STATUS OF KEK STUDIES OF SUPERCONDUCTING THIN FILMS BY PULSED LASER DEPOSITION

 S. Mitsunobu , S. Inagaki, H. Nakanishi, K. Saito, T. Takatomi, M. Wake, and M. Yoshida

High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki, Japan

M. Fukutomi, National Institute for Material Science (NIMS) , Tsukuba, Ibaraki, Japan

PLD Thin Film Superconductor at KEK

TE011 Mode Cavity

• Nb Thin Film on Nb, Sapphire

• Nb3Sn Thin Film on Nb

• MgB2 Thin Film on Nb, Ti, Cu

TM010 Mode Cavity

• MgB2 Thin Film on Nb, Cu and Cu Quadrant Cavity

Conceptual Drawing of Pulse Laser Deposition

Laser system of PLD

Vacuum Chamber of PLD

PLD Processing

TE011 Mode Cavity

Nb3Sn on Nb Nb on Nb Nb on Sapphire

Nb film on Sapphire

Al2O3

NbNb2O5

Nb thin film on sapphire have low Q value of less than 100.TE011 peak was not observed.

TE011

Nb thin film on Nb substrata

Nb

Nb2O5

NbNb2O5

Surface resistance of Nb thin film on Nb

Nb3Sn thin film show no superconducting transition around 18K.

104

MgB2

• We have been engaged in fabrication of high-Tc or MgB thin films on metallic substrates. At the international workshop on thin films in Padova, we showed our basic idea to make an accelerating-mode cavity. In the last of this paper, we report a subsequent development, mainly a partial success in fabricating superconducting film on a quadrant cavity of an accelerator structure. In the beginning, we describe some results concerning fabrication of films on Niobium and Titanium surfaces.

PRECURSOR POST-ANNEALING METHOD

After assembling the cavity with the component parts, we deposit MgB on the inner surface by pulsed laser deposition (PLD) and heat up to temperatures around 600 oC.This procedure is called precursor post-annealing method.

This method consists of two steps.

(i) The target is a pellet with a mixture of MgB2 and Mgpowder with the stoichiometry ofMg:B=2:1. Excimer laser with λ=248 nm (KrF), 400 mJ,

and 5–10 Hz is irrradiatedon the target in 6x10-5 Torr Ar atmosphere at room temperature for 2 hr. The thickness of the

fabricated film is around 1 µm.

(ii) Typically, the precursors are heated in 1 atm Ar gas at 550–650 oC for 10 min. This process must be optimized so that magnesium evaporation should be well controlled to become MgB2 in the phase diagram .

In the preceding experiments, we deposited chromium on the copper surface, then yttria-stabilized-zirconia (YSZ), and MgB2 . However, we now know that if the copper oxide is completely removed from the surface, the chromium buffer layer can be omissible.

Two parts machined vertical to the axis are assembled to make a cavity. The inside surface

is to be covered with MgB2 films.

RF Measurments of MgB2 ON Nb and Ti

TM010 mode cavity

• Nb TM010 cavity made from Nb rod.• Each half cell inner surfaces were formed

MgB2 precursor films by PLD.• The half cell joint by bolts.• The equator part was formed by conical

target.• The cavity was heated by cylindrical heater

upto 620 digC.

TM010 cavity

LaserMover

Camera

MirrorVacuumChamber

Side View Top View

Mover

Mirror

Camera

Laser

PC(python+PyVISA+Tkinter)

GPIB

Target

Cone Shape MgB2 TargetDisc Shape MgB2 Target

Rotation

Horizontal Vertical

Q-value of the cavity fabricated with process (pi).

Microwave surface resistance of a quadrant.

SUMMARY

We are making MgB2 films by a precursor post-annealingmethod on copper and Nb cavities of an accelerator structure.Some of them have already showed superconductingproperties.We concentrate our effort to find the better conditionto get films of better quality. We hope we can make a high

power test in future.MgB2 thin films on Nb disks show partial shielding effect

which is important for high field cavity.Nb and Nb3Sn PLD films study started.