

– 8 MW (600 MeV, 13.3 mA) Proton Linac– Measurements of =0.29 (ANL) and 0.175 (LANL)

spoke cavities

3. Q0 disease with low- cavities


1. 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.


700 MHz SRF Section Specs 0.64 0.82Eacc (MV/m) 5.5 4.1 (6.4)

Real estate Eacc 1.54 1.26 (1.89)

Epeak (MV/m) 19.1 12.7 (19.1)

No. Cavities 90 312Aperture (cm) 13 16Aperture/(rms-beam size)

35 45

Thermal load @ 1.9 K (kW)

2.0 6.1 (9.2)

Numbers in parentheses are for 1.7 GeV case


APT = 0.64, 700 MHz, 5-Cell Cavity

Cavity length : 116 cmCell diameter : 40 cmBeam aperture : 13 cmEp/Eacc= 3.38Hp/Eacc= 69.6 Oe/(MV/m)


Facilities for SRF Activities at LANSCE

43 mHigh-pressure rinse in a clean room.

240 m2 Class-100 Clean roomUltra-pure water system with 5700 liter storage tank

Cryostat with movable radiation shield

2 Inserts for 0.96 m diameter, 3 m deep cryostat

Control, tuning

30 m

Pumps to pump down the cryostat to < 2 K


Results of 6 Prototype Cavities at 2 K. Defects, field emission and MP(?) limited performance.

Best results of all the APT 5-cell cavities (up to 1/29/01)

1.E+08

1.E+09

1.E+10

1.E+11

0 2 4 6 8 10 12 14

Eacc (MV/m)

Q0

AES (12/21/00)@J LABAyako (9/27/00)Eleanore (12/6/00)Germaine (4/12/00)@J LABSylvia (8/3/00)@J LABSylvia (11/1/00)LANL (1/18/01)APT Spec.

Quench

Defect at the middle cell equator

=0.64

Calculated MP band by W. Hartung

APT Goal

835 Oe40 Epk

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


Fitting of an Rs-T curve with the theoretical formula gives you Rres.

In this case, Rres was determined to be 6.4 n

Cavity: Sylvia


Comparison with TESLA Cavities

• Difference of Ep/Eacc and Hp/Eacc.

• Difference of . APT(0.64) vs. TESLA (1).– It might be more susceptible to multipacting

(MP) due to the squeezed shape

63.1

6.426.69

/EH/EH

69.10.2

38.3/EE/EE

TESLAaccp

APTaccp

TESLAaccp

APTaccp

APT cavity shape is not well optimized!


Comparison with TESLA Cavities

1.E+08

1.E+09

1.E+10

1.E+11

0 10 20 30 40

Eacc* (MV/m)

Q0TESLA 500 TESLA 800

Best TESLA 9-Cell Cavity

Eacc*: Hp/Eacc=42.6 was used.

APT

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

2 K


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.


2. 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.


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


Tests on ANL = 0.29, 340 MHz, 2-gap Spoke Cavity

Cell diameter : 44.2 cmCavity length : 30 cmBeam aperture : 3.2 cm

Ep/Eacc : 3.18Hp/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


Vertical Test Results of ANL =0.29, 340 MHz, 2-Gap Spoke Cavity

ANL b=0.29 spoke cavity Q vs. Eacc

1.E+08

1.E+09

1.E+10

0 2 4 6 8 10 12 14Eacc (MV/m)

Q0

4K Aft. Helium Process 2K Aft. Helium Process ADTF Spec.

2 K

4K

AAA Goal

Ep 40 MV/mHp 1063 Oe


AAA =0.175, 350-MHz, 2-Gap, Spoke Cavity

Cell diameter : 39.2 cmCavity length : 20 cmBeam aperture : 5 cm

Ep/Eacc : 2.82Hp/Eacc : 69 Oe/MV/m

Two cavities were fabricated at ZANON, Italy.


Vertical Test Results of LANL =0.175, 350-MHz, Spoke Cavities

With Nb blank flanges on large radial ports

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

Eacc, max = 13.5 MV/mEp, max= 38.0 MV/mHp, max = 994 Oe

Details in PAC03 Proceedings.

4 K

Thoroughly cleaned with high-pressure water rinse after BCP.


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

.)sin(8

.2

)sin(2

2

332

0

2

3320

20

Periodacc Lq

mcTEE

LmcTEq

L

• Our longitudinal beam-dynamics

design approach has been to keep 0<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 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.

2

3

accE MaximumperiodL


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.


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 m at <15°C


Q disease tests with EZ01 at 4 K

1.E+08

1.E+09

1.E+10

0 2 4 6 8 10 12 14 16

Eacc (MV/m)

Q0

After 160-195K for 120 h12/2/02After 100 K for 13 h 12/3/02

After 100 K for 11 h (total 24 h)12/4/02After 100 K for 12 h (total 36 h)12/5/02Kept at 4 K over the weekend12/9/02After 100 K for 12 h (total 48 h)12/10/02After 100 K for 12 h (total 60 h)12/11/02After 100 K for 12 h (total 72 h)12/12/02After 100 K for 12 h (total 84 h)12/13/02Kept 100 K over the weekend12/16/02After 100 K for 12 h (total 96 h)12/17/02After 100 K for 12 h (total 108 h)12/18/02After 100 K for 12 h (total 120 h)12/19/02

Chronological order

Q0 – Eacc Curves were taken after every 12 hours of holding at 100 K


Q0 vs holding time at 100 K

0.0E+00

2.0E+08

4.0E+08

6.0E+08

8.0E+08

1.0E+09

1.2E+09

1.4E+09

0 20 40 60 80 100 120 140

Total holding time (h)

Q0

Q0 Dependence on the Holding Time at 100 K


Increase of surface resistance (Rs) due to Q disease vs holding time at

100 K

0102030405060708090

100

0 20 40 60 80 100 120 140

Total holding time (h)

Add

ition

al R

s (n

ohm

s)

Additional Surface Resistance Due to Q disease


Summary of the Q disease tests• With our 350-MHz spoke cavities, little Q disease occurs

after 150-m 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.


Summary• Six =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 =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.

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

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