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Measurements of plasma bremsstrahlung and plasma energy density produced by electron cyclotron resonance ion source plasmas

  • Author(s): Noland, Jonathan David
  • Advisor(s): Verboncoeur, John
  • Lieberman, Michael
  • et al.
Abstract

The goal of this dissertation was to gain an understanding on the relative

importance of microwave power, neutral pressure, and magnetic field

configuration on the behavior of the hot electrons within an Electron Cyclotron

Resonance Ion Source (ECRIS) plasma. This was carried out through measurement of

plasma bremsstrahlung with both NaI(Tl) (hv > 30 keV) and CdTe (2 keV < hv <

70 keV) x-ray detectors, and through measurement of the plasma energy density

with a diamagnetic loop placed around the plasma chamber. We also examined the

anisotropy in x-ray power by simultaneously measuring the x-ray spectra in two

orthogonal directions: radially and axially, using NaI(Tl) detectors.

We have seen that for a 6.4 GHz ECRIS, both the x-ray power produced by confined

electrons and the plasma energy density behave logarithmically with microwave

power. The x-ray flux created by electrons lost from the plasma, however, does

not saturate. Thus, the small increase in plasma density that occurred at high

microwave powers (> 150 W on a 6.4 GHz ECRIS) was accompanied by a large

increase in total x-ray power. We suggest that the saturation of x-ray power and

plasma energy density was due to rf-induced pitch-angle scattering of the

electrons. X-ray power and plasma energy density were also shown to saturate

with neutral pressure, and to increase nearly linearly as the gradient of the

magnetic field in the resonance zone was decreased. All of these findings were

in agreement with the theoretical models describing ECRIS plasmas.

We have discussed the use of a diamagnetic loop as a means of exploring various

plasma time scales on a relative basis. Specifically, we focused much of our

attention on studying how changing ion source parameters, such as microwave

power and neutral pressure, would effect the rise and decay of the integrated

diamagnetic signal, which can be related to plasma energy density. We showed

that increasing microwave power lowers the e-fold times at both the leading edge

and the trailing edge of the microwave pulse. Microwave power, however, had

almost no impact on the ignition times of the plasma.

The plasma energy density e-fold times were insensitive to both neutral pressure

and magnetic field setting. Neutral pressure, however, had a dramatic effect on

the time of first appearance of the diamagnetic signal ("plasma ignition time").

In addition to neutral pressure, ignition times were also a function the

relative abundance of electrons in the plasma chamber at the beginning of a

microwave pulse. In all instances, the rise time of the integrated diamagnetic

signal was seen to be faster than the decay time.

By comparing the unintegrated diamagnetic signal to the ratio of reflected to

forward microwave power we theorized that the initial, exponential rise in the

diamagnetic signal at the leading edge of a microwave pulse was due to rapid

changes in both the average electron energy and density. During the slowly

decaying portion of the diamagnetic loop signal, only the hot tail of the

electron population was increasing. This theory was supported by time resolved,

low energy x-ray measurements that showed that the period of rapid change of the

ratio of reflected to forward microwave power coincided with a rapid change in

average photon energy.

We have also showed that x-rays production in an ECRIS plasma was highly

anisotropic, with radial x-ray counts being much greater than axial x-ray

counts. This was shown to be true for both the "ECR" (operating at 6.4 GHz) and

the higher performance "AECR-U" (operating at 14 GHz). Based on this, we can

make the qualitative statement that the electron energy was also highly

anisotropic, with a much larger perpendicular energy than parallel energy. The

degree of anisotropy was shown to increase with the operating frequency of the

ion source. This increase was most likely attributable to the higher power

density and greater confinement associated with higher performance machines, and

implies that superconducting ECRIS operating at very high frequencies will have

extremely anisotropic x-ray power deposition.

The radial spectral temperature on the "AECR-U" was over twice as large as the

axial spectral temperature. However, in the "ECR" the radial and axial spectral

temperatures were similar. Hence, the anisotropy in spectral temperature also

showed dependence on the magnetic field strength and operating frequency of the

ECRIS. The combination of higher energies, and intensity of high energy x-rays

in the radial direction has important implications in the x-ray heat load

estimates for superconducting ECR ion source cryostats.

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