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.
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.